physical fitness and low back pain

NATIONAL RESEARCH CENTRE FOR THE WORKING ENVIRONMENT

FACULTY OF HEALTH SCIENSES UNIVERSITY OF COPENHAGEN

PHYSICAL FITNESS AND LOW BACK PAIN

Performancebased and selfassessed physical fi tness as risk indicator of low back pain among

health care workers and students

PhD thesis by Jesper Strøyer Andersen

Physical Fitness and Low Back Pain

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PHYSICAL FITNESS AND LOW BACK PAIN Performancebased and selfassessed physical fi tness as risk indicator of low back pain among health care workers and students

PhD thesis by Jesper Strøyer Andersen

National Research Centre for the Working EnvironmentFaculty of Health Sciences, University of Copenhagen, DenmarkNovember 2007


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CONTENTS

Preface ............................................................................................................................... 7

Summary in English .......................................................................................................... 9

Dansk resumé (summary in Danish) ............................................................................ 11

List of papers .................................................................................................................... 13

1. Introduction ..................................................................................................................14

2. Materials and Methods................................................................................................20

2.1 Study populations and designs ...........................................................................20

2.2 Performancebased physical fi tness ....................................................................20

2.2.1 Maximal Oxygen Uptake ............................................................................20

2.2.2 Back Muscle Strength ..................................................................................20

2.2.3 Back extension Endurance ..........................................................................22

2.2.4 Back fl exion Endurance ...............................................................................22

2.2.5 Flexibility .......................................................................................................22

2.2.6 Balance ...........................................................................................................23

2.3 Selfassessed physical fi tness ...............................................................................24

2.4 Low back pain ........................................................................................................24

2.5 Covariates ...............................................................................................................25

2.6 Statistics ...................................................................................................................25

3. Results ............................................................................................................................27

3.1 Basic characteristics of the study populations ...................................................27

3.2 Associations between Performancebased physical fi tness and LBP .............27

3.3 Associations between Selfassessed physical fi tness and LBP ........................29

3.4 Covariates with associations to LBP ...................................................................30

3.5 Characteristics of selfassessed physical fi tness ................................................30

3.5.1 Distribution, sex differences and interitem correlations ......................30

3.5.2 Convergence and divergence of selfassessed and

performancebased fi tness .........................................................................31


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4. Discussion .....................................................................................................................33

4.1 Performancebased physical fi tness as risk indicator of LBP ..........................33

4.2 Selfassessed physical fi tness as risk indicator of LBP .....................................35

4.3. Selfassessed compared with performancebased physical fi tness ...............36

4.3.1 Associations between selfassessed and performancebased

physical fi tness .............................................................................................36

4.3.2 Predictive validity ........................................................................................38

4.4 Methodological considerations ............................................................................39

4.4.1 Limitations and strengths of the thesis .....................................................39

4.4.2 Differences in test procedures and conditions ........................................40

4.4.3 Testretest reliability of selfassessed physical fi tness ............................40

5. Conclusions ...................................................................................................................42

6. Perspectives ..................................................................................................................43

References .........................................................................................................................45

Paper 1 ............................................................................................................................. 53

Paper 2 ............................................................................................................................. 72

Paper 3 ............................................................................................................................. 89


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PREFACE

This PhD project was initiated at the National Research Centre for Working Environment (the former National Institute of Occupational Health), Copenhagen, Denmark, at the department of physiology in September 2003. The thesis was submitted to the faculty of Health Sciences, University of Copenhagen, November 2006 and defended November 2007. The studies were conducted in accordance with the declaration of Helsinki. The project was fi nancially supported by the National Research Centre for Working Environment, Copenhagen, Denmark.

Academic advisors:Associate Professor in Social Medicine Kirsten Schultz Larsen MD, PhD, Institute of Public Health, University of Copenhagen.Senior Researcher Ole Olsen MSc, the National Research Centre for Working Environment Associate Professor in Public Health, Kirsten Avlund PhD, Dr.Med.Sci. at the University of Copenhagen, Institute of Public HealthSenior Researcher Bente Schibye PhD, National Institute of Occupational Health

Opponents:Professor ClasHåkan Nygaard, Tampere School of Public Health, University of Tampere, Finland Professor Jan Hartvigsen, Insitute of Sports Science and Clinical Biomechanics, Odense, University of Southern Denmark.Lektor Dan Meyrowitsch, Institute of Public Health, Department of Epidemiology, University of Copenhagen


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ACKNOWLEDGEMENT

Lots of thanks and thoughts to my former superior and fi rst mentor of the PhD project, Bente Schibye, for inspiring and supporting me in the decision to do this PhD project. As the result of a traffi c accident a month after my matriculation as a PhD student, she was unfortunately unable to return to work again. I have missed not only her critical questioning and physiological knowledge, but also the inspiring and friendly atmosphere surrounding her. Thanks to Kirsten Avlund for being two mentors in one person when Bente was prevented from working, for her optimistic ways, and for keeping me on track when my children had kept me awake the half night. Very unfortunately, also Kirsten was unable to remain my mentor due to sickness in October 2006 – lots of thoughts to her. Thanks to Ole Olsen for stepping in halfway and joining as a mentor, and for his critically statistical advice. Thanks to Kirsten Schultz Larsen for the stepping in at the last moment without any knowledge about my project or me, and for all the hours she spent reading, correcting and discussing my thesis. The thesis is based on an almost incredible number of fi eld measures. Thanks to all my very good colleagues at AMI who spent so much time in the fi eld with me: Nis Hjortskov Jensen, Morten Essendrop, Christian HyeKnudsen, Anne Faber, Klaus Hansen, Dorte Ekner, Jørgen Skotte, Hanne Giver, Susan Warming, Jette Nygaard Jensen, Kirsten Nabe Nielsen Thanks to Sisse Warming, Mette Øllgaard Jakobsen, Pernille Mikkelsen and Ute Bültmann for the computerizing of VAS and test data. Thanks to Karl Bang Christensen for statistical advice and nice talks about soccer. Thanks to all other good colleagues in the SOSU group, the former AEA and ITA departments, and at AMI for support and a convivial working atmosphere. Thanks to Annemarie Eskelund and Lone Donbæk Jensen for constructive and inspiring teamwork. During my PhD period I have had the luck to become the father of three lovely children. Thanks to Marie, Laurits and Emil and to you Lotte for putting my life into perspective.


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SUMMARY IN ENGLISH

Background: The prevalence of Low Back Pain (LBP) is particularly high in the health care sector. Various physical and psychological workrelated aspects typical of the sector have been identifi ed as risk factors of LBP. Despite high physical work demands being characteristic of the health care sector, to date, there is no conclusive evidence that a high level of physical fi tness can prevent LBP. Isometric back extension endurance is one of the physical fi tness parameters that most studies has found to associate with LBP, but as many other studies do not fi nd that low endurance increases the risk of LBP, there is no consensus on its ability to predict LBP.

Aim: The overall aim of this PhD thesis was to examine whether performancebased and selfassessed physical fi tness is associated with the development and prevalence of LBP among persons who work or are training to work in the health care sector. An additional aim was to test convergence and divergence validity of selfassessed dimensions of physical fi tness against performancebased physical fi tness parameters and the infl uence of LBP on this relationship.

Methods: The association between physical fi tness parameters (performancebased and selfassessed) and the increase in LBP intensity (>2) after 30 months (followup) was examined in a cohort of persons working with physically and mentally disabled persons (n=327). Convergence and divergence validity was tested among social and healthcare helpers and assistants in training (healthcare students), persons working with physically and mentally disabled persons, and hospital staff. Associations between 12month prevalence of LBP and physical fi tness parameters and the infl uence of LBP on the association between selfassessed and performancebased physical fi tness were examined in a crosssectional study of healthcare students (in the fi rst week of their training) (n=612). In all studies (the prospective study, the validity study and the crosssectional study), back endurance, fl exibility and balance were tested. In addition to the aforementioned tests, the level of aerobic fi tness, back extension strength, and back fl exion strength were measured in the validity study. Selfassessed physical fi tness was measured identical in all study populations using illustrated visual analogue scales (VAS), (a newlydeveloped and untested tool for selfassessment). The reproducibility of this new tool (selfassessed physical fi tness (week by week)) was tested in a separate group of healthcare students (n=159). Logistical regression analysis was used in the prospective and crosssectional study. In the validity study linear regression analyses were performed (adjusted for age and sex) to examine the convergence and divergence between related and nonrelated selfassessed and performancebased parameters.

Results: Isometric back extension endurance measured using a modifi ed version of BieringSørensen’s procedure was the only physical fi tness parameter that showed any kind of prospective association with increased LBP intensity. The overall association (likelihood ratio test) was only close to signifi cance (p=0.067), whereas those with a medium level of back extension endurance were at a signifi


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cantly higher risk of increased LBP intensity during followup (OR=2.7, p=0.034) in relation to the group with the highest endurance. Those with the lowest score in endurance came only close to having a signifi cantly higher risk (OR=2.37, p=0.076) compared with those with high level endurance. Selfassessed level of aerobic fi tness showed a reverse association with LBP. Those with a moderate level of selfassessed aerobic fi tness had a reduced risk of increased LBP intensity compared to those with a high level of selfassessed physical fi tness (OR=0.37, p=0.02), whereas those with the lowest level did not have a signifi cantly reduced risk (OR=0.58, p=0.23). Selfassessed muscular strength, endurance and fl exibility all showed the same tendency in relation to LBP as selfassessed level of fi tness; however, the associations were not signifi cant. In the crosssectional study an experience of LBP during the previous year was signifi cantly associated with lower score of selfassessed aerobic fi tness, fl exibility and balance, and performancebased fl exibility. In the validity analysis, we found that level of selfassessed aerobic fi tness, muscle strength and fl exibility had a small to moderate association with the corresponding performancebased parameters. An association between the selfassessed fl exibility score and the result of the performancebased fl exibility test result was also confi rmed in the crosssectional study. Additionally, selfassessed endurance and balance scores were also associated with the corresponding performancebased test result. LBP did not confound any of the associations between selfassessed and performancebased physical fi tness. The reproducibility of selfassessed level of fi tness and muscle strength was good (ICC=0.80), whereas endurance, fl exibility and balance showed only moderate reproducibility (ICC=0.620.69).

Discussion: The fi nding of the preventive effect of high isometric back extension endurance in relation to increased LBP intensity is supported by several other studies showing that low back extension endurance, in particular, is signifi cantly associated with an increased risk of LBP. The study did not support a dosesresponse relation between back extension endurance and increased risk of increased LBP intensity, as the risk for moderate and low endurance was shown to be approximately the same. The reduced risk of increased LBP intensity among those with a moderate level of selfassessed fi tness in relation to those with a high level of selfassessed fi tness was surprising and contradicted our hypothesis. One possible explanation could be that those who estimate their level of fi tness as low are also more aware of their fi tness level and, accordingly, have a behavioural pattern where they avoid the physically straining work tasks that they believe they lack the physical fi tness to perform. They thereby reduce the risk of excessive strain and, consequently, reduce the risk of increased back pain. In the long term, this is not a sustainable solution for avoiding LBP, as the individual’s level of physical fi tness will fall in accordance with the reduced mechanical stimulation, whereby the load required to strain the tissue is also reduced. The weak to moderate agreement between performancebased and selfassessed physical fi tness and the contradicting associations with LBP in the prospective study point towards performancebased and selfassessed physical fi tness both measuring something different, despite their association.


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DANSK RESUMÉ (SUMMARY IN DANISH)

Baggrund: Forekomsten af lænderygbesvær (LBP) er særlig høj indenfor pleje og sundhedssektoren. Adskillige fysiske og psykiske arbejdsrelaterede faktorer som hyppigt optræder i denne sektor er identifi ceret som risikofaktorer for LBP. På trods af at pleje og sundhedssektoren er karakteriseret ved høje fysiske arbejdskrav, er der endnu ikke skabt evidens for, at god fysisk kapacitet forebygger LBP, da resultaterne er inkonsistente hvad angår sammenhængen mellem lav fysisk kapacitet og øget risiko for LBP. Isometrisk rygekstensionsudholdenhed er et af de fysiske kapacitetsparametre med fl est påviste sammenhænge til LBP, men da fl ere andre studier ikke fi nder, at dårlig udholdenhed øger risikoen for LBP, er der endnu ikke konsensus om dens evne til at forudsige fremtidigt LBP.

Formål: Det overordnede formål med ph.d.afhandlingen var at undersøge, om målt og selvvurderet fysisk kapacitet er associeret med udviklingen og forekomsten af LBP blandt personer, som er beskæftiget eller under uddannelse indenfor pleje og sundhedssektoren. Ydermere var formålet at teste konvergens og divergensvaliditeten af selvvurderede dimensioner af fysisk kapacitet mod testede fysiske kapacitetsparametre og om LBP infl uerede på disse sammenhænge.

Metoder: Sammenhængen mellem fysiske kapacitetsparametre (testede og selvvurderede) og stigningen i LBPintensiteten (>2) efter 30 måneder (followup) blev undersøgt i en kohorte af personer, der arbejdede med fysisk og psykisk handicappede mennesker (n=327). Konvergens og divergensvaliditeten blev testet blandt kommende social og sundhedsassistenter og hjælpere (SOSUstuderende), personer, der arbejdede med fysisk og psykisk handicappede samt hospitalspersonale. Associationer mellem 12måneders prævalensen af LBP og fysiske kapacitetsparametre (testede og selvvurderede) og indfl ydelsen af LBP på sammenhængen mellem korresponderede målte og selvvurderede kapacitetsmål blev undersøgt i et tværsnitsstudie blandt SOSUstuderende (i deres første uge på uddannelsen) (n=612). I alle studierne (forløbsstudiet, validitetsstudiet og tværsnitsstudiet) blev rygudholdenheden, rygbevægeligheden samt balanceevnen testet. Udover disse tests blev konditionen og den isometriske rygstyrke målt i validitetsstudiet. Selvvurderet fysisk kapacitet blev i alle studiepopulationer målt på samme måde ved hjælp af illustrerede, visuelle, analoge skala?er (VAS), (som var et nyudviklet uprøvet selvvurderingsværktøj). Reproducerbarheden af selvvurderet fysisk kapacitet (uge til uge) blev testet i en separat gruppe SOSUstuderende (n=159). Konvergens og divergensvaliditetsanalysen blev udført ved hjælp af lineær regression (aldersjusteret og stratifi ceret i forhold til køn) for at undersøge konvergensen og divergensen mellem relaterede og ikke relaterede selvvurderede og testede parametre.

Resultater: Isometrisk rygekstensionsudholdenhed målt med en modifi ceret version af BieringSørensens procedure var den eneste fysiske kapacitetsparameter, der udviste egenskaber som risikofaktor i forhold til stigende LBPintensitet. Den overordnede association mellem rygekstensionsudholdenhed og øget LBPintensitet (likelihood ratio test) var kun tæt på statistisk signifi kans (p=0,067). Personer


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med middelniveau af rygekstensionsudholdenhed var i signifi kant større risiko for at udvikle øget LBPintensitet ved followup (OR=2,7; p=0,034) set i forhold til personer med høj udholdenhed. De personer, som havde dårligst udholdenhed, var kun tæt på at være i signifi kant højere risiko (OR=2,37; p=0,076) end for personer med højt niveau. Selvvurderet kondition viste en modsat sammenhæng med LBP. Personer, der vurderede deres kondition til middel, var i reduceret risiko for øget LBPintensitet i forhold til personer med høj selvvurderet fysisk kapacitet (OR=0,37; p=0,02), mens dem med dårligst niveau ikke var i signifi kant mindre risiko (OR=0,58; p=0,23). Selvvurderet muskelstyrke, udholdenhed og bevægelighed viste alle samme tendens i forhold til LBP som selvvurderet kondition, men associationerne var ikke statistisk signifi kante. I tværsnitsstudiet hang oplevet LBP indenfor det sidste år signifi kant sammen med lavere scorer i selvvurderet kondition, bevægelighed og balance og testet rygbevægelighed. I validitetsanalysen fandt vi, at kondition, muskelstyrke og bevægelighed associerede svagt til moderat med de korresponderede målte parametre. Sammenhængen mellem selvvurderet fl eksibilitet og testet rygbevægelighed blev også signifi kant bekræftet i tværsnitsstudiet. Ydermere var selvvurderet udholdenhed og balance signifi kant associerede med de korresponderede målte parametre, og LBP var ikke en konfounder i sammenhængen mellem selvvurderede og testede fysiske kapacitetsparametre. Reproducerbarheden for selvvurderet kondition og muskelstyrke var god (ICC=0,80), mens udholdenhed, bevægelighed og balance viste moderat reproducerbarhed (ICC=0,620,69).

