dennerlein,forearmlff,aiha2003

Seediscussions,stats,andauthorprofilesforthispublicationat:http://www.researchgate.net/publication/5632213

Fatigueintheforearmresultingfromlow-levelrepetitiveulnardeviation.

ARTICLEinAIHAJOURNALNOVEMBER2003

DOI:10.1202/515.1Source:PubMed

CITATIONS

21

DOWNLOADS

86

VIEWS

135

4AUTHORS,INCLUDING:

JackTighDennerlein

NortheasternUniversity

179PUBLICATIONS1,724CITATIONS

SEEPROFILE

PeterJohnson

UniversityofWashingtonSeattle

95PUBLICATIONS817CITATIONS

SEEPROFILE

Availablefrom:PeterJohnson

Retrievedon:21July2015
http://www.researchgate.net/publication/5632213_Fatigue_in_the_forearm_resulting_from_low-level_repetitive_ulnar_deviation?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_2http://www.researchgate.net/publication/5632213_Fatigue_in_the_forearm_resulting_from_low-level_repetitive_ulnar_deviation?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_3http://www.researchgate.net/?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_1http://www.researchgate.net/profile/Jack_Dennerlein?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_4http://www.researchgate.net/profile/Jack_Dennerlein?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_5http://www.researchgate.net/institution/Northeastern_University?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_6http://www.researchgate.net/profile/Jack_Dennerlein?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_7http://www.researchgate.net/profile/Peter_Johnson13?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_4http://www.researchgate.net/profile/Peter_Johnson13?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_5http://www.researchgate.net/institution/University_of_Washington_Seattle?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_6http://www.researchgate.net/profile/Peter_Johnson13?enrichId=rgreq-517968e9-fae0-42ce-b2e8-21aab5f0f897&enrichSource=Y292ZXJQYWdlOzU2MzIyMTM7QVM6MTAxMDY2MTI1MzQ4ODY2QDE0MDExMDc0NTE1MjQ%3D&el=1_x_7

Copyright 2003, American Industrial Hygiene Association

AIHA Journal 64:799805 (2003) Ms. #515

APPLIED

STU

DIES

AIHA Journal (64) November/December 2003 799

AUTHORSJack Tigh DennerleinaVincent M. CiriellobKirsty J. Kerina,cPeter W. Johnsond

aDepartment of EnvironmentalHealth, Harvard University,School of Public Health, 665Huntington Ave., Boston, MA02115;bLiberty Mutual ResearchInstitute for Safety, 71 FranklandRoad, Hopkinton, MA 01748;cCircadian Technologies Inc., 24Hartwell Ave., Lexington, MA02421;dDepartment of EnvironmentalHealth, University ofWashington, School of PublicHealth and CommunityMedicine, Seattle, WA 98195

Fatigue in the Forearm ResultingFrom Low-Level RepetitiveUlnar Deviation

This study measured low-frequency fatigue (LFF) in the extensor carpi ulnaris (ECU) muscle

while workers completed a repetitive ulnar deviation task. Using a repeated measures design,

10 healthy women participated in three conditions, each lasting 2 consecutive days: a control

condition in which subjects remained inactive, and two repetitive work conditions involving

repeated ulnar deviation of the wrist at 20 and 25 repetitions per minute at individual

workloads deemed acceptable for 8 hours through a psychophysical protocol. LFF of the ECU

muscle and self-reported levels of fatigue were recorded eight times throughout the control and

workdays before (time 0), during (2, 4.25, 6.75, 8 hours), and after (9, 10, and 11 hours)

exposure. The ratio of the isometric force produced by electrical stimulus at 20 pulses per

second (pps) to the isometric force produced by 50 pps provided the measure of LFF. The

ratios were lower on workdays compared with the control days, indicating the presence of LFF

during repetitive work. During repetitive work the ratios decreased during the day, indicating the

muscles fatigued as the day progressed. The psychophysically determined workloads, although

not creating noticeable discomfort to the subjects, were high enough to create low levels of

muscle fatigue.

Keywords: extensor carpi ulnaris, low-frequency fatigue, muscle, musculoskeletal

disorders, repetitive work

This project was fundedin part by The KresgeCenter for EnvironmentalHealth (ES00002) at theHarvard School of PublicHealth; the LibertyMutual Harvard Programin Occupational Healthand Safety; and theLiberty Mutual ResearchInstitute for Safety.

Since the middle of the 1980s, chronicmusculoskeletal disorders of the upperextremity associated with repetitive workhave increased in both prevalence and in-

cidence.(1) These disorders are especially evidentin the food packing industry, where workers re-peatedly use ulnar deviation movements duringmeat, poultry, or seafood cutting tasks.(2,3) Al-though the injury mechanisms of chronic mus-culoskeletal disorders are not well understood,studies have identified several physical risk factorsincluding repetition rate,(4) force,(5) posture,(6) vi-bration,(7) and the patterns of the work. Thequestion that remains, however, is what levels offorce and repetition are acceptable.

The methods of Snook et al.(810) and Cirielloet al.(1113) used psychophysical methods to de-termine acceptable workloads for tasks involvingrepetitive motions of the wrist at various repeti-tion rates over an 8-hour day. Subjects workedfor extended periods of time7 hours per day,

5 days per week, up to a total of 23 days in somecases. They performed repetitive movementsagainst a load. Each subject defined an individualmaximum acceptable force operationally, as themaximum force he or she could tolerate withoutdeveloping unusual self-reported upper extremi-ty discomfort, such as soreness, stiffness, ornumbness. From these experiments, Snook etal.(810) and Ciriello et al.(1113) developed accept-able limits on repetitive work; however, thesestudies lack the support of physiological data.

Muscle fatigue when measured before, dur-ing, and after exercise is a measure of physiolog-ical change of a muscle and may provide a bio-marker for cumulative exposure to repetitivework. The primary risk factors for work-relatedmusculoskeletal disordersforce, duration offorce, repetition, and patterns of workare alsocauses of fatigue. Muscle fatigue and muscle painsyndromes are common in the workplace andmay precede more serious disorders.(1416) For

800 AIHA Journal (64) November/December 2003

APPL

IED

STU

DIE

S

TABLE I. Subject Daily Work and Experimental ProceduresMonday Tuesday Wednesday Thursday Friday

Week 1Week 2Week 3Week 4Week 5

training

PP @ 25 RPMFMP @ 25 RPM

setuptrainingPP @ 15 RPMPP @ 25 RPMFMP @ 25 RPM

control

PP @ 15 RPM

controltrainingPP @ 20 RPMFMP @ 20 RPM

trainingPP @ 20 RPMFMP @ 20 RPM

Note: PP 5 psychophysical protocol; FMP 5 fatigue measurement protocol

example, a relationship between muscle tissue damage and in-creased fatigability during occupational work has been shown us-ing electromyographic zero-crossing techniques in the trapeziusmuscle.(17,18) Given the low force levels, the long hours of repeti-tive work, and the fact that motor units are recruited and used atlow frequencies in everyday life,(19) low-frequency fatigue (LFF) islikely to be the most common type of muscle fatigue in the mod-ern workplace.(20)

LFF is a transient decrement in the force from a muscle inresponse to low-frequency (120 Hz) stimulation. Edwards etal.(21) electrically stimulated the abductor pollicis brevis muscle andmeasured the resultant force output at the thumb. The measure-ment of LFF involves stimulating the muscle at both low (120pulses per second [pps]) and high frequencies (50100 pps); thefatigue measure is based on a ratio of the force output of themuscle resulting from the low- and high-frequency stimulation.The force response of the muscle to high-frequency stimulationrecovered rapidly, whereas the force response to the low-frequencystimulation was suppressed and took a longer time to recover.Hence, recovery from LFF generally takes hours(20,22) and can evenpersist more than a day.(19,23) For these measurements the electricalstimulation recruits the same muscle fibers within a muscle syn-chronously, unlike a voluntary contraction, which recruits differ-ent fibers throughout the muscle asynchronously.

To date, the development of muscle fatigue during repetitivework has not been investigated over the course of a full workday.The goal of this study was to repeat the ulnar deviation tasks ofSnook et al.(9) for a set of female participants executing a repetitiveulnar deviation wrist movement for 8-hour days at two repetitionrates, and concurrently measure muscle fatigue in the extensorcarpi ulnaris muscle. Because the workloads are set by the partic-ipants to be acceptablethat is, not creating any unnecessary dis-comfort, soreness, or painthe null hypothesis was that no musclefatigue would occur during the workday and across 2 consecutiveworkdays.