Diskussion: At høj isometrisk rygekstensionsudholdenhed er præventiv i forhold til øget intensitet og forekomst af LBP understøttes af andre studier. Undersøgelsen understøttede ikke en dosisresponssammenhæng mellem rygekstensionsudholdenhed og øget risiko for øget LBPintensitet, idet risikoen for middel og dårlig udholdenhed var nogenlunde den samme. Den påviste reducerede risiko for øget LBPintensitet blandt personer med middelhøj selvvurderet kondition (i forhold til personer med høj) var overraskende og i modsætning til vores hypotese. En mulig forklaring kan være, at personer, som vurderer deres kapacitet til at være lavere, er særligt opmærksomme på deres kapacitet og derfor udvikler et adfærdsmønster, hvor de undgår de fysisk belastende arbejdsopgaver, de ikke mener, de besidder fysisk kapacitet til at udføre. Derved reducerer de risikoen for at blive overbelastet, og risikoen for at få mere ondt i ryggen mindskes. På længere sigt er det dog ikke en holdbar løsning til at undgå LBP, fordi den fysiske kapacitet vil blive mindre i takt med, at den mekaniske stimulering reduceres, hvorved den ydre kraft, der skal til for at overbelaste bevægeapparatet, også mindskes. Den svage til moderate overensstemmelse mellem målt og selvvurderet fysisk kapacitet samt de modsat rettede sammenhænge med LBP i det prospektive studie tyder på, at testet og selvvurderet fysisk kapacitet måler noget forskelligt på trods af, at de er associerede.


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LIST OF PAPERS

This thesis is based on three papers. They will be referred to in the text by their roman numerals.

I. The role of physical fi tness as risk indicator of increased Low Back Pain intensity among people working with physically and mentally disabled persons: A 30month prospective study.

Jesper Strøyer1, Lone Donbæk Jensen2. Submitted to SPINE November 2006, and accepted for publication September 2007.

1National Research Centre for the Working Environment, Denmark, 2University Hospital of Aarhus, Denmark.

II. Construct Validity and Reliability of Selfassessed Physical Fitness. Jesper Strøyer1, Morten Essendrop2, Lone Donbæk Jensen3, Susan War

ming4, Kirsten Avlund5, Bente Schibye1. Perceptual and Motor Skills, 2007, 104, 519533.

1National Research Centre for the Working Environment, Denmark, 2Group Clinical development, ALKAbello Hørsholm, Denmark, 3Department of Occupational Medicine, University Hospital of Aarhus, Denmark, 4Clinical Unit of Health Promotion Bispebjerg University Hospital, Denmark, 5Institute of Public Health, University of Copenhagen

III Is the crosssectional association between selfassessed physical fi tness and performancebased physical fi tness among health care students infl uenced by low back pain?

Jesper Strøyer1, Annemarie Lyng EskelundHansen2 , Kirsten Schultz Larsen3 and Niels Erik Ebbehoej2.

Manuscript.

1National Research Centre for the Working Environment, Denmark. 2Clinic of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, 3 Institute of Public Health, Department of Social Medicine, University of Copenhagen.


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1. INTRODUCTION

Low back pain (LBP) is both a Danish and a global public health problem and results in many days of sick leave, reduced work capacity or, ultimately, expulsion from the labour market, leading to extensive human and socioeconomic consequences (44,129). Health care workers seem to be particularly exposed to LBP. International studies reported their 12month prevalence of LBP as above 60%, compared to employees in general who showed a prevalence closer to 50% (22,63,70,92,99,113,117). Of the social and healthcare helpers and assistants who were under education (healthcare students) in Denmark in 2004, 51% (n=5700) had experienced LBP during the previous year (95), whereas a much higher 12month prevalence of 69% was found among experienced healthcare who were in employment during the same period in Denmark (n=8038) (personal communication). This difference between those under education and those in employment indicates that the high prevalence of LBP may be caused by factors experienced when performing the work at the workplace. From comparisons with school teachers it has been shown that 43% of the musculoskeletal disorders in the back and knee among employees in the home care and 24hour care centres can be ascribed to the occupational environment. This implies that these musculoskeletal disorders can be prevented (9). In addition, on the bases of more than 300 studies, the Occupational Safety and Health Administration (OSHA) concludes that with implemented ergonomic interventions in different occupations, 66% of the sick leave due to musculoskeletal disorders can be prevented in the health care sector (1). Thus, in the health care sector the potential of prevention seems to be large. Healthcare workers are highly exposed to both physical and psychosocial work factors. Awkward, rotated and fl exed work positions take place during patient handling and patient care, and cleaning are often carried out at diffi cult accessible spaces (18,62,63,123). In addition, the working environment are characterised by high workload, high time pressure, low job control, low social support and confl icts with patients (11,18,19,57,62,63). Many attempts have been made to establish predictors of LBP with the aim of enhancing prevention. Physical workload is identifi ed as a risk indicator of LBP, both as a general factor compared to jobs with low physical demands, and as more specifi c work factors such as frequent bending or twisting of the back, heavy lifting and patient handling (15,20,49,53,61,65,67,82,106). Psychological work factors, for instance low social support, are also suggested as risk indicators of LBP; however, the interpretation of the evidence regarding the risk of LBP are contradictory (26,51,54,78). The inconsistency in results regarding psychological work factors might be due to the many different ways of measure psychological work factors or lack of controlling for potential confounding from occupational biomechanical demands as discussed by Davis and Heaney (26).

A conceptual model for the complex interaction between different work factors present in the work environment, the way they interact with the individual and the way it leads to pain, impairment and further to disability is illustrated in the model in fi gure 1 (96). The model which is further presented and described in the


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comprehensive work by the National Research council (97) deals with two broad categories: workplace factors (the workplace) and characteristics of the person (the person). The person is identifi ed as the central biological entity subject to biomechanical loading with the various physical, psychological and social features associated with the individual that may infl uence the biological and clinical response, and disability response. The issue for this thesis is the association individual factors in the interaction with the biomechanical loading (which is a result of the external loads) and the internal tolerances. The theory is that this interaction is crucial to generate pain. The biomechanical mechanism that explains how the external load leads to mechanical loadings which further result in mechanical strain, tissue damage and LBP was described by McGill (88). McGill defi nes a failure tolerance which is the limit an applied load shall exceed to cause injury or failure of the tissue. The distance between the applied load and the failure tolerance is the margin of safety. McGill defi nes injury as the full continuum from the most minor of tissue irritation to the grossest of tissue failure and presumes that such damage generates pain. A bigger margin of safety implies a reduced risk of pain. According to this theory, LBP caused by physical work demands can be prevented by two strategies: one is to decrease the applied physical load (external loads in fi gure 1); the other is by increasing the failure tolerance. Both strategies increase the margin of safety and thereby reduce the risk of tissue damage and LBP.

To reduce the physical load in the health care sector, as suggested in the fi rst strategy is diffi cult because the job (as for example health care worker) involves working physically with people who may be unmanageable and unpredictable objects. The introduction of mechanical lifts devises has reduced some of the major loads, but it is diffi cult to standardize physical work tasks in the health care sector so that technical aids can be used exclusively. Another way of reducing the applied load is by better patient handling techniques. Better patient handling technique has shown to reduce the applied load signifi cantly in isolated methodological studies (81,111); however, to date, no effect on LBP was found when instruction in patient handling was implemented in the workplace (50,52,114).The second strategy, which is the focus of this PhD, is the opportunity to improve the failure tolerance by increasing the person’s level of physical fi tness. In the present thesis physical fi tness is defi ned as the dimensions of aerobic fi tness, muscle strength, muscle endurance, fl exibility and balance. This defi nition was inspired by the early work of Fleishman (37) and the defi nition by Miller (91). Both concepts refl ect the belief in a multidimensional nature of physical fi tness. Physical fi tness was in the study clearly distinguished from physical activity and physical training.Physical fi tness belongs to the “Individual factors” box of the model (fi gure 1) and as the arrows in the model indicate it can interact with the physiological pathway on several levels. In the thesis the focus is on the connection with the biomechanical box. By increasing the individual’s physical fi tness the relative load decreases as do the internal loads which match the external loads.

The relationship between low level physical fi tness and increased risk of future


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LBP as the theory by McGill and the conceptual model support, has been attempted to be established in several studies introducing a wide spectrum of physical fi tness tests: Aerobic fi tness that is the physical fi tness component with the most unambiguous defi nition of maximal oxygen uptake (VO2) per kilo of body weight (mlO2*min1*kg1) has not been found as a independent risk factor of LBP (13,28,72,118). Despite the very exact defi nition, the testing of aerobic fi tness is not unproblematic under fi eld test conditions because the direct measurement of maximal oxygen uptake is both time and equipment consuming and the maximally testing can furthermore imply a health risk for the participants, which makes it less applicable as a test among the working population and for people in all ages. Consequently, many of the epidemiological studies used indirectly methods for the assessment of aerobic fi tness as the widely used Åstrand test (7,8) or the UKK 2KM Walk test (100), although they are still time consuming in large scale studies. The indirectly test introduces markedly standard errors for predicting the maximal oxygen uptake that makes the relationship with LBP harder to fi nd. In general more knowledge exists about the effect of physical activity, which traditionally has been assessed by questionnaires, compared to aerobic fi tness in relation to

Figure 1. A conceptual model of the possible roles and infl uences that various factors may have in the development of musculoskeletal disorders. (National Research Council and the Institute of medicine. Musculoskeletal Disorders and the Workplace. Washington, D.C.: National Academy Press, p. 1492, 2001. Originally adapted from National Research Council. WorkRelated Musculoskeletal Disorders: Report, Workshop Summary, and Workshop Papers. Steering Commitee for the workshop on WorkRelated Musculoskeletal Injuries: The Research Base. Washington, DC: National Acedemy Press. 1999)


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the incidence and especially treatment of LBP (102). However, due to the genetic factor of aerobic fi tness (104) and the fact that physical exercising at a low level not necessary increases the maximally oxygen uptake, the effect of physical exercise and aerobic fi tness in relation to health outcomes have to be distinguish. The association between leisure time physical activity and LBP is not within the scope for this thesis. The remaining components of physical fi tness are more wide defi ned than aerobic fi tness and a diversity of methods have been applied to test for associations with future LBP.Muscle strength in relation to LBP have primarily been measured by isometric strength tests (3,12,16,21,77,79,83,98,107,124) but also isokinetic tests have been used (31,41,45,72,75,98,122). Despite that fi ve studies found poor trunk muscle strength as predictive for low back pain (10,16,42,83,122) the evidence is inconclusive for a relation between trunk muscle strength and the risk of low back pain (47) due to inconsistent results. The remaining studies found no relationship between muscle strength and LBP or found the opposite result, that high muscle strength was a risk factor for LBP (77,83). Muscle endurance has both been measured in dynamic setups (77,79,105,109) and static setups (3,17,41,45,64,68,79,115,118,122). The dynamic test procedures vary much between studies whereas static endurance in relation to LBP is almost solely evaluated by the back extension endurance test by Biering Sørensen (16) that is a kind of golden standard although it has several modifi ed but closely related versions (27). A static back fl exion endurance test has also been evaluated as predictor of LBP (68). Three of the four studies using dynamic setups (77,79,105) and six of the ten studies using the static setup of Biering Sørensen (3,41,64,68,115,118) and the study testing static back fl exion endurance (68) found no relationship between muscle endurance and the risk of LBP. That fi ve studies found low level of isometric back extension endurance to be a risk factor of LBP (17,45,64,79,122) indicate that this particular test could have the quality as a risk factor of LBP, although inconclusive evidence is stated (48). Another physical fi tness component that traditionally has been linked to the risk of LBP is the fl exibility of the spine that has been evaluated with a variety of methods. Best known for epidemiological approaches are the modifi ed Schöber test (80), the fi ngertofl oor method (40) and the sitandreach test (4). In general, all methods and LBP outcomes together, there is weak evidence for no relation between fl exibility and the risk of LBP. Although most of the studies that examined this relationship found no relation with LBP (3,14,17,42,45,79,83,105,115,118,122), one study found high fl exibility to increased risk of LBP (among men) (16) whereas two other studies found the opposite result that low fl exibility increased the risk of LBP (3,122). The last component of physical fi tness that is examined in this thesis is balance which is the most sparsely examined physical fi tness parameters in relation to risk of future LBP. Balance is a very complex motor skill and there is no solid consensus regarding the defi nition of balance control or globally “gold standards” for measuring it (103). No studies have to my knowledge found balance to be associated with future LBP among healthy persons (118)In conclusion no moderate or strong evidence is to date identifi ed for any compo


18

nents of physical fi tness as predictors of LBP despite a variety of studies, although some components have showed more promising ability as a predictor of LBP than others.Physical fi tness is traditionally assessed using objective performancebased tests as in the aforementioned studies. However, despite performancebased tests having several advantages, including high reproducibility and sensitivity to change, they also have the disadvantages of being timeconsuming, and requiring adequate space, special equipment and trained examiners. Hence, performancebased tests are not always suitable for epidemiological studies involving a large number of subjects. In such cases, selfassessed physical fi tness can be a practical and costeffective alternative. The question is whether performancebased physical fi tness can be substituted by selfassessed physical fi tness.Several instruments for selfassessment of physical fi tness are designed, and compared with performancebased parameters of physical fi tness (2,32,69,74,90,101,127,130). However, despite welldesigned studies and the multidimensional structure of physical fi tness (91), participants have experienced diffi culties in differentiating between different physical fi tness components when they completed the selfassessment and the validity of the different components showed great variety. Visual analogue scales (VAS) is a feasible, valid, and reliable method when assessing subjective experience of pain (84) but VAS has also showed to be applicable in other areas such as fatigue (76), appetite (38) and disability (5). In the present thesis the use of VAS for selfassessment of physical fi tness was tested.


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AIMThe overall aim of this thesis was to expand the knowledge about physical fi tness as a risk indicator of LBP. Physical fi tness parameters were assessed by both objective tests and by selfassessment using a newly developed questionnaire using visual analogue scales.In addition the agreement between the performancebased tests and the selfassessed questions was evaluated, and the infl uence of LBP and individual factors on this relationship was further studied, to examine the applicability of the selfassessment instrument for use in future larger surveys.

Aims of the specifi c papers:

I. The aim of paper I was to test if a low level of selfassessed or performancebased physical fi tness was associated with an increase in LBP intensity at 30month followup among people working with physically and mentally disabled persons.

II. The aim of paper II was to analyse the convergence and divergence between selfassessed and performancebased physical fi tness and to test the reliability of the instrument for selfassessment of physical fi tness using VAS.

III. The aim of paper III was fi rst to analyse if the association between LBP and selfassessed physical fi tness differed from the association between LBP and performancebased physical fi tness. Second, it was analysed if LBP confounded the adjusted associations between corresponding dimensions of selfassessed and performancebased physical fi tness.


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2. MATERIALS AND METHODS

2.1 STUDY POPULATIONS AND DESIGNSThe different study populations all comprised people who were either employed in areas or training to work in areas that involved working physically with people. A short description of the study populations and the performancebased dimensions and methods we applied are listed in table 1. Although all study populations comprised both women and men, it should be noticed that the number of men are low in all populations. The associations of performancebased and selfassessed physical fi tness with LBP outcomes were analysed in a prospective design with a 30month followup (“the prospective study” or paper I). The validity and reliability analyses were based on secondary analyses of crosssectional data from several surveys comprising performancebased and selfassessed parameters assessed within the same month (“the validity study” or paper II). The differences and similarities between selfassessed and performance based physical fi tness in relation to each other and LBP were further examined among healthcare students in their fi rst week of training (“the crosssectional study” or paper III). 2.2 PERFORMANCEBASED PHYSICAL FITNESSAll the performancebased tests were selected according to the following criteria: that, as far as possible, they were reported as valid and reliable (inter and intratester), that they provided well distributed scores with limited “ceiling” and “fl oor” effects, and that they were easy to administer in a fi eld setting. There should be minimal health risk involved with participation in the test battery; additional, participants should be able to perform the tests in everyday clothing.

2.2.1 Maximal Oxygen UptakeAerobic fi tness (ml O2*kg1* min1) was measured using the Åstrand ergometer bicycle test (7,8) in conjunction with the Åstrand agecorrection factor (24). The subjects cycled on an ergometer bicycle for 6 minutes on a preselected power that was estimated to give a steadystate heart rate of at a least 120 beats per minute. The standard error for predicting the maximal oxygen uptake was reported as between 10% and 15% (8,24), and this has been shown to have good reliability (r=0.830.93) (24,66).

2.2.2 Back Muscle StrengthMaximal isometric voluntary contraction (MVC) of the back extensor and fl exor muscles was measured with a straingauge dynamometer fi xed to the wall with the subjects fi xed in a standing position (33,34). Lever arm for torque calculations at L4/L5 level was defi ned as the vertical distance from the middle of the strap to the upper edge of the iliac crest. The reliability of the isometric strength tests was reported as good (Intraclass Correlation Coeffi cient (ICC)=0.910.96) (33).


21


22

2.2.3 Back extension EnduranceIsometric back extension endurance was measured using a modifi ed version of the Sørensen test (16,27,93). The subjects were placed on their stomach with their navel over the edge of a padded sloping board, which was 70 cm in length and 15 cm high in the raised end. The subject’s feet were pressed down to the fl oor by an assistant and the subject was instructed to fold his/her arms across the chest and hold the upper body in a horizontal position for as long as possible. The sloping board implied that the hip fl exion was approximately 12º during the test. The determination time differed between the studies. In paper II the test was continued for a maximum of 360 seconds, whereas the time limit in paper III was reduced to 180 seconds due to a tight schedule. The time limit of 180 seconds was decided on the basis of previous test data from which we expected that approximately 50% of the test persons would hold the position for 180 sec. Consequently, the test result was treated as a continuous variable when using 360 seconds and as a dichotomous variable when using 180 seconds (passed/not passed 180 sec). Among healthy subjects and with a time limit of 240 seconds, the reliability was reported to range from 0.54 to 0.99 (ICC) and among physically active LBP subjects the ICCs ranged from 0.82 to 0.96 (66,93) and, in general, can be regarded as moderate to good.

2.2.4 Back fl exion EnduranceThe methods used to test isometric back fl exion endurance differed between the studies. In paper I and II , isometric back fl exion endurance was tested with the subjects in a supine position with 90Ÿ hip and knee fl exion and with their feet supported on a chair (56,60). The subjects were instructed to curl up until a band fi xed around the chest, at the height of the inferior angulus of scapulae, was free of the fl oor. This position was held for as long as possible or to a maximum of 360 sec. The reliability of the test has been reported as good (r=0.93) (56,60). In paper III, isometric back fl exion endurance was measured using the test developed by McGill (85,89). The subject was positioned in a situp posture with the back resting against a jig angled at 60º from the fl oor. Both knees and hips were fl exed 90º, the arms were folded across the chest, and the toes were secured under toe straps. To begin, the jig was pulled back 10 cm and the person held the isometric posture for as long as possible or to a maximum of 180 sec. The test was evaluated as passed or not passed the 180 sec maximum. An unspecifi ed reliability coeffi cient of 0.97 on the basic of fi ve people only was reported by McGill (89).

2.2.5 FlexibilitySagittal fl exibility was measured using the modifi ed fi ngertofl oor method (40,108) and calculated as the distance from the fi ngertips to the fl oor in a fully fl exed position when standing on a 30 cm measuring box without shoes. Positive values indicate that the subject was unable to reach the top of the measuring box (fl oor level) during a full forward bending; negative values indicate that the subject was able to reach further down the side of the box (below fl oor level) during a full forward bending. The reliability was reported as good (ICC=0.93) (40).


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2.2.6 BalanceIn paper I and II the ability to balance and coordinate the trunk muscles was evaluated by a balance test in a sitting position on a wobble board. This test was developed by Essendrop and HyeKnudsen at the National Institute of Occupational Health, Denmark. The wobble board was placed on a table, which allowed the legs to hang down freely over the edge of the table. Two trials were given to familiarize the subject with the wobble board and thereafter the subject was asked to keep the wobble board in balance through movements of the hip and the back. If any body segment or the edge of the wobble board touched the table the stopwatch was stopped, but not zeroed, and one attempt was counted. Number of attempts was counted until the subject had been balancing for one minute or a maximum of 15 attempts was reached. McGill et al. have previously used a similar setup (87). In a testretest setup at the National Institute of Occupational Health, Copenhagen, we tested the reliability, which was found to be good (r=0.90, n=36 health care students, not published).


24

In paper III, the balance was tested as the ability to stand on one leg with the eyes open for 60 seconds (onelegstanding test) (116,120). The participants placed the heel of the opposite foot against the inner side of the supporting leg at the level of knee joint. The arms hung as relaxed as possible down the sides. The subjects were fi rst instructed to familiarize themselves with the balance position and to choose the supporting leg where they felt most comfortable. The subjects were told to stand in the position for as long as possible and as steadily as possible. The reliability was reported as acceptable for fi eld testing of fi tness (ICC=0.76)(120).

2.3 SELFASSESSED PHYSICAL FITNESS

Identical designs for selfassessment of physical fi tness were applied in all studies. The fi ve components of physical fi tness that were considered essential when coping with physically demanding tasks were selfassessed: aerobic fi tness, muscle strength, endurance, fl exibility, and balance. The answers were measured by fi ve VAS of 100 mm with illustrations and verbal anchoring of the extreme situations (Figure 2). The subjects were asked: “How would you score the following components of physical fi tness in relation to people of your own age and sex?” and they indicated their replies as vertical marks on the VAS. The VAS scores were computerized by a digitizer (Intuos A4 regular, Wacom Co Ltd.) which showed a high degree of intraobserver reliability (ICC2.1=1.00, 115 double VAS readings) and interobserver reliability (ICC2.1= 1.00, 26 VAS readings by six observers). The selfassessment instrument was designed in a group of PhD’s, associate professors and candidates with an educational background of sports science or physiotherapy from the department of work physiology at the national institute of occupational health. The fi nal graphic design was made by Christian HyeKnudsen.

2.4 LOW BACK PAIN

Low back pain was defi ned as tiredness, discomfort or pain in the low back region with or without radiating symptoms to the leg or legs. The low back region was defi ned as the region of the back between L1 and the gluteal folds. In the prospective study (paper I), cases were defi ned as subjects with an increase of more than two at followup in selfreported rating of the LBP intensity during the past twelve months (one indicating no pain and 10 indicating worst possible pain). It has been shown that an increase in LBP intensity above two was clinically relevant among chronic LBP patients (35). Due to the high prevalence of LBP in the study population it would not had been possible to include only the “never experienced LBP” subgroup if the analyses should have suffi cient statistical power. Only subjects with LBP intensity below 6 at baseline were included in the analyses to ensure that all subjects had the possibility of being a case. In the crosssectional study (paper III), the 12month prevalence of LBP was included in the analyses with physical fi tness parameters as independent variable. Although the 12month prevalence deals with the experience of LBP during the previous year it was regarded as a measure of current status of nonspecifi c LBP. LBP questions were assessed using the Standardized Nordic Musculoskeletal Questionnaire (30,73).


25

2.5 COVARIATES

The defi nition and classifi cation of selected covariates are described. For a complete description of the covariates see the original papers.In paper I, seniority was defi ned as years working with physically and mentally disabled people. 010 yr (low), >10 yr (high). Body mass index (BMI) was classifi ed into three groups: <=24.9 (normal weight), 2530 (overweight), >30 (severe overweight). Leisure time physical activity was assessed by a modifi ed version of the item of Saltin and Grimby (110): 04 hours/week (low level activity), >4hours/week (high level activity). Previous LBP history was defi ned as: no period ever with 3month persistent LBP (no); at least one 3month period with persistent LBP (yes). The frequency during the workday of bent back was classifi ed to: neversometimes (seldom); oftenvery often (often). The frequency during the workday of rotated back was classifi ed to: neversometimes (seldom); oftenvery often (often). Physical workload during patientrelated tasks was assessed by a continuous scale (014) and dichotomised into: 05 (light); 614 (strenuous). The psychosocial scales: infl uence at work, quantitative job demands, emotional job demands, and cognitive job demands, were measured using the Copenhagen Psychosocial Questionnaire (COPSOQ) (71) and classifi ed into tertiles.In paper III, A history of physical demanding job was defi ned as more than 6 month in the past on the basis of an open question about their previous occupations and duration. Educational attainments was categorized as low (≤9 years) or high (>9 years). Height was measured by an electronic height measuring unit (SOEHNLE Professional GmbH & Co. KG Postfach 1308, D 71536 Murrhardt). BMI was categorized as in paper I.

2.6 STATISTICS

The prospective associations with increased LBP intensity as outcome (paper I) was tested with multivariate logistic regression analyses. Only physical fi tness parameters with a sex and age adjusted association to the outcome at p<0.20 were tested further. This was done in parallel multivariate logistic regression analyses including those covariates in each analysis that showed an isolated association (sex and age adjusted) to the outcome at p<0.20. Sex and age were fi xed factors in all the analyses. The choice of the relatively high signifi cance level as the screening criterion for variable selection was to ensure that all important variables were identifi ed (55). Due to the relatively few data and thereby limited statistical power no interactions terms were included in the analyses. The validity of selfassessed physical fi tness examined in paper II was evaluated by convergent and divergent validity analyses based on ageadjusted linear regression analysis of selfassessed physical fi tness parameters with performancebased parameters (43,59). Convergent validity was evaluated by the size of the ageadjusted correlation coeffi cient between corresponding parameters according to the criteria of Innes, (59) who defi nes r<0.30 as poor, r≥0.30 as moderate, and r≥0.60 as good convergent validity. If the convergent validity was satisfactory, the divergent validity was further evaluated by correlations of selfassessed compo


26

nents with noncorresponding parameters of performancebased physical fi tness. Because different components of physical fi tness would not be totally unrelated, the criterion for a satisfactory divergent validity was a markedly lower correlation between noncorresponding parameters than corresponding parameters and was not necessarily a nonsignifi cant relationship. The construct validity analyses were stratifi ed by sex because of the very skewed distribution between women and men. The reliability was assessed by ICC values that take the number of subjects and test sessions into account (29), and by confi dence intervals of the difference between test sessions to test for systematic changes. The ICC values were evaluated according to the reliability criteria of Innes (58), who defi nes ICC<0.75 as poor to moderate, ICC>0.75 as good, and ICC≥0.90 as “required for clinical application to ensure valid interpretations of fi ndings”. A signifi cance level of p<0.05 was chosen. In paper III the association between LBP and the physical fi tness parameters was described stratifi ed to sex but analysed in the total group of both sex. The correlations between LBP and the continuous physical fi tness variables (the selfassessed components and performancebased fl exibility) were tested by GLM analyses with the physical fi tness score as the dependent variable and LBP and covariates as independent variables. The correlations between the dichotomy physical fi tness variables (the back endurance and balance tests) were tested by a logistic regression model with the test result (failed/passed) as dependent variables and LBP and covariates as independent variables. A sex adjusted and a full model analysis, that included all the covariates, were performed.The ability of selfassessed endurance, fl exibility and balance to discriminate the persons who failed the corresponding performancebased test from the ones who passed the test, were tested by general linear models (GLM) with the selfassessed score as the dependent variable and the performancebased (passed/failed) and covariates as independent variables. To examine the effect of including the covariates, an analysis only adjusted to sex was performed fi rst, then an analysis including the remaining covariates except from LBP, and then fi nally a full model including LBP. Signifi cance level of p<0.05 was chosen. The SAS statistical software was used for all the analyses (PROC GLM was used for linear regression analyses and PROC GENMOD for logistic regression analyses).


27

Table 2. Basic characteristics of the study participants. Mean (SD) or n (%)

Paper Study status of Sex Age BMI population occupation Women Men years kg*m2

I IA in job 271 (83%) 56 (17%) 47 (9) 25 (5) II IIA in job 530 (81%) 123 (19%) 45 (9) 25 (5) II IIB students 91 (95%) 5 (5%) 31 (12) 24 (5) II IIC in job 170 (91%) 17 (9%) 34 (9) 24 (4) II IID students 149 (94%) 10 (6%) 35 (11) 24 (4) III IIIA students 511 (83%) 101 (17%) 33 (10) 25 (5)

3. RESULTS

The main results from the different studies are presented under the following headings: Basic characteristics of the study populations, Associations between Performancebased physical fi tness and LBP, Associations between Selfassessed physical fi tness and LBP, Covariates with associations to LBP, and Characteristics of selfassessed physical fi tness.

3.1 BASIC CHARACTERISTICS OF THE STUDY POPULATIONS

Some basic characteristics of the study participants are listed in table 2. In all the study populations the women were in the majority, ranging from 81 to 95%. The people working with physically and mentally handicapped persons of study IA and IIA V were older than the other study populations, which were relatively equal in age (3135 years). No differences were found in BMI.

3.2 ASSOCIATIONS BETWEEN PERFORMANCEBASED PHYSICAL FITNESS AND LBP

Isometric back extension endurance measured using the Sørensen method was the only performancebased parameter that showed any kind of association with increased LBP intensity in the prospective study (paper I). In the multivariate analyses of the prospective study comprising people working with physically and mentally disabled persons, the general association with increased LBP intensity was only close to signifi cance (p=0.067) (table 4). However, medium level isometric back extension endurance resulted in a more than twofold higher risk of increased LBP intensity after the 30month followup compared to those with high level back extension endurance (OR=2.7, p=0.034, Wald test) (table 4). Low level isometric back extension endurance showed an almost similar but insignifi cantly higher risk of increased LBP intensity (OR=2.4, p=0.076, Wald test). After stratifi cation by frequency of rotated back, which was the only covariate included in the multivariate model except for age and sex, neither the subgroup exposed to low physical demands nor the subgroup exposed to high physical demands was signifi cantly associated with increased LBP intensity.


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Table 3. Associations of physical fi tness parameters at baseline with increased LBP intensity at 30month followup, adjusted to age and sex (paper I).