METHODS

Ten female participants between the ages of 19 and 52 (mean35 years, SD 15 years) were recruited from the general publicusing newspaper advertisements. Screening ensured that subjectswere not routinely overly exposed to repetitive motion in theirdaily activities, and each participant was examined by a nurse prac-titioner for symptoms associated with musculoskeletal disorders ofthe upper extremity. Each participant read and signed an experi-mental informed consent form. Both the Harvard School of PublicHealth Human Subjects Committee and the Liberty Mutual Re-search Institute for Safety Internal Review Committee approvedexperimental protocols and the participant consent and recruit-ment forms. These subjects were a subset of a larger group un-dergoing repetitive ulnar deviation tasks reported elsewhere.(11,24)

Subjects entered a 17-day protocol dispersed over 4 weeks (Ta-ble I). The first day was devoted to subject orientation and max-imal strength measurements. On the second and third days control(no-exposure) baseline muscle fatigue measurements were record-ed throughout the day. Subjects then entered a 1-week trainingprotocol and then a 1.5-week psychophysical protocol to deter-mine their self-selected acceptable workloads (see Snook et al.(9)

and Ciriello et al.(11) for details) for ulnar deviation at 15, 20, and25 repetitions per minute (RPM). After a day of rest, fatigue mea-surements were recorded during 8 hours of repetitive work at 20RPM for 2 consecutive days and then again, after a weekendbreak, as subjects performed 8 hours of repetitive work at 25 RPMfor 2 consecutive days. Randomization of conditions was not pos-sible due to the psychophysical protocol of gradually increasingthe subjects up to the higher repetition rates.

The participants completed the repetitive ulnar deviation tasksat a set of workstations consisting of an adjustable chair and foot-rest with a visual display monitor mounted at eye level (see Snooket al.(9) for details). The workstation contained a magnetic particlebrake with a force and angle transducer attached to the shaft. Theshape of the handle, attached to the shaft, was rectangular andsimilar to knife handles used in meat-packing activities (152 mmlong, 29 mm wide, 22 mm deep with curved edges for comfort).Subjects sat at the workstation, placed their right forearm hori-zontally on the armrest, and grasped the handle with a power grip.The handle length was adjusted so that the point of rotation ofthe equipment was in line with the axis of the wrist joint. Thechair and footrest were adjusted such that the angle between theupper arm and the forearm was approximately 1208 and the legswere firmly supported by footrests. At the sound of an audibletone, subjects moved the handle through 808 of rotation using acombination of forearm movement and ulnar deviation of thewrist. As mentioned previously, handle resistance was determinedduring the psychophysical test days. To simulate a full workday,subjects worked for seven 55-minute segments. There were 5-minbreaks after segments 1, 3, and 5; 15-min work breaks after seg-ments 2 and 6; and a 30-min lunch break after segment 4. Workceased after segment 7, and subjects entered a 3-hour recoveryperiod.

Fatigue measurements of the extensor carpi ulnaris (ECU)muscle, a primary muscle associated with ulnar deviation,(25) wereobtained at the beginning of the day (0 hour) and then through-out the day at 2, 4:15, 6:45, 8, 9, 10, and 11 hours thereafter.During workdays work would cease at hour 8, and the additionalmeasures monitored recovery (hours 9, 10, and 11).

Electrically stimulating the ECU muscle and measuring the re-sulting isometric force output at the wrist provided the measureof muscle fatigue. Each measurement was obtained by having thesubjects sit with their right forearms in wrist force measurementapparatuses especially designed to measure isometric force of thewrist in the direction of ulnar deviation (Figure 1). The apparatus


APPLIED

STU

DIES

FIGURE 1. The wrist-restraining jig to measure the isometricforce of the ECU muscle from the electrical stimulation. Theapparatus kept the wrist steady and measured the resultingisometric force produced at the distal end of the fifthmetacarpalphalangeal joint.

secured the forearm just proximal to the wrist allowing for artic-ulation of the wrist joint only. The forearm was in a neutral pro-nation-supination posture with the thumb pointing upward. Thehand was placed on a platform which had a 25 lb (112 N) loadcell (Omega Engineering LCCA-25, Stamford, Conn.) mountedunderneath. The hand was positioned so the metacarpalphalangealjoint of the fifth digit was directly above the load cell. The studyparticipants sat upright with feet on the floor, knees at 908, shoul-ders parallel to the floor, and the arm relaxed forming a 908 angleat the elbow with the forearm parallel to the floor.