N % cases OR 95% CI p (Wald) p (LR)Performancebased physical fi tness Back extension endurance 0.12 High 116 8 1 . Medium 104 15 2.26 0.945.47 0.069 Low 96 15 2.18 0.875.44 0.095 Back fl exion endurance 0.33 High 108 9 1 . Medium 97 12 1.58 0.633.92 0.33 Low 104 14 1.92 0.794.67 0.15 Sagittal fl exibility 0.77 High 104 14 . Medium 103 11 0.74 0.321.71 0.48 Low 109 11 0.85 0.371.96 0.70 Balance 0.47 High 93 11 1 . Medium 89 11 0.97 0.372.52 0.94 Low 97 16 1.56 0.633.86 0.33 Selfassessed physical fi tness Aerobic fi tness 0.059 High 109 17 1 . Medium 112 8 0.37 0.160.87 0.02 Low 106 12 0.58 0.261.29 0.18 Muscle strength 0.27 High 111 15 1 . Medium 107 13 0.82 0.381.78 0.62 Low 109 9 0.51 0.221.18 0.11 Endurance 0.29 High 115 16 1 . Medium 112 10 0.54 0.241.21 0.13 Low 100 12 0.66 0.291.46 0.30 Flexibility 0.32 High 121 16 1 . Medium 106 9 0.54 0.241.23 0.14 Low 100 12 0.74 0.341.62 0.45 Balance 0.96 High 111 12 1 . Medium 113 12 0.91 0.402.08 0.83 Low 103 14 1.02 0.442.34 0.97

Wald: Wald test for level vs. reference level. LR: Likelihood ratio test for type3 signifi cance.

However, the ORs among those highest exposed were markedly higher than the lowest exposed (3.9/3.2 vs. 2.0/1.8). Isometric back fl exion endurance, sagittal fl exibility and balance showed no signifi cant prospective associations with increased LBP intensity.In the crosssectional study (paper III), LBP was included in the analyses as independent variable and tested for associations with selfassessed and performance


29

based physical fi tness (table 5). Both in the sexadjusted and in the full model analyses (adjusted to educational attainments, history of physical demanding job, BMI, height, age and sex), LBP during the previous year was signifi cantly associated with poorer fl exibility. Although, the associations of LBP with the back endurance tests and the balance test did not reach statistical signifi cance (p=0.120.15), a general picture was found that the percentages of persons who passed the tests were highest in the group without experience of LBP during the previous year.

3.3 ASSOCIATIONS BETWEEN SELFASSESSED PHYSICAL FITNESS AND LBP

In the prospective study, persons with medium level selfassessed aerobic fi tness were at a signifi cantly lower risk of increased LBP intensity than those with high level selfassessed aerobic fi tness (OR=0.37, p=0.02) (table 4). Persons with low level selfassessed aerobic fi tness did not reduce their risk signifi cantly (OR=0.58, p=0.23) and the general association of aerobic fi tness was also insignifi cant (p=0.066) (table 4). Selfassessed muscle strength, endurance, fl exibility and balance were not strongly enough associated with increased LBP intensity to be included in multivariate analyses (p>0.20). However, it was notable that the ORs of selfassessed strength, endurance and fl exibility were all below 1 (table 3), indicating an association between low level of LBP and decreased risk of increased LBP intensity, which was the same as that found for aerobic fi tness.In the crosssectional study of health care students, experience of LBP in the previous 12month was highly signifi cantly associated with lower score of selfassessed aerobic fi tness, both in the sexadjusted and the full model analysis (table 5). Additionally, LBP was signifi cantly associated with lower selfassessed fl exibility and balance. The reduction in signifi cance level when the association between LBP and selfassessed balance was analysed in the full model compared with the sexadjusted was due to the inclusion of BMI in the analysis.

Table 4. Multivariate logistics regression analyses of physical fi tness parameters adjusted to sex, age, and rotated back with increased LBP intensity at 30month followup (paper I).

Full model OR 95%CI p

Performancebased back extension endurance 0.067 A High level 1 Medium level 2.71 1.086.79 0.034 Low level 2.37 0.916.14 0.076 Selfassessed aerobic fi tness 0.066A

High level 1 Medium level 0.37 0.150.88 0.02 Low level 0.58 0.271.38 0.23 ALikelihood ratio test.


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Table 5. Crosssectional associations between LBP and physical fi tness variables (paper III) 12month prevalence of Association between LBP LBP and the physical fi tness All n=612 parameter† yes no sex full model‡ adjustedSelfassessed physical fi tness Aerobic fi tness (mm), mean (SD) 43 (19) 51 (20) p<0.0001 p<0.0001 Muscle strength (mm), mean (SD) 50 (18) 52 (18) p=0.39 p=0.28 Endurance (mm), mean (SD) 53 (20) 56 (19) p=0.14 p=0.18 Flexibility (mm), mean (SD) 49 (21) 55 (19) p=0.0012 p=0.0082 Balance (mm), mean (SD) 55 (19) 59 (20) p=0.0061 p=0.026 Performancebased physical fi tness Back ext. endurance, n (%) passed 180s 114 (50%) 217 (58%) p=0.17 p=0.15 Back fl ex. endurance, n (%) passed 180s 93 (40%) 182 (49%) p=0.10 p=0.15 Flexibility (cm above fl oor level), mean (SD) 1 (10) 3 (9) p=0.0011 p=0.0066 Balance, n (%) passed 60s 188 (82%) 299 (86%) p=0.13 p=0.12†GLM performed for continuously variables and multivariate logistic regression analyses for dichotomous variables, physical fi tness as dependent variable. ‡ Adjusted to educational attainments, history of physical demanding job, BMI, height, sex and age.

3.4 COVARIATES WITH ASSOCIATIONS TO LBP

Frequency of rotated back was the only covariate of the prospective study that was suffi ciently associated with increased LBP intensity to be included in the multivariate model (table 3). It remained insignifi cant in the multivariate analyses but changed the estimate of performancebased back extension endurance (table 4). Among women, the risk of increased LBP intensity was almost three times higher in the prospective study, but it did not reach signifi cance (OR=2.9 p=0.083) (data not shown). The association between LBP and covariates was not tested in the prospective study

3.5 CHARACTERISTICS OF SELFASSESSED PHYSICAL FITNESS

3.5.1 Distribution, sex differences and interitem correlationsThe full lengths of the VAS were satisfactory utilized among both women and men. We observed that the participants showed a quick and intuitive feeling of where to score their physical fi tness on the VAS. Normal distributions of all VAS scores were assumed due to skewness and kurtosis values between one and minus one (study population IIAC). Men’s mean score of selfassessed aerobic fi tness, muscle strength, endurance, and balance were signifi cantly higher than the women’s in the study populations of both paper II and III. The men scored their fl exibility signifi cantly lower than did the women of paper II, whereas no difference was found according to sex in paper III. It appears from table 6 that strong interitem correlations were observed between the selfassessed physical fi tness components (r=0.290.58, paper II).


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Table 6. Interitem correlation coeffi cients* of selfassessed physical fi tness scores. In total 935 women and men (study population IIAC).

N=935 women and men* 1 2 3 4 51. Aerobic fi tness 0.41 0.58 0.34 0.312. Muscle strength 0.49 0.29 0.323. Endurance 0.35 0.384. Flexibility 0.365. Balance

*All correlations were signifi cant at p<0.0001.

3.5.2 Convergence and divergence of selfassessed and performancebased fi tness In women, poortomoderate convergent validity was found for selfassessed aerobic fi tness, muscle strength and fl exibility (r=0.300.36), whereas endurance and balance showed poor convergent validity (r=0.050.16) (table 7). The divergent validity of selfassessed aerobic fi tness, muscle strength and fl exibility was satisfactory due to markedly weaker correlations with noncorresponding performancebased parameters (table 7). In men, aerobic fi tness and muscle strength showed moderatetogood convergent validity (r=0.510.64) and fl exibility showed lowtomoderate convergent validity (r=0.31). Selfassessed aerobic fi tness and muscle strength showed satisfactory divergent validity, whereas selfassessed fl exibility was highly correlated, although not signifi cantly, to oxygen uptake and isometric back fl exion strength.The analyses of paper III supported a convergence between the performancebased fl exibility result and the selfassessed fl exibility score (table 8) due to the ability of the selfassessed score to discriminate between the persons who performed in the lower half from those who performed in the higher half. The associations of both back endurance tests and the balance test with their corresponding selfassessed scores were also highly signifi cant, and no confounding of LBP was found in any of the analyses. The reliability of selfassessed aerobic fi tness and muscle strength was good (ICC=0.80), whereas the reliability of fl exibility, endurance, and balance was only moderate (ICC=0.620.69) (Paper II).


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Table 7. Ageadjusted correlations coeffi cients of selfassessed components with performancebased parameters of physical fi tness. (paper II). Group/Measure Selfassessed components of physical fi tness Aerobic Muscle Endurance Flexibility Balance fi tness strengthWomen Study population IIC (n=170) Maximal VO2 0.36‡ 0.11 0.20† 0.02 0.02 Back extension strength 0.01 0.30‡ 0.11 0.08 0.05 Back fl exion strength 0.01 0.34‡ 0.16 0.05 0.01 Study population IIA+B (n=620) Back extension endurance 0.21‡ 0.12† 0.16‡ 0.22‡ 0.08 Back fl exion endurance 0.20‡ 0.08* 0.18‡ 0.12† 0.09* Sagittal fl exibility 0.17‡ 0.17‡ 0.14‡ 0.36‡ 0.06 Balance 0.07 0.01 0.01 0.06 0.05

Men Study population IIC (n=17) Maximal VO2 0.64† 0.41 0.77‡ 0.32 0.32 Back extension strength 0.04 0.51* 0.19 0.18 0.45 Back fl exion strength 0.03 0.67† 0.31 0.50 0.61* Study population IIA+B (n=128) Back extension endurance 0.18* 0.10 0.15 0.27† 0.20* Back fl exion endurance 0.04 0.01 0.08 0.11 0.08 Sagittal fl exibility 0.30‡ 0.13 0.13 0.31‡ 0.08 Balance 0.02 0.13 0.03 0.08 0.25Note. Vertical, performance based measures of physical fi tness. Correlation coeffi cients between corresponding parameters in boldface. * p<0.05, † p<0.01, ‡ p<0.001.

Table 8. Stepwise adjusted associations between performancebased and selfassessed physical fi tness (paper III). All n=612 Association with selfassessed fi tness VAS score sex full model‡ test result n mean (SD) adjusted without LBP with LBPBack ext. endurance (passed 180s) failed 263 51 (19) p<0.0001 p=0.002 p=0.001 passed 330 57 (20) Back fl ex. Endurance (passed 180s) failed 314 50 (19) p<0.0001 p<0.0001 p<0.0001 passed 279 59 (19) Flexibility (dichotomized)† low 293 48 (20) p<0.0001 P<0.0001 p<0.0001 high 297 58 (18) Balance (passed 60s) failed 90 49 (23) p<0.0001 p=0.0004 P=0.0008 passed 503 59 (18) Mean (SD). †The Sagittal fl exibility scores were dichotomized according to the distribution in the absence of a “passed value”. ‡ Adjusted to educational attainments, history of physical demanding job, BMI, height, sex and age in a GLM analysis.


33

4. DISCUSSION

4.1 PERFORMANCEBASED PHYSICAL FITNESS AS RISK INDICATOR OF LBP Among the four performancebased parameters we measured in the prospective study, only isometric back extension endurance showed signifi cant association with increased LBP intensity at followup.

The result that lower back extension endurance was associated with the risk of increased LBP intensity is in concordance with our hypothesis of an association between poor physical fi tness and higher risk of increased LBP intensity. However, there was no indication of a doseresponse relationship and the almost similar ORs for low and medium level compared to high level back extension endurance suggest that those with low and medium level were in same risk of increased LBP. The mechanism leading to LBP among those with lower physical fi tness was described by McGill (88). Poor physical fi tness implies a higher relative physical workload during the workday (other factors being equal). Consequently, the margin of safety, defi ned as the distance between the failure tolerance and the applied physical workload decreases, and the risk of exceeding the failure tolerance increases. A load that exceeds the failure tolerance produces injury, which causes pain.

McGill showed also that previous history of LBP is related to lingering defi cits in biomechanical, physiological, and motor control characteristics (87). Hence the level of physical fi tness may be due to a history of LBP. Persons with low physical fi tness may simply be less physically active during leisure time. It is unknown which approach dominates, but it is hoped that the aggravation of LBP can be prevented in both groups by physical fi tness training.

The observation in this study that low back extension endurance measured using the Sørensen method, associates with increased risk of LBP are in agreement with some previous studies (16,17,45,79). However, other studies have found no association between back extension endurance and LBP (3,64,115,122). A signifi cant association between LBP and back extension endurance was found in both representative (16,45), and specifi c populations (79), among men (16,17), and in mixed populations (45,79) and using different LBP outcome defi nitions. These results strongly indicate that isometric back extension endurance could be an important risk indicator of LBP, regarding different LBP outcomes. Although there is a lack of representative studies among those studies who found no relationship between physical fi tness level and LBP there are too many studies with negative fi ndings to consider back extension endurance as a consistent risk indicator of LBP. One reason for the strong indication of a causal relationship between back extension endurance and LBP is that the Sørensen method or a modifi ed version has become the preferred method for measurement of back extension endurance (27,93), which has increased the comparability between studies. In contrast no


34

consensus exists regarding the defi nition and measurement of balance (103). Especially in fi eld research where the use of heavy and advanced technical equipment is limited, only very few tests are applicable. In the prospective study balance was measured using a newly developed method that was designed to test a balance parameter with particular relation to the low back region; however, no association with increased LBP intensity was found and the lack of comparability with other studies limits a deeper exploration. Although the balance test we chose for the crosssectional study has been evaluated to be a reliable and feasible healthrelated balance test (119,120) and to be associated with back health (among women) (121) we found no association with LBP in the crosssectional study (paper III).

Back fl exion endurance and sagittal fl exibility was not associated with increased LBP intensity in the prospective study. According to my knowledge, only one study has examined the association of isometric back fl exion endurance with future LBP (68) and another one the association of dynamic trunk fl exion with future LBP (later register based disability due to LBP) (109). Both studies found no association with future LBP. Contradictory results about the association between sagittal fl exibility and LBPrelated outcomes have been reported in three studies. Sørensen (16) found that high sagittal fl exibility among men measured by using the Schober test, associated with fi rsttime occurrence of LBP, and that high fl exibility among women measured using the fi ngertofl oor method was associated with recurrent LBP. Takala (122) and Adams (3) found that low fl exibility was associated with the occurrence of LBP, while high fl exibility among women was associated with medical consultation. In addition low sagittal fl exibility has been found to be a predictor of future back pain as reported in a study of 3,020 aircraft manufacturing employees (14). Several other studies fi nd no relationship between fl exibility and future LBP. One of the hypotheses underlying this thesis was that the level of physical work factors may infl uence the association between physical fi tness parameters and LBP. This hypothesis was not signifi cantly supported by the results of the prospective study. However, when the prospective data were stratifi ed by “rotated back”, as an indicator of physical work load, the insignifi cant ORs among the highly exposed became twice as high compared to the insignifi cant ORs among the low exposed. This fi nding was considered to be an indication of poor back extension endurance as a stronger risk indicator among those exposed to high physical demands compared to those with low physical demands at work. More research is needed to further explore these fi ndings, especially because a recent study by Hambergvan Reenen et al. fi nd no differences in risk estimates in relation to LBP between two groups with poor back extension endurance that were exposed to either low or high physical demands (46). It would be interesting to design a future study with enough statistical power to include interaction terms between physical fi tness and physical work factors.

The crosssectional study could not add knowledge to this area due to the lack of work factors among students; however, the inclusion of the person’s previous experience with a physical demanding job in the analysis was to test if it infl uenced


35

the crosssectional association between LBP and the physical fi tness parameters. It did not.