The ECU muscle was stimulated through two surface elec-trodes 1 cm in diameter placed along the belly of the muscle withthe anode distally with respect to the cathode.(26) To ensure con-sistent electrode placement across days, henna tattoos, which re-main for at least 1 to 2 weeks, marked the position of the elec-trodes on the skin. Photographs and sketches of the forearm alsodocumented the electrode placement. The electrodes were con-nected to a Grass Electronics S48 muscle and nerve stimulator(West Warwick, R.I.) through a stimulus isolation unit (GrassSIU5) and a constant current unit (Grass CCU1). The S48 stim-ulator controlled the frequency and the duration of the stimulus(100 ms). The constant current unit controlled the stimulus toensure a constant current was always applied to the muscle. Theelectrode placement and stimulation intensity were determined onthe orientation day (day 1). Electrode placement was determinedby repeatedly positioning the cathode electrode over the ECUmuscle until the muscle response, based on visual observation,appeared to be maximal with minimal recruitment of other mus-cles. Once this site was determined, the stimulus intensity wasincreased to the subjects maximal level of acceptable tolerance.Three subjects from the larger study(11) (N513) had low tolerancelevels to the stimulation such that a reliable muscle response couldnot be obtained. Hence, they were not included in this portionof the study. The current settings remained constant throughoutthe experiment.

The fatigue measurements consisted of recording the force pro-duced by electrically stimulating the ECU muscle for 2 sec at 50pulses per second (pps) and then after a 2-sec rest another 2-secstimulus at 20 pps. This was repeated after 2 sec of rest providing

two sets of contractions. The force signal was recorded throughan analogue-to-digital converter (PCMIO16E-4, National Instru-ments, Austin, Texas) and stored on a personal computer at 1000samples per second. The 20 to 50 pps ratios were calculated forthe two sets of contractions and averaged to produce the singlemeasure for each time period. The ratio of the force produced by20 pps to the force produced by the 50 pps indicated the amountof muscle fatigue.(20)

Signs of upper extremity symptoms and subject discomfortwere also monitored throughout the workday through a self-ad-ministered symptom-survey form.(8,9,12) Subjects completed theform each hour, recording any soreness, stiffness, or numbness(rated 0 to 3: 05no soreness, 15a little sore, 25moderately orsomewhat sore, 35very sore) that they experienced in their hands,wrists, or forearms. The protocol called for the experimenter tointervene when a subject reported a level 2 symptom at the be-ginning of the day (prework), a level 3 at any time, or three con-secutive level 2 scores. Intervention consisted of a private reviewof the instructions with the subject and an examination of theforearm for any signs of pain, inflammation, or discomfort. Inaddition, the subjects perceptions of fatigue were measured witha modified Borg Scale. The Borg Scale was presented visually ona 10-cm line with Borgs verbal anchors spaced proportionallywith respect to their numerical score. Subjects were asked to markthe position on the line that corresponded to their perceived levelsof fatigue. Subjects were asked to rate their perceptions of fatiguelocalized to the forearm and their general overall levels of per-ceived fatigue.

Due to technical problems and late arrival of one subject on 1day, there were some missing repeated measures; therefore, thePROC MIXED regression analysis procedure in SAS (SAS V 8.2SAS Computing Software, Cary, N.C.) was used to determinewhether there were any significant differences and trends withinthe data over the experimental days and between conditions. Theindependent variables were Condition: control (no exposure), 20RPM (exposure), or 25 RPM (exposure); Day within condition:day 1 or 2; and Hour of Measurement: 0, 2, 4.25, 6.75, 8, 9, 10,11. The dependent variables were the ratio of the force producedby 20 pps stimulation to the force produced by the 50 ppsstimulation.

RESULTS

Table II lists the workload torque values selected by the 10 sub-jects as being acceptable for completing 8 hours of repetitivework at the two repetition rates. The torque levels were withinthe ranges reported in previous psychophysical protocols (see Cir-iello et al.(11) for a summary) and were a subset of a larger groupof participants presented in Bennie et al.(24) No subject reportedsigns of upper extremity symptoms and subjective discomfortabove 1 (a little sore) at any point during the experiment. Theincidence of level 1 symptom reporting was less than 1% of therecorded levels.