4.2 SELFASSESSED PHYSICAL FITNESS AS RISK INDICATOR OF LBP

It was unexpected and contrary to our hypothesis that medium level of selfassessed aerobic fi tness signifi cantly decreased the risk of increased LBP intensity at followup compared with those with high level selfassessed aerobic fi tness. Selfassessed muscle strength, endurance and fl exibility indicated a similar relationship but did not reach a statistically signifi cant level.

The crosssectional study (paper III) showed, in contrast to the prospective study, an association between experience of LBP during the previous year and lower selfassessed aerobic fi tness, fl exibility and balance in both the sexadjusted and full model. An explanation of the association in the prospective study could be that selfassessed physical fi tness was different associated with the LBP variable in the two baseline populations. However, this was not the case. Baseline analyses of the study participants in the prospective study (only those with followup data) showed that poor selfassessed physical fi tness was associated with higher LBP intensity (r=0.07(0.11), p=0.0450.21 raw correlations between the fi ve dimensions of selfassessed physical fi tness and reported mean LBP intensity during the previous year). Some of the explanation can be found in the characteristics of those who drop out during the followup time. They were characterized by having more pain but the same level of selfassessed physical fi tness as the remaining group (paper I). This suggests that the drop out persons fi t better with our hypothesis of low physical fi tness as associated with increased LBP than the remaining group in the study.

A hypothetical explanation of the fi nding of poor selfassessed fi tness as preventive against increased LBP intensity could be that those who selfassess their physical fi tness to be low are particularly aware of their reduced capacity and act accordingly. Consequently, they may change their behaviour at the workplace to reduce their physical work demands, which further reduces the risk of their failure tolerance being exceeded, due to a increased margin of safety (88). As a result, the risk of increased LBP intensity at followup decreased, not because of their lower level of physical fi tness, but because of a changed behaviour that reduces their physical exposure. Not even detailed physical exposure assessment during the study period, which is an extremely expensive solution, would register such changes in behaviour with certainty. An alternative way to exploring the association would be to combine the quantitative survey with a qualitative approach, for example by interviewing those with low selfassessed level of aerobic fi tness who reported a decreased intensity of LBP at followup.To further elucidate the association of physical work demands and the association between LBP and level of physical fi tness, the variable “history of physical


36

demanding job” was included in the crosssectional analyses (paper III) of health care students in training (free of physical work demands). However, the inclusion of physical demanding job in the analyses between LBP (12monmth prevalence) and selfassessed physical fi tness did not affect any of the associations. Another variable introduced in the analyses of paper III was educational attainments as a proxy for the social context for the individual. Educational attainments were only signifi cantly associated with selfassessed aerobic fi tness but did not affect the association between LBP and selfassessed aerobic fi tness or any other of the selfassessed dimensions.

Other methodological approaches could be introduced to understand the nature of selfassessed physical fi tness better and to explore why poor selfassessed fi tness decreases the risk of LBP aggravation. The introduction of a cognitivebehavioural approach as that used when explaining the cognitivebehavioural model of fear of movement/(re)injury (128) could inspire to a model that explain the link between the awareness of the physical fi tness level and a possible change in behaviour which may lead to either reduced or increased risk of LBP.More knowledge is needed about which psychological factors and how they infl uence the selfassessment of physical fi tness using VAS to elucidate what we really measure and how context dependent the measures are compared to objectively measured performancebased physical fi tness (39,94,131).

4.3. SELFASSESSED COMPARED WITH PERFORMANCEBASED PHYSICAL FITNESS

4.3.1 Associations between selfassessed and performancebased physical fi tnessNo selfassessed dimensions were strongly related to their corresponding performancebased measures. Weak to moderate convergence was found for the dimensions of aerobic fi tness, muscle strength and fl exibility which also had satisfactory divergence, except for men’s selfassessed fl exibility. The three selfassessed dimensions which showed the best convergence with their performancebased counterparts had correlations ranging 0.300.36 which corresponded to a shared variance of 913%. A higher convergence was found among the 17 men in study population IIC (2641% of the variance was shared) but due to the group size these results can be regarded only as indicative. This degree of convergence seems too low for an interchange of performancebased physical fi tness with selfassessed fi tness at either individual level or when comparing smaller groups, which neither was expected. However, in larger epidemiological studies comprising a large number of persons, selfassessed aerobic capacity, muscle strength and fl exibility may give a useful estimate of the actual physical fi tness level.The analyses of association between corresponding physical fi tness parameters in paper III supported an association between the result of the fl exibility test and the VAS score which was highly signifi cant. Additionally, the analyses also showed signifi cant associations between isometric back extension and fl exion endurance, and balance (oneleg standing), which were not found in the validity study.


37

However, the results are diffi cult to compare directly due to the dichotomy test result for the test used in paper III and the consequently different statistical analyses. Interpreting the results of the agreement examined in paper III one might argue that if the VAS scores are highly intercorrelated it would not makes any difference which VAS score we use to discriminate between those who failed and passed a test. In general, this was not the picture. For all performancebased tests, except for back extension endurance, the corresponding VAS score discriminated best (largest difference) between those who passed and failed the test compared to the noncorresponding VAS scores (data not shown). In relation to the back extension endurance test, selfassessed aerobic fi tness and selfassessed endurance showed both a signifi cant difference of 6 mm between those who failed and those who passed the test. Thus indicating a failed divergence validity. This result fi ts well with the fi nding in paper II of a lacking validity for selfassessed endurance that showed a high interitem correlation with selfassessed aerobic fi tness.

The face validity was in general good. Most of the participants only used few seconds to consider where to put the marks on the VAS and they differentiated clearly between the different dimensions, even though high interitem correlations were found. However, the absence of convergence for selfassessed endurance in relation to the performancebased back extension and fl exion endurance tests may be due to a lack of content validity of the endurance questions in relation to the isometric back endurance tests. The endurance question was illustrated with two persons climbing a mountain, and did not correspond with local back muscle endurance parameters. In future studies redesigning the instrument to measure a more specifi c muscle endurance component will be considered. Selfassessed balance did not correspond with its performancebased counterpart in the validity study. However, as discussed in paper II it was a positive fi nding, at least among women, that selfassessed balance did not correlate with noncorresponding performancebased measures, which indicated that selfassessed balance may have the promising quality of possessing specifi city. Paper III supported this fi nding by the highly signifi cant association between the balance test result and selfassessed balance, only among women. It seems that the oneleg balance test correspond more well with people’s perception of balance, although a differential reporting of selfassessed balance, in particular might be present.

This additional result supports that selfassessed balance share enough variance with performancebased balance to be a rough estimate and that the degree of concordance can be dependent on sex. If both performancebased and selfassessed balance is differential in relation to sex it should be considered to use different balance test for evaluating women and men. This could increase the variation of the test score among each sex and thereby increase the statistical applicability. No other instruments for selfassessment of physical fi tness among healthy people used VAS to score the replies, whereas an earlier study was found that used VAS for measuring of functional capacity (overall, chosen and preselected function) among patients with rheumatoid arthritis receiving longterm treatment (112).


38

Two different strategies are used for selfassessment of physical fi tness. The one is to ask for a relative scoring of the respondent’s physical fi tness using peers, as we did when the respondents were asked to compare their fi tness with people at same age and sex. The other strategy is to use absolute standards, by asking the respondents to estimate how well they can perform a specifi c physical fi tness task or how much or for how long time they can sustain a given physical fi tness task (e.g. walking, biking, carrying bags, walking steps, etcetera). Due to the different types of instruments, the variability, and especially the different degree of awareness of physical fi tness in the population being measured, it is diffi cult to compare the few studies that examine the association between selfassessed and performancebased physical fi tness. For instance one study comprises soldiers which is a group with particularly good conditions for knowing their own level of fi tness (69) while another study comprises subjects with exactly same age and only the extreme groups selected on the basis of a physical fi tness test performed 15 years earlier. Thus the consideration for age was eliminating in addition with an implementation of an artifi cial big contrast in the data (as the authors mentioned them selves) (90). One of the better validated (two studies) and promising instruments with better convergence between corresponding parameters is the one described by Abadie (2) which found valid assessment among younger persons (<50 years) of cardiorespiratory endurance (aerobic fi tness) (r=0.430.61), muscular strength (r=0.47) and muscular fl exibility (r=0.53). The components of physical fi tness they found most valid was the same as we found most valid in our validity study (paper II) but the validity of their instrument was better than the one we found. However, the instrument by Abadie proved unsatisfactory for use among elderly people (2,126) and the divergence among younger persons has not been reported. No studies were found that examined the association between one of the above mentioned instruments for selfassessment of physical fi tness with LBP, neither crosssectionally nor prospectively.

4.3.2 Predictive validityThe comparison of selfassessed and performancebased fi tness’ ability as risk indicators of increased LBP intensity in the prospective study can be regarded as a test of the predictive validity of the selfassessed instrument. None of the selfassessed physical fi tness parameters were able to show the expected association between low level and increased LBP intensity. Hence, poor predictive validity according to the hypothesis was found for aerobic fi tness, muscle strength, endurance, and balance. As previously mentioned it is debatable if low or high fl exibility prevents against LBP or if an association exists at all, however, neither high nor low level of selfassessed fl exibility was associated with increased risk of increased LBP intensity.

The low correlation of the fi ve physical fi tness VAS items with their corresponding performancebased parameters and the high correlations between the selfassessed physical fi tness dimensions suggest that the VAS items could be combined in a scale measuring overall physical fi tness. The fact that four out of fi ve VAS items show the same inverse association with LBP intensity also points in this direction.


39

The internal consistency was assessed using the Cronbach coeffi cient alpha (25). On the basis of data from study population IIAC (n=935) a value of 0.76 was calculated and using data from study IIIA (n=612) yielded a value of 0.79. This supports the idea of combining the fi ve VAS items in one scale. The argument for combining these items in a scale is that it results in a more precise and consistent measurement of physical fi tness, yielding better analyses of associations with relevant outcomes. Several scale construction approaches exist (36). For these continuous VAS items factor analysis would be logical to start with, and analysis of differential item functioning could be done in this framework. Item response theory models (125) for VAS items have also been proposed (23).

4.4 METHODOLOGICAL CONSIDERATIONS

4.4.1 Limitations and strengths of the thesisOne of the main limitations of this thesis was the low numbers of men present in the study populations (519%). This implies that the results are much more comparable to other groups or studies of women than of men. However, the ratio of men refl ects the actual ratio of men employed and studying in the health care sector. In the validity study the analyses were stratifi ed to sex because of different ratios of men in the subpopulations of paper III. Due to a higher ratio of men in the prospective (paper I) and crosssectional study (paper III) compared to the validity study (paper II), the data from both sexes were in these studies examined together in ageadjusted analyses, although many of the data are presented in the tables stratifi ed by sex.

The many study populations used in the thesis can both be seen as a limitation and as a strength. The validation study would have been stronger if all performance based parameters were assessed in the same population. The validity of selfassessed aerobic fi tness and muscle strength was tested in one population while the validity of endurance, fl exibility, and balance was tested in another. Consequently, the differences in validity between these two groups of selfassessed parameters could be due to differences between the study populations. However, the different study populations used in this thesis made it possible to examine the consistency of the distribution pattern, the sex dependency and the crosssectional association with LBP. This ensured that the basic characteristics for the instrument for selfassessment of physical fi tness were not linked to one particular subpopulation that might possess specifi c qualities. Although the thesis contained many study populations, the study populations, as described in the method section, all comprised people who were either employed in areas or training to work in areas that involved working physically with people. Thus, compared to the variety of existing occupations the study populations were relatively homogenous.

Selection bias occurred in study population IIIA with a followup population only comprising 30% of the basis population. The drop out groups all experienced more LBP and were younger compared to those who remained in the survey. The consequences of this selection on the results are diffi cult to determine, but it might


40

be more diffi cult to fi nd an association with LBP among the remaining healthier part of the employees, resulting in an underestimation of the association strength. In the crosssectional study comprising health care students in their fi rst week of training (paper III) the response rate was high (89% with completed the questionnaire, 80% were tested and 69% with complete test data and questionnaire) which imply that the results obtained were representative of the population of health care students examined.

4.4.2 Differences in test procedures and conditionsWhen comparing the results between the prospective study comprising people working with disabled persons and the crosssectional study comprising health care students we must be aware that beyond differences in study designs and study populations also methodological differences existed. Those differences are important to elucidate if performancebased functional tests are viewed as behavioural assessments in which a subject’s performance during testing is considered within the unique assessment context (43). The procedures for measuring performancebased back fl exion endurance and balance differed between studies. The isometric back fl exion endurance test described by Ito and Hyytianen (56,60) applied in the prospective study was replaced by the test described by McGill 2003 (87) in the crosssectional study (i.e. baseline in a RCT) because the test was found to be more standardizable due to the situp posture and furthermore the posture reduced the load of the neck muscles (i.e. some complained about soreness in the neck after test termination). The way of measuring balance was changed from the wobbleboard procedure in the prospective study to the simpler onelegstanding test in the crosssectional study to ensure that all participants were able to perform the test regardless of LBP status, motor skills and courage. The onelegstanding test was also believed to be less infl uenced by the participants’ ability to concentrate which was an important factor in the crosssectional study because the students were tested together in a gym which unavoidably causes more noise and it was unfortunately not possible to avoid some interactions between the students who were tested and those who waited for their turn at one of the fi ve test stations. However, the test sessions were carried out in a very positive and competitive atmosphere. The limited time allocated to the test sessions was the reason to the shorter test termination time for back extension endurance and back fl exion endurance (maximum of 180 seconds) in the crosssectional study compared to the prospective (maximum of 360 seconds). More people than expected reached the time limit (of 180 seconds) which may be result of the competitive atmosphere during the test session. This choise between a wellestablished but less suitable method, and an untested but more specifi c method is a problem that often appears when selecting performance tests for a survey.

4.4.3 Testretest reliability of selfassessed physical fi tnessAlthough the reliability of the selfassessment instrument was found to be acceptable it was somewhat lower than we desired. Together with the retest questionnaire the respondents also received two additional questions concerning LBP: “Have you experienced LBP during the previous 7 days?” and “score your average LBP intensity during the previous 3 month” which allowed us to examine


41

if respondents with low agreement (>20mm) between the repeated occasions had specifi c characteristics regarding age, sex or LBP. However, no systematic trends were found between low agreement of selfassessed physical fi tness and LBP status, changes in LBP status (comparing 7day and 3month prevalence at the two occasions), age or sex. The only systematic trend seen was that those with low agreement in one dimension more frequently showed low agreements in other dimensions too.


42

5. CONCLUSIONS

Physical fi tness as risk indicators of LBPThe performancebased and selfassessed physical measurements used in the study did not show any strong associations with LBP outcomes. Only low back extension endurance was found to be moderately associated with increased LBP intensity at followup. This result is in agreement with previous fi ndings indicating that back extension endurance measured by the Sørensen method could be considered a risk indicator of LBP. Opposite to our hypothesis, low level of selfassessed aerobic fi tness was shown to reduce the risk of increased LBP intensity. No other performancebased or selfassessed physical fi tness parameters showed statistical signifi cant associations with increased LBP intensity.

The applicability of the instrument to selfassessment of physical fi tnessThe weak to moderate agreement between performancebased and selfassessed physical fi tness and the contradicting associations with LBP in the prospective study point toward selfassessed physical fi tness to be a concept different from performancebased physical fi tness. The strong interrelationships found between the fi ve different dimensions of selfassessed physical fi tness further support such an understanding.

More knowledge about how work factors and other individuals factors infl uence the selfassessed dimensions of physical fi tness are needed before the instrument for selfassessment of physical fi tness can be implemented in surveys as an alternative to performancebased physical fi tness. The instrument for selfassessment might possess characteristics that differ so much from performancebased physical fi tness’ that it shall be considered as a related but independent risk indicator of LBP.


43

6. PERSPECTIVES

More prospective studies are still needed to establish more knowledge about which physical fi tness components that can prevent fi rst time occurrence, recurrent or aggravation of LBP. Future study populations should optimally comprise both men and women who are exposed to varied level of physical work demands during their workday to take the physical exposure level into account. Also intervention studies are needed to examine how much we can infl uence the LBP aggravation by inducing better physical fi tness and if a changed level of physical fi tness is as important as the level it self. As a step in that direction the analyses of the followup data of study population IIIA will show if it is possible to improve the individual capacity already during the training and thereby hopefully reduce the risk of new LBP episodes upon entry the labour market.A way of further elucidate the changes in the selfassessed fi tness over time and especially in relation to LBP would be to combine our quantitative designs with qualitative analyses. These quantitative analyses could comprise interviews with the marginal groups which selfassess their capacity as low and opposite to our hypothesis had an unchanged or even decreased LBP intensity. This would give valuable information about possible changes in behaviour during the followup as we hypothesized could be one of the causes of the unexpected relationship between low selfassessed fi tness and decreased risk of increased LBP intensity. Another possible approach for further research is to use the selfassessment instrument among chronic LBP patient or patients in a rehabilitation process in which cases the use of performancebased test are limited. Finally I will support the statement by Bill Marras that the treatment of LBP and the fi nding of the causality is a multidisciplinary discipline. Hence we all have to combine our knowledge irrespective of our scientifi cally approach to make an effective prevention strategy.


44


45

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53

The role of physical fi tness as risk indicator of increased Low Back Pain intensity among people working with physical and mentally disabled persons: a 30month prospective study.

Authors: Jesper Strøyer1 and Lone Donbæk Jensen2

1 National Research Centre for the Working Environment, Denmark, 3Department of Occupational Medicine, University Hospital of Aarhus, Denmark

Running title: Physical fi tness and LBPJournal: SPINE Accepted for publication September 2007.

Corresponding author: Jesper Strøyer, M.Sc., Ph.D. stud. National Research Centre for the Working Environment, Denmark. Lersø Parkalle 105 DK2100 Copenhagen, Denmark Email: [email protected] Tel: (+45) 3916 5477 Fax: (+45) 3916 5201

mailto:[email protected]


54

Abstract

Study Design. A prospective cohort study. Objective. To study if low level of physical fi tness was associated with increased low back pain (LBP) intensity at 30month followup. Summary of Background Data. The evidence of low physical fi tness as a risk factor for LBP is inconclusive due to contradictory results. Methods. The study participants were 327 employees (women=271, men=56) at institutions for physically and mentally disabled persons. Physical fi tness was measured by tests of back extension and fl exion endurance, fl exibility and balance and by selfassessed aerobic fi tness, muscle strength, endurance, fl exibility and balance, using visual analogue scales. Low back pain, lifestyle parameters, and physical and psychosocial work factors were assessed by questionnaires at baseline and at followup. Outcome was defi ned as an increase above 2 steps in average LBP intensity during the previous year (010). Results. Persons with low level back endurance showed an insignifi cantly higher risk of increased LBP intensity (OR=2.4, p=0.076), whereas persons with medium level back endurance were at signifi cantly higher risk (OR=2.7, p=0.034) compared with high level back endurance. The general association between isometric back extension endurance and increased LBP intensity was insignifi cant (p=0.067). Persons with medium level selfassessed aerobic fi tness were at lower risk of increased LBP intensity compared to those with high level (OR=0.37, p=0.02), although the general association of aerobic fi tness was insignifi cant (0.066). Performancebased back fl exion endurance, fl exibility, and balance and selfassessed muscle strength, endurance, fl exibility, and balance were not associated with increased LBP intensity. Conclusions. The signifi cant association between medium level back extension endurance and increased LBP intensity supports the fi nding of other studies that particularly back extension endurance is an important physical fi tness component in preventing LBP and that the subcomponents of physical fi tness are related in different ways to LBP.Keywords: Selfassessed physical fi tness, Visual Analogue Scale, back extension endurance, physical job demands, prospective study. Key Points• The evidence of physical fi tness as a risk factor for LBP remains unclarifi ed

due to contradictory results in the literature.• Associations of low levels of performancebased and selfassessed physical

fi tness with increased LBP intensity were studied in a prospective cohort study with a 30month followup.

• Persons with medium level back endurance had at signifi cantly higher risk of increased LBP intensity at followup compared with persons with high level back endurance.

• The study supports the fi ndings of other studies that back extension endurance, in particular, is an important physical fi tness component in the prevention of LBP and that the subcomponents of physical fi tness are related in different ways to LBP.


55

Mini Abstract/Précis Performancebased and selfassessed physical fi tness associations with increased LBP intensity were examined prospectively. Persons with medium level back endurance were at signifi cantly higher risk of increased LBP intensity at followup compared with high level endurance. No other selfassessed or performancebased parameter prevented against an increase in LBP intensity at followup. Introduction Low back pain (LBP) is a common musculoskeletal disorder leading to extensive human and socioeconomic consequences 1;2. Many attempts have been made to establish predictors in order to enhance prevention. Physical work load, such as working with the back in bent and rotated positions and patient handlings have been identifi ed as risk factors for LBP 37. Low physical fi tness has also been suggested as a risk factor for LBP; however, evidence is weak due to contradictory results in the literature 813. There are many reasons for these contradictions e.g. the ratio between women and men differs between studies, some stratify to sex and others do not, the type of population, the way the physical fi tness parameters are measured, the defi nition of LBP, the followup length, and the level and assessment of physical workload which may infl uence the relationship between physical fi tness and LBP 14. Furthermore, the strength of the association between physical fi tness and LBP may depend on the chosen subcomponent of physical fi tness, which implies that a general consensus about physical fi tness as a risk factor of LBP is irrelevant. One of the subcomponents showing promising results as a risk factor of LBP is performancebased back extension endurance12;1517 , although several studies fi nd no association between the test performance and LBP10;11;13;18. This study comprised people working in healthcare which are exposed to relatively high physical demands. Additionally, the healthcare sector is characterized by a high prevalence of LBP 1921. Our aim was to test if a low level of different subcomponents of physical fi tness was associated with an increase in LBP intensity at 30month followup among people working with physically and mentally disabled persons.


56

Materials and methodsDesign and population. The study was a prospective cohort study comprising a selfadministered questionnaire and performancebased physical fi tness tests at baseline and a questionnaire at 30month followup. The cohort participation rate at baseline and followup are outlined in fi gure 1. We invited 1106 employees from all institutions for physically and mentally disabled persons in the county of Aarhus, Denmark to participate. Of these 800 (72%) accepted, 620 (78%) underwent a physical fi tness test. At the 30month followup, 440 of the tested persons completed the questionnaire. The study group was reduced to 327 persons (women=271, men=56) before entering the analyses, because 48 persons had incomplete LBP data and 65 persons reported LBP intensity above 5 at baseline which excluded them from the analyses according to the case/noncase criteria. All participants gave a written, informed consent and the local ethics committee at the University of Copenhagen, Denmark approved the study.

Outcome VariableIncreased LBP intensity. LBP was defi ned as tiredness, discomfort or pain in the low back region. The low back region was defi ned as the region between L1 and the gluteal folds. Cases were defi ned as subjects with an increase of more than two steps in selfreported rating of the LBP intensity during the past twelve months followup (“State your average level of LBP during the previous 12 months on the scale below”, one indicating no pain and 10 indicating worst possible pain). It has been shown that an increase in LBP intensity above 2 was clinically relevant in chronic LBP patients 22. Table 1 shows the distribution of the LBP intensity at baseline in the source population and the number of cases at followup at each level of LBP intensity. Subjects with low back pain intensity above 5 at baseline were excluded from the analyses to give all the participants a change to become a case. The cutoff limit of 5 also ensured that all cases had had a markedly relative increase in pain intensity. Of the 327 persons included in the analyses at followup, 27 % improved their LBP intensity, 42 % were unchanged and 31 % got worse. As table 1 shows, 41 persons (12 %) got worse by more than 2 and were classifi ed as cases (women=37 and men=4).

DeterminantsPerformancebased physical fi tness. Before the test session, the participant was interviewed, using a standardized questionnaire guide, to elicit musculoskeletal pain, diseases and other circumstances that could involve a health risk or might affect the results of the tests. The most frequent occurrences for exclusion were: musculoskeletal pain at the test day in the regions tested, history of severe low back pain, under treatment for high blood pressure, fever, headache and pregnancy. Isometric back extension endurance (Modifi ed Sorensen test): the subjects were lying prone on a sloping board (70x40x15 cm). The feet were pressed down to the fl oor by an assistant and the subjects held their upper body in a horizontal position with the arms folded across the chest and with a hip fl exion of approximately 12º. Isometric back fl exion endurance: a band was fi xed around the subject’s chest at the height of the inferior angulus of scapulae. In supine position with the feet supported on a chair (90º hip and knee fl exion) and the arms folded across the


57

chest, the subject was instructed to curl up until the band was free of the fl oor. The two tests were held for as long as possible, but to a maximum of 360 sec. 23;24. Flexibility: The modifi ed fi ngertofl oor method was used25;26. The subjects stood on a 30 cm high box and bent forward while pressing a horizontal measurement slide downwards. Sagittal fl exibility was defi ned as the distance from the fi ngertips to the box level in the fully fl exed position. Thus negative values indicated that the subject was able to reach further down than the box level. Balance: was evaluated by a balance test with the subject sitting on a wobble board that was placed on a table allowing the legs to hang freely over the edge of the table. Two trials were given to familiarize the subjects with the test. Subjects were asked to keep the wobble board in balance through movements of the hip and back. If any body segment touched the table, the stopwatch was stopped but not zeroed, and one attempt was counted. Number of attempts was counted until the subject had been balancing for one minute or a maximum of 15 attempts was reached. A similar setup has recently been developed27.

Selfassessed physical fi tness. Aerobic fi tness, muscle strength, endurance, fl exibility, and balance were selfassessed using Visual Analogue Scales (VAS) of 100 mm with illustrations and verbal anchoring of the extreme situations (fi gure 2). The respondents were asked: “How would you score the following components of physical fi tness compared to people of your own age and sex?”. Replies appeared as vertical marks on the VASs. The questions were developed by the Department of Work Physiology at the National Research Centre for the Working Environment, Denmark 28. The VASs were completed the day before the tests and computerized using a digitizer. The classifi cation of the performancebased and selfassessed physical fi tness parameters were based on the distribution in the population (tertiles) and appear in table 2. The three classifi cation levels enabled us to test for signs of doseresponse relationships.

CovariatesSeniority (yr): defi ned as years working with physically and mentally disabled people. 010 yr (low), >10 yr (high). Body mass index (BMI): the subjects were classifi ed into three groups according to BMI (kg/m2): <=24.9 (normal weight), 2530 (overweight), >30 (severe overweight). Leisure time physical activity: assessed by a slightly modifi ed version of the question of Saltin and Grimby 29 with illustrations: 04 hours/week (low), >4hours/week (high). Previous LBP history: defi ned as: no period ever with 3month persistent LBP (no) or at least one 3month period with persistent LBP (yes). Previous LBP history is a known risk factor of low back pain 30.

Physical factors at Work. Bent back: the frequency during the workday of bent back: neversometimes (seldom) or oftenvery often (often). Rotated back: The frequency during the workday of rotated back was assessed; neversometimes (seldom) or oftenvery often (often). Physical workload during patient related tasks: was assessed by a continuous scale (014) and dichotomised into 05 (light) or 614 (strenuous).


58

Psychosocial factors at work. Psychosocial work factors were measured by the Copenhagen Psychosocial Questionnaire (COPSOQ) 31. Infl uence at work consisted of four questions dealing with decision latitude. Quantitative job demands consisted of fi ve questions on quantitative job demands concerning the relationship between the amount of work and the time allotted to do the work. The quantitative job demands scale was reduced from seven to fi ve items on the authors’ recommendation 32. Emotional job demands consisted of three questions regarding how the work affected the respondent emotionally. Cognitive job demands consisted of four items concerning memory and refl ection demands. The psychosocial work factors were classifi ed in three levels according to the distribution in the population (tertiles).

Participation and dropout. Dropout occurred four times during the study period (fi gure 1). The fi rst was the group of nonrespondents at baseline (N=306), whom we have no information about, the second was the persons who completed the questionnaire but did not undergo the performancebased tests (N=180). Compared with the persons with test data at baseline (620) the persons without test data had more sick leave due to LBP during the previous year (p=0.005), they had to be spared more at work because of LBP during the previous year (0.007), they had a higher LBP intensity during the previous 3 months (p=0.005) and additionally, they were younger (4.8 yrs, p<0.001) with lower seniority (3.9 yrs, p<0.001). The third dropout was the persons with complete performancebased test and questionnaire data at baseline but without questionnaire data at followup (N=180). Compared with the tested persons at baseline who completed the questionnaire at followup (n=440), the persons without followup data had more sick leave due to LBP during the previous year at baseline (p=0.027), they were younger (3.4 yrs, p=0.001) and had lower seniority (3.5 yrs, p<0.001). However, no differences were seen in any of the selfassessed or performancebased physical fi tness parameters. The fourth dropout was the combined group of persons with incomplete LBP data to be included in the analyses and persons with LBP intensity above 5, in total 113 persons. The dropout group was older (2.1 yrs, p=0.028) and performed worse in the isometric back extension endurance test (p=0.001).

Data analyses. The data were analyzed for associations between each physical fi tness parameter (determinant) at baseline and an increased intensity of pain in the low back region (dependent variable) at followup. Separate logistic regression analyses were performed using the GENMOD procedure of the SAS 8.02 software. The criterion for including a determinant or a covariate in the multivariate model was an association with the LBP outcome at p<0.20 level in an age and sex adjusted logistic regression analysis. This relatively high signifi cance level was chosen as screening criterion for variable selection to ensure that all important variables were identifi ed 33. The fi nal model included all signifi cant covariates, sex and age. Due to the relatively small study population, we had to be aware of the number of variables entered in the fi nal model as the recommended ratio between cases and independent variables is 1:10 34. No interaction terms were included in the model. The effects of the physical fi tness parameters were tested separately; hence, we must consider chance fi ndings. All variable levels were coded so that the reference level (OR=1) represented the hypothetical advantageous level concerning incre


59

ased LBP. All determinants and covariates were defi ned as class variables in the logistic regressions analyses. The Wald test was performed to test the signifi cance of each classifi cation level compared with the reference level, and the Likelihood Ratio test was performed to test for a general effect (type 3) if the variable had more than two levels. A signifi cance level of p<0.05 was chosen.

ResultsThe drop out analyses showed that we dealt with a restricted study population of relatively healthy persons.Results from the age and sex adjusted logistic regression analyses appear in table 3. Performancebased back endurance (p=0.12), selfassessed aerobic fi tness (p=0.059) and frequency of rotated back (p=0.17) fulfi lled the criterion for inclusion in the multivariate analyses. Sex was signifi cantly related to increased LBP intensity (p=0.048, OR=2.9 of being a woman), whereas age did not associate with increased LBP intensity. Performancebased back fl exion endurance, fl exibility and balance were not associated with increased LBP intensity, neither were selfassessed muscle strength, endurance, fl exibility and balance. Results from the multivariate analyses appear in table 4. When adjusted to “rotated back”, age and sex, persons with low level back endurance showed an insignifi cantly higher risk of increased LBP intensity (OR=2.4, p=0.076, Wald test), whereas persons with medium level back endurance were at signifi cantly higher risk (OR=2.7, p=0.034, Wald test). The general association between isometric back extension endurance and increased LPB intensity was insignifi cant (p=0.067, Likelihood ratio test).To elucidate the effect of adjusting by frequency of rotated back, the analysis was stratifi ed by frequency of rotated back. After stratifying, neither the subgroup exposed to low physical demands nor the subgroup exposed to high physical demands were signifi cantly associated with increased LBP intensity. However, the OR in the highly exposed group was markedly higher compared to the low exposed group (3.9/3.2 vs. 2.0/1.8) (table 5).Persons with medium level selfassessed aerobic fi tness were at lower risk of increased LBP intensity compared to those with a high level (OR=0.37, p=0.02, Wald test), although the general association of aerobic fi tness was insignifi cant (0.066, Likelihood ratio test) (table 4).