The ratio of the force produced by the 20 pps to the forceproduced by the 50 pps decreased with exposure to repetitivework, hence, the muscles exhibited LFF. The ratios on the work-days were on average significantly lower (p,.01) than the ratioson the control days, on average by 3 and 4% for the 20 RPM and25 RPM days, respectively (Figure 2). The difference between the20 RPM and 25 RPM days was small, roughly 1% smaller on the25 RPM exposure. Significance was not observed between theconsecutive days within each condition (p5.4).


APPL

IED

STU

DIE

S

TABLE II. Subject Parameters Including Strength and the Individually Selected Workload Torques

Subject No.Age

(Years)MVC(N-m)

Torque @ 20 RPM(N-m)

Torque @ 25 RPM(N-m)

20 RPMTorque Product

(N-m/min)

25 RPMTorque Product

(N-m/min)

0456789

101213

52484621211924514923

6.88.56.97.09.59.97.14.13.98.6

1.771.311.970.370.211.522.940.760.942.28

1.380.941.530.190.221.382.850.80.581.39

35.426.239.47.44.2

30.458.815.218.845.6

34.523.538.34.85.5

34.571.320.014.534.8

Mean (SD) 35 (15) 7.23 (2.03) 1.41 (0.86) 1.13 (0.78) 28 (5.8) 28 (6.5)

Note: MVC 5 maximum voluntary contraction torque applied to the handle; RPM 5 repetitions per minute.

FIGURE 2. The fatigue values of the ratios over the threeconditions and the 2 days of testing. The ratios are lower on theworkdays, indicating a low level of LFF present in the musclesresulting from the repetitive workloads.

FIGURE 3. The ratio of the force produced by the 20 and the50 pps stimulus before (H0) and after exposure (H8) and after a3-hour recovery (H11). The ratios decrease during the exposure;however, they remain relatively constant on the control days.

The ratios decreased during the workdays but not on the con-trol days (Figure 3). For the control condition the ratios remainedrelatively constant with respect to the time of day, with hour notbeing a significant factor (p..5). For the work conditions, how-ever, the ratios significantly decreased throughout the day withrespect to hours of exposure (p,.005).

LFF did not appear to last into the next day of testing duringthe exposure conditions. Although the ratios did tend to be about1% smaller on the second day of testing, this decline was not sig-nificant (p5.4, Figure 2). The ratio at the beginning of day 2 ofthe 20 RPM condition appeared to be lower, as shown in Figure3, the measures of the LFF ratio at the beginning of the day (H0)were not significantly different across conditions (p5.82) or acrossdays (p5.4) of testing.

The ratings of perceived fatigue in the forearm were unaffectedby condition (p5.15, Figure 4). However, the perceived fatiguedid increase as the day progressed (p,.001) even on the control

days. These levels of perceived fatigue were quite small, with ter-minal ratings at hour 8 ranging between very slight to slight. Re-sponses to the general overall fatigue question were similar in thatthere were no differences observed over the day, and the valuesdid increase as the day progressed.

DISCUSSION

When the workload of a repetitive task of the wrist was deter-mined using a psychophysical protocol, LFF was observed inthese 10 women. These physiological data support the hypothesisthat a full workday of repetitive work creates a physiologicalchange, rejecting the stated null hypothesis. These data indicatethat the range of psychophysical protocol levels reported bySnook(810) and Ciriello(1113) were high enough to create fatigueduring the day, but low enough that subjects were almost fullyrecovered by the next day. The decreases in the ratio of the forcesproduced by the 20 and 50 pps stimulations indicate that the mus-cles of the forearm fatigued during the 20 and 25 RPM condi-tions, even when subjects worked at their self-selected workloads.As indicated by the pain symptom responses, these workloads


APPLIED

STU

DIES

FIGURE 4. The perceived levels of forearm fatigue measuredthrough the day and the corresponding verbal anchors. Thelevels of self-reported fatigue were very small (maximum of 1.6on a 10-point scale) and increased only significantly with thetime of day. The self-reported fatigue levels did not correspondwith the measurement of LFF.

FIGURE 5. The ratios measured at the end (H8) of the 20 and25 RPM exposures relative to the workload normalized to thesubjects strength (n540). The wide range of workloads addedto the variation in the fatigue levels. In general, the fatigueincreased as the workload increased across subjects (p5.001).However, fatigue levels clustered