DiscussionEmployees with medium level back extension endurance were at signifi cantly higher risk of increased LBP intensity after 30 months compared to those reporting a high level. No signifi cant associations were found between any level of the remaining performancebased tests and increased LBP intensity. Employees with medium level selfassessed aerobic fi tness were at signifi cantly lower risk of increased LBP intensity at followup compared to those with high level. No other selfassessed physical fi tness components were associated with increased LBP intensity.When evaluating the fi ndings of multiple analyses we cannot exclude that the results can be due to chance fi nding. However, it was not nine identical analyses


60

but tests of fi ve different dimensions of physical fi tness, where some of the dimensions were assessed by two different methods. The relationship between high level of selfassessed aerobic fi tness and higher risk of increased LBP intensity was not expected. The association was not due to an opposite association between selfassessed and performancebased aerobic fi tness. In a previous study that examined the validity of the instrument for selfassessment of physical fi tness using VAS28, positive associations were found of selfassessed aerobic fi tness, muscle strength and fl exibility with their corresponding performancebased measures, although the associations were only weak to moderate.That low aerobic fi tness prevented against increased LBP intensity and that selfassessed muscle strength, endurance and fl exibility showed similar tendencies indicate that the selfassessed physical fi tness parameters in general are differently related to increased LBP intensity than the performancebased parameters. The respondents may take several psychosocial factors into account when they answer the questions of selfassessed fi tness 3537, factors that we did not control for in the analyses and that infl uence the relationship to increased LBP intensity. Another explanation we can not exclude is despite the weak to moderate convergent validity between some performancebased and selfassessed parameters the two methods may measure different latent constructs with different relationships to changes in LBP intensity. A third explanation could be that those who selfassess their physical fi tness to be low are particularly aware of their reduced capacity and consequently change their behavior at the workplace in a way that reduce their excessive loading. Thereby they may reduce the excessive physical load and the subsequent occurrence of LBP. Consequently, this group may reduce their risk of increased LBP intensity due to a changed behavior that reduces their physical exposure. A way of elucidating this hypothesis would be to include a quantitative approach. By interviewing the marginal group who reported low selfassessed fi tness in combination with decreased or unaltered LBP intensity changes in behavior may be explained.We can neither rule out the possibility that some participants, and especially those with low physical fi tness, were motivated by the baseline measurements and increased their leisure time physical activities subsequently. A weakness of the study is the relatively few cases, which reduces the power of the analyses. A better response rate would have enhanced the possibility of more cases and thereby more power. We believe that the low power and consequently higher risk of a statistic type 2 error, also was the explanation for the nonsignifi cant relationship we found between low level isometric back extension endurance and increased LBP intensity, and not because of a physiological difference between the persons in the low and medium level groups. The odds ratios were almost the same for medium and low level (2.71 vs. 2.37 compared with high level) which might refl ect the existence of a threshold value opposite to a dose response relationship between back extension endurance and increased low back pain intensity.The study population at followup included only 30% of the respondents at baseline, which underlines that the study population was a selected group. The dropout of a younger group with more LBP during the followup period suggests that those without the ability to sustain the work demands were selected out of the occupation, and those who stayed had particular qualifi cations. It might be


61

refl ected by the result that only medium level back endurance was signifi cantly associated with increased LBP intensity, whereas low level only tended to be so. The consequences of this selection on the results are diffi cult to interpret, but it may be more diffi cult to fi nd an association with LBP among the remaining healthier part of the employees, resulting in an underestimation of the association strength. To test how sensitive the results of the logistic regression analyses were to the classifi cation of the physical fi tness variables, the analyses were succeedingly performed with the physical fi tness variables as continuously variables and then dichotomized (in contrast to the tertile classifi cation). The sensitivity analyses showed that the signifi cance of the results was sensitive to the cutoff point’s defi nitions, especially regarding selfassessed aerobic fi tness. However, back extension endurance was the performancebased components with the strongest association to higher risk of aggravated LBP and low level selfassessed physical fi tness was in general associated with a lower risk of aggravated LBP, irrespective of the cutoff point defi nition. The study population was originally supposed to be relatively highly exposed to physical demands to ensure the relevance of high physical fi tness to correspond to the demands. However, only 27% were classifi ed as having high physical demands according to the variable “rotated back”, and despite contrast in the material normally being a quality, we would have preferred a higher ratio of employees with high physical job demands to ensure the need for high physical fi tness. Our outcome defi nition was an increase in LBP intensity and not the incidence or recurrence of LBP. Hence, when comparing our results with other studies, we must consider that differences in results can be due to differences in case defi nitions. The preventive effect of high level isometric back extension endurance and development of LBP supports the fi ndings of other studies 12;16;17;38, and although several studies fail to prove the relationship between back endurance and LBP 10;11;13;18, the number of positive studies suggests that this is a promising physical fi tness factor in preventing LBP.Two earlier studies indicated that the relationship between physical fi tness and LBP may be stronger among persons exposed to high physical demands 39;40. This was not supported in a recent study by Hambergvan Reenen et al 41 which found equal and statistically signifi cant risks for two subgroups with a poor back extension endurance, but exposed to different levels of physical demands. When the analysis of back extension endurance in the present study was stratifi ed by exposure level, both strata were nonsignifi cantly associated with increased LBP intensity. However, interestingly, the ORs among the highly exposed group were almost twice as high as among the low exposed group, which indicates that an effect of the physical exposure level might exist. This study supports the fi ndings of other and that back extension endurance is an important physical fi tness component in the prevention of LBP. Although nonsignifi cant fi ndings between performancebased tests and increased LBP dominated, we suggest enhancing the focus of physical fi tness among healthcare personal, especially the muscles involved in back functioning in order to withstand the physical exposure during the workday. It might be a step in the right direction to reduce the high prevalence of LBP among healthcare personnel.


62

Tables and fi gures

Table 1. The distribution of LBP intensity during the previous year reported at baseline. Only subjects with pain intensity below 6 were included in the analyses.

LBP intensity Cases baseline increase of pain>2 unitsLevel N % N %1 130 33 14 332 54 14 9 213 57 15 11 264 48 12 3 75 38 10 4 106 23 6 1 27 20 5 0 08 12 3 0 09 9 2 0 010 1 1 0 0Total 392 100% 42 100%


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Table 2. The classifi cation of the performancebased and selfassessed physical fi tness parameters.

Predictor variable levels Range NPerformancebased physical fi tness Back extension endurance (sec) Low 12105 96 Medium 107157 104 High 158360 116 Back fl exion endurance (sec) Low 045 104 Medium 4687 97 High 88360 108 Sagittal fl exibility (cm above fl oor level) Low 332 109 Medium (6) 2 103 High (22) (7) 104 Balance test (number of attempts 15=maximum) Low 15 97 Medium 614 89 High 15 93 Selfassessed physical fi tness using VAS Aerobic fi tness (mm) Low 341 106 Medium 4254 112 High 5597 109 Muscle strength (mm) Low 1047 109 Medium 4863 107 High 6499 111 Endurance (mm) Low 549 100 Medium 5065 112 High 66100 115 Flexibility (mm) Low 442 100 Medium 4360 106 High 6199 121 Balance (mm) Low 445 103 Medium 4660 113 High 61100 111


64

Table 3. Age and sex adjusted analyses of physical fi tness parameters and covariates with increased LBP intensity at 30month followup. $Percentage of cases at each classifi cation level, † Wald test performed for each level,‡ Likelihood Ratio test performed for general effect (type 3),*adjusted only to age, #adjusted only to sex.

Variables N=327 % cases$ OR 95% CI p Wald† p LR‡

Determinants Performancebased physical fi tness Back extension endurance 0.12 High 116 8 % 1 . Medium 104 15 % 2.26 0.945.47 0.069 Low 96 15 % 2.18 0.875.44 0.095 Back fl exion endurance 0.33 High 108 9 % 1 . Medium 97 12 % 1.58 0.633.92 0.33 Low 104 14 % 1.92 0.794.67 0.15 Flexibility 0.77 High 104 14 % 1 . Medium 103 11 % 0.74 0.321.71 0.48 Low 109 11 % 0.85 0.371.96 0.70 Balance 0.47 High 93 11 % 1 . Medium 89 11 % 0.97 0.372.52 0.94 Low 97 16 % 1.56 0.633.86 0.33 Selfassessed physical fi tness Aerobic fi tness 0.059 High 109 17 % 1 . Medium 112 8 % 0.37 0.160.87 0.02 Low 106 12 % 0.58 0.261.29 0.18 Muscle strength 0.27 High 111 15 % 1 . Medium 107 13 % 0.82 0.381.78 0.62 Low 109 9 % 0.51 0.221.18 0.11 Endurance 0.29 High 115 16 % 1 . Medium 112 10 % 0.54 0.241.21 0.13 Low 100 12 % 0.66 0.291.46 0.30 Flexibility 0.32 High 121 16 % 1 . Medium 106 9 % 0.54 0.241.23 0.14 Low 100 12 % 0.74 0.341.62 0.45 Balance 0.96 High 111 12 % 1 . Medium 113 12 % 0.91 0.402.08 0.83 Low 103 14 % 1.02 0.442.34 0.97 Covariates BMI 0.67 <=25 184 13 % 1 . 2530 88 10 % 0.81 0.351.86 0.61 >30 50 16 % 1.3 0.533.16 0.56


65

Leisure time physical activity 0.89 Low 297 12 % 1 . High 30 13 % 1.08 0.353.32 0.89 Low back pain History (3month period) 0.56 No 287 12 % 1 . Yes 40 15 % 1.34 0.513.48 0.55 Frequency of work time with bent back position 0.37 Neversometimes 222 11 % 1 . Oftenvery often 96 16 % 1.38 0.682.80 0.37 Frequency of work time with rotated back position 0.17 Neversometimes 233 11 % 1 . Oftenvery often 87 17 % 1.65 0.813.35 0.16 Physical workload during patientrelated tasks 0.25 Light (05) 102 9 % 1 . Strenuous (614) 222 14 % 1.57 0.713.47 0.26 Infl uence at work 0.77 High 96 10 % 1 . Medium 135 14 % 1.35 0.593.09 0.47 Low 96 13 % 1.19 0.482.93 0.71 Quantitative demands 0.97 High 114 11 % 1 . Medium 106 13 % 1.08 0.482.45 0.85 Low 107 13 % 1.11 0.492.51 0.80 Emotional demands 0.60 High 99 10 % 1 . Medium 145 12 % 1.13 0.492.62 0.77 Low 83 16 % 1.55 0.633.78 0.34 Cognitive demands 0.45 High 98 10 % 1 . Medium 88 17 % 1.64 0.693.92 0.27 Low 140 11 % 1.07 0.462.49 0.88 Seniority 0.24 High 155 11 % 1 . Low 172 14 % 1.54 0.743.18 0.25 Age* 0.41 <36 yr 36 6 % 1 . 3645 yr 92 15 % 3.11 0.6714.5 0.15 4655 yr 135 13 % 2.75 0.6012.5 0.19 >55 yr 64 11 % 2.15 0.4211.0 0.36 Sex# 0.048 Male 56 5 % 1 . Female 271 14 % 2.93 0.879.90 0.083


66

Table 4. Multivariate associations of physical fi tness parameters at baseline adjusted to sex, age, and rotated back during the workday with increased LBP intensity at 30month followup. Full modelA

OR CI pPerformancebased back extension endurance 0.067 B

High level 1 Medium level 2.71 1.086.79 0.034Low level 2.37 0.916.14 0.076 Selfassessed aerobic fi tness 0.066B

High level 1 Medium level 0.37 0.150.88 0.02Low level 0.58 0.271.38 0.23AAdjusted to rotated back during the workday, sex, and age. BLikelihood ratio test.

Table 5. Multivariate association of performancebased back extension endurance with increased LBP intensity stratifi ed by level of frequency of workday with rotated back. Adjusted by sex and age.

Low physical exposure High physical exposure n=233 n=87 OR CI p OR CI pPerformancebased back extension endurance 0.43 0.25High level 1 1 Medium level 1.97 0.665.85 0.22 3.88 0.6124.6 0.15Low level 1.81 0.546.06 0.33 3.26 0.5918.1 0.18


67

Figure 1

Figure 1. Flow chart of the study population.


68

Figure 2

Figure 2. The design of the selfassessment instrument using VAS. The subjects were asked to score the physical fi tness components with reference to people of their own age and sex by setting a vertical mark on each VAS.


69

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Is the crosssectional association between selfassessed physical fi tness and performancebased physical fi tness among healthcare

students infl uenced by low back pain?

Jesper Strøyer1, Annemarie Lyng EskelundHansen2 , Kirsten Schultz Larsen3 and Niels Erik Ebbehoej2. 1National Research Centre for the Working Environment, Denmark. 2Clinic of Occupational and Environmental Medicine, Bispebjerg University Hospital, Copenhagen, 3 Institute of Public Health, Department of Social Medicine , University of Copenhagen.

Manuscript

Prepared for submission to: Occupational Medicine

Corresponding author:

Jesper Strøyer Andersen, M.Sc., PhD. Stud.

National Research Centre for the Working Environment, Denmark

Lersø parkallé 105,

DK2100, Copenhagen Ø

Email: [email protected]

mailto:[email protected]


90

AbstractBackground Confl icting results has been found for the predictive value of selfassessed and performancebased physical fi tness regarding low back pain (LBP). Further exploration to better understand the nature of selfassessment of physical fi tness and its applicability in occupational medicine is needed. Aims First, differences in associations of LBP with selfassessed and performancebased physical fi tness were examined. Second, it was examined if the association between corresponding dimensions of selfassessed and performancebased physical fi tness were confounded by LBP. Methods Of the 885 healthcare students invited, 612 completed a questionnaire and a physical fi tness test program in their fi rst week of training. Aerobic fi tness, muscle strength, endurance, fl exibility and balance were selfassessed using visual analogue scales. Back endurance, fl exibility and balance were in addition tested. Results LBP was signifi cantly associated with selfassessed aerobic fi tness, fl exibility and balance, and performancebased fl exibility in both the sexadjusted and full model analyses. The signifi cance levels of the associations between selfassessed endurance, fl exibility and balance with their corresponding test results were not affected to any appreciable degree when LBP or any other covariate (history of physical demanding job, educational attainments, BMI, height and age) were included in the analyses. The signifi cant association between selfassessed and performancebased balance was only due to the women’s scoring. Conclusion LBP showed almost similar associations with selfassessed and performancebased physical fi tness. LBP did not confound the highly signifi cant associations between corresponding selfassessed and performancebased physical fi tness parameters among healthcare students. More knowledge is needed about which factors that infl uence the selfassessment of physical fi tness.

Keywords: Visual analogue scale, Biering Sørensen test, fi ngertofl oor method, oneleg standing balance.


91

IntroductionThe prevalence of Low Back Pain (LBP) is particularly high in the healthcare sector (15). Various physical workrelated aspects typical of the sector (2;68) have been identifi ed as risk factors of LBP (915). About 43% of the musculoskeletal disorders in the low back and knee among home care helpers has been ascribed to the occupational environment (16;17).

Despite high physical work demands being characteristic of the healthcare sector, to date, there is no conclusive evidence that a high level of physical fi tness can prevent LBP (1823). It has been discussed if the inconsistent results concerning the relation between physical fi tness and LBP are due to the great variability in physical fi tness tests applied, the different types of populations investigated or insuffi cient control of physical exposure variables. Another reason could be that physical fi tness is indirectly associated with LBP through the individual’s strategy for handling the physical workload experienced in the occupational environment. Such a hypothesis emphasis the need for a more complex understanding of the relationship between physical fi tness and the cause of LBP, including the interplay between individual factors, background variables and the perception of own physical fi tness.

Instruments for selfassessment of physical fi tness have primarily been used in large epidemiological studies as a substitute for physical fi tness measurements. Selfassessed physical fi tness measures have shown moderate correlations in relation to corresponding performancebased measures. At the same time strong interitem correlations between the different components of selfassessed physical fi tness has been found (2430). These fi ndings, together with the unexpected results of a previous followup study pointing to low level selfassessed physical fi tness as protective against aggravated LBP (31), indicate that performancebased and selfassessed physical fi tness might represent related but different concepts of physical fi tness.

If selfassessed physical fi tness is to be considered as a risk indicator of LBP in the healthcare sector, substituting performancebased physical fi tness, more knowledge are needed about which factors that infl uence the associations with LBP and performancebased physical fi tness before the selfassessed tool can be implemented in prevention programme.

In the present study it was fi rst analysed if the association between LBP and selfassessed physical fi tness differed from the association between LBP and performancebased physical fi tness. Second, it was analysed if LBP confounded the adjusted associations between corresponding dimensions of selfassessed and performancebased physical fi tness.

MethodsThe study was based on secondary analyses on baseline data of a randomized controlled intervention study among healthcare students in the community of Copenhagen in 20042005. Students from 37 classes (n=885) were invited to complete


92

a comprehensive questionnaire, and to participate in physical fi tness test in their fi rst week of training. A total of 612 (69%) students with complete physical fi tness test data were analysed. All participants gave written informed consent and the local ethics committee at the University of Copenhagen, Denmark approved the study (journal: 2003413508).

Prevalence of LBP was defi ned according to a questionnaire (32;33) as tiredness, discomfort or pain in the low back region with or without radiating symptoms to the leg or legs (34) during the previous 12 month. A history of physical demanding job was defi ned as more than 6 months in the past, on the basis of an open question about their previous occupations and duration. Educational attainments was categorized as low (≤9 years) or high (>9 years).

Five components of physical fi tness (Aerobic fi tness, muscle strength, endurance, fl exibility, and balance) were selfassessed according to peers using Visual Analogue Scales (VAS) of 100 mm with illustrations and verbal anchoring of the extreme situations (fi gure 2)(30). The respondents were asked: “How would you score the following components of physical fi tness compared to people of your own age and sex?”. The VASs were scored the day before the tests and computerized using a digitizer.

Four different physical fi tness tests were conducted in classes during a 90 minutes test session. Isometric back extension endurance was tested using a modifi ed Sorensen test (3538). The subjects lie prone on a sloping board (70x40x15cm) with the feet pressed down to the fl oor by an assistant. Their upper body was in a horizontal position, the arms folded across the chest, and the hip fl exed 12º. This position was hold for a maximum time of 180 sec. Isometric back fl exion endurance was assessed with the subjects positioned in a situp posture with the back resting against a jig angled at 60º from the fl oor, the arms folded across the chest, and with knees and hips 90º fl exed (39). To begin the jig was pulled back 10cm and the person holds the isometric posture in line with the jig as long as possible, but maximally 180 sec. The modifi ed fi ngertofl oor method was used to measure sagittal fl exibility (40;41), defi ned as the distance from the fi ngertips to the fl oor level when the subject fl exed the spine and hip maximally without bending the knees. The person stood on a 30 cm high box without shoes. Balance was evaluated by the onelegstanding test testing the ability to stand on one leg with the eyes open in 60 seconds (42;43). The two back endurance tests and the balance test were coded as passed/not passed. The fl exibility test was analysed as a continuously variable or dichotomized, dependent of the analyses. Weight was measured with hidden display and height was measured by an electronic height measuring unit (SOEHNLE Professional GmbH & Co. KG Postfach 1308, D 71536 Murrhardt). Body mass index (BMI) was calculated as weight (kg)*height (cm)2 and classifi ed into three groups. A short onetoone screening interview was performed before the testing to identify reasons for wholly or partly exclusion. E.g.: musculoskeletal pain at the test day in the regions of testing; history of severe low back pain; under treatment for high blood pressure; fever; headache; or pregnancy.


93

Descriptive analyses were performed for all the variables. Differences according to sex were tested with students ttest and chisquare test for continuously and dichotomous variables respectively. The association between LBP and the physical fi tness parameters was described stratifi ed to sex but analysed in the total group of both sex. The correlations between LBP and the continuous physical fi tness variables (the selfassessed components and performancebased fl exibility) were tested by GLM analyses with the physical fi tness score as the dependent variable and LBP and covariates as independent variables. The correlations between the dichotomy physical fi tness variables (the back endurance and balance tests) were tested by a logistic regression model with the test result (failed/passed) as dependent variables and LBP and covariates as independent variables. A sex adjusted and a full model analysis, that included all the covariates, were performed.The ability of selfassessed endurance, fl exibility and balance to discriminate the persons who failed the corresponding performancebased test from the ones who passed the test, were tested by general linear models (GLM) with the selfassessed score as the dependent variable and the performancebased (passed/failed) and covariates as independent variables. Flexibility was dichotomized according to the distribution due to the lack of a “passed” value. To examine the effect of including the covariates, an analysis only adjusted to sex was performed fi rst, then an analysis including the remaining covariates except from LBP, and then fi nally a full model including LBP. Signifi cance level of p<0.05 was chosen. The SAS statistical software (v. 9.1) was used for all the analyses (PROC GLM was used for linear regression analyses and PROC GENMOD for logistic regression analyses).

ResultsOf the 885 healthcare students invited to participate in their fi rst week at school, 790 (89%) agreed to complete the questionnaire. Among the 705 (80%) students who agreed to be tested, 612 (69%) students performed all physical fi tness test and were included in the analyses, whereas 89 students where excluded from the analysis due to an incomplete physical fi tness test program. Four students were excluded due to pregnancy or disease. The students who refused to be tested (n=85) were not different from those who accepted (n=705). The students with incomplete test data (n=89) scored their selfassessed aerobic fi tness, muscle strength and balance signifi cantly lower than those who completed the test program, whereas their 12month prevalence was not signifi cantly higher (47% vs. 38%).

The LBP prevalence was the same in women (39%) and men (34%) (table 1). Selfassessed aerobic fi tness, muscle strength, endurance and balance were signifi cantly higher in men than in women. The women were signifi cantly more fl exible, whereas the men had signifi cantly better isometric back fl exion endurance. The men were signifi cantly taller compared to women, whereas BMI was identical.

Table 2 displays the selfassessed and performancebased physical fi tness scores among those with and without LBP during the previous year, both stratifi ed by sex and in the total group. The associations between the physical fi tness parameters and LBP and the effect of including the covariates in the model were analysed


94

in the total group. Higher prevalence of LBP during the previous year was signifi cantly associated with lower selfassessed aerobic fi tness, lower selfassessed fl exibility, lower selfassessed balance and lower performancebased fl exibility in both the sexadjusted and full model analyses (table 2). Only the signifi cance of the association between LBP and selfassessed balance differed between the sexadjusted and the full model analysis which was due to the inclusion of height in the model (p=0.05). BMI was highly signifi cantly associated with most of the physical fi tness variables but did not affect the strength of the associations between physical fi tness and LBP in any of the analyses. Educational attainments were associated with selfassessed aerobic fi tness (p=0.02) but did not infl uence aerobic fi tness’ association with LBP.

The ability of selfassessed endurance, fl exibility and balance to discriminate between persons who failed or passed the corresponding performancebased test was tested by adjusted correlations between selfassessed scores and performancebased test results (passed/failed). Educational attainments, history of physical demanding job, BMI, height, sex and age as covariates were included (table 3). Highly signifi cant associations were found between all physical test results and their corresponding selfassessed mean scores. The highly signifi cant association between the selfassessed balance score and the balance test result was only due to the women’s ability to discriminate, as no difference was found between the men who failed and those who passed the balance test. The inclusion of LBP in the model did not change the signifi cance level in any of the associations between the selfassessed scores and corresponding test results (table 3). The only appreciable change in signifi cance level was seen when the covariates (except from LBP) was included in the analysis of selfassessed endurance with back extension endurance (p<0.0001 vs p=0.002) which was elucidated to be due to the inclusion of BMI in the model (p=0.06).To examine how specifi c the corresponding selfassessed physical fi tness score was compared with the noncorresponding scores for each performancebased test, the noncorresponding mean scores were computed for all VAS scores were computed in relation to each compared in the groups who passed and failed To examine if the noncorresponding VAS scores discriminated as well between those who failed or passed the tests all VAS score means were calculated in relation to success of each test result. For all performancebased tests, except for back extension endurance, the corresponding VAS score discriminated best (largest difference) between those who passed and failed the test compared to the noncorresponding VAS scores (data not shown). Selfassessed aerobic fi tness discriminated as well (6 mm) as selfassessed endurance between those who failed and passed the back extension test. DiscussionLBP showed almost similar associations with selfassessed and performancebased physical fi tness parameters. More selfassessed than performancebased parameters were signifi cantly associated with LBP, however all the physical fi tness parameters showed the same picture of better physical fi tness among those wit


95

hout experience of LBP during the previous year compared to those with LBP. The similarities were further pronounced by the signifi cant associations of both selfassessed and performancebased fl exibility with LBP and the lack of signifi cance for selfassessed and performancebased endurance. However, LBP was signifi cantly associated with selfassessed balance in contrast to a nonsignifi cant association with performancebased balance.

LBP did not confound the associations between corresponding parameters of selfassessed and performancebased physical fi tness. The history of physical demanding job was included in the analyses to test if the ability to selfassess physical fi tness was linked to a previous experience of a physical demanding job. However, it was insignifi cantly associated with all physical fi tness variables and did not confound any of the associations. Educational attainments did neither affect the discriminate ability of selfassessed physical fi tness. The rationale for including educational attainments was that it could be a proxy for the individual’s social context. The social context might infl uence the way to consider own physical fi tness in interplay with the experiences and strategy for handling musculoskeletal pain. BMI was included because it is known factor by many people and thereby could be a reason for the ability to selfassess physical fi tness if BMI in addition associates with performancebased physical fi tness. BMI only slightly confound the association between selfassessed endurance and back extension endurance, whereas the remaining associations were unaffected.

The highly signifi cant associations between corresponded parameters, irrespective of variables included in the analyses, indicate an independent and robust association between selfassessed and performancebased physical fi tness, although the major part of the variation in selfassessed physical fi tness still remain to be explained. However, the ability of selfassessed aerobic fi tness to discriminate according to the result of the back extension strength test indicate that selfassessed endurance was not specifi c enough to be used as a measure of back extension strength. This result is in concordance with a previous result (30) that did not fi nd a satisfactory convergence between selfassessed endurance and back extension endurance.

The general higher means of the selfassessed physical fi tness parameters among men compared to women was identically with the result of a previous study using the same instrument among people working in the healthcare (30). Due to the lack of reference values for the applied fi tness test, a survey in a reference population is needed to examine if this difference is due to differential perceptions of physical fi tness between men and women or is due to the fact that men are relatively more fi t. Differential reporting by women and men was not found regarding selfreported disability (44). More women than men reported disability and functional limitation which corresponded with their poorer performance scores. When using peers as reference, as in the present study, women and men in a reference population are supposed to score identical values in average, if the instrument is free of differential and systematic bias.


96

One of the limitations of this thesis was the low numbers of men who appear in the study population (17%). However, the ratio of men refl ects the actual ratio of men studying in the health care sector. Another weakness is the application of two different statistical analyses when comparing the selfassessed physical fi tness scores with the performancebased test result. It can not be excluded that differences in signifi cance level between performancebased tests with dichotomy and continuously test results can be due to different statistical power when performing logistic regression (nonparametric statistics) compared with GLM (parametric statistics). An advantage was the high participation rate of 80 %. In the analyses we included the students with complete test data only (69%) to avoid that differences in statistical results between physical fi tness parameters were due to different subsamples analysed. The strength of the present study was that the subjects were chosen independently of their physical fi tness status. Some studies examine the correspondence between selfassessed and performancebased physical fi tness among soldiers (25) or marginal groups selected on the basis of earlier fi tness test results (29) which induce an artifi cial big contrast in the analyses and limit the comparability. Healthcare students should not possess particular qualities of selfassessing physical fi tness, advocating for the instruments applicability in other occupations than among healthcare students. Although, studies only involving subjects with the same education or occupation in general limit the generalization of the results to some degree.

To conclude, almost similar associations of LBP with selfassessed and performancebased physical fi tness were found. LBP did not confound the highly signifi cant associations between corresponding selfassessed and performancebased physical fi tness parameters among healthcare students. The results support that a real and independent association between selfassessed and physical fi tness exists and that selfassessed physical fi tness can be measured without taking, nonspecifi c LBP, educational attainments and morphological factors into account. As concluded by other authors as well (45;46), it seems that although selfassessed and performance measures are associated they are also distinct and should in higher degree be combined than substituted. A performancebased measure is not necessary superior (46) or more objective (47) compared with selfassessed measures in relation to an health outcome which should be elucidated in future surveys. Hence more knowledge is needed about how work factors and other individual factors infl uence the selfassessed dimensions of physical fi tness before the instrument for selfassessment of physical fi tness can be implemented in surveys as an alternative to performancebased physical fi tness. The population studied is a unique group of students tested in the beginning of their training before they are educated and before entering the labour marked. It would be highly interesting to follow changes in physical fi tness and LBP status during their training and intervention period and subsequently when entering the labour marked.


97

Table 1. Characteristics of the study participants.

Women Men n=511 n=101

Age 32.3 (10) 37.6 (34)*** 12month prevalence of LBP 196 (39%) 34 (34%)Selfassessed physical fi tness Aerobic fi tness (mm) 46 (20) 56 (21)*** Muscle strength (mm) 50 (18) 58 (17) *** Endurance (mm) 52 (19) 65 (19) *** Flexibility (mm) 52 (20) 56 (21) Balance (mm) 56 (19) 64 (19) ***Performancebased physical fi tness back ext. endurance (% passed 180s) 286 (56%) 51 (50%) back fl ex. endurance (% passed 180s) 225 (44%) 57 (56%)* Flexibility (cm above fl oor level) 3.3 (9) 1.8 (11)*** Balance (% passed 60s) 432 (85%) 85 (84%)Height (cm) 164 (6) 178 (7) ***BMI (kg*m2) Normal (<25) 289 (57%) 61 (60%) Overweight (2530) 138 (27%) 26 (26%) Severe overweight (>30) 80 (16%) 14 (14%)Educational attainments (>9 years school) 127 (25%) 31 (31%)History of physical demanding work (>6month) 171 (37%) 34 (35%)

Mean (SD) or n (%). *P<0.05, **P<0.01, ***P<0.001 between men and women.


98


99


100

Figure 1

Figure 1. The design of the selfassessment instrument using VAS. The subjects were asked to score the physical fi tness components with reference to people of their own age and sex by setting a vertical mark on each VAS.


101

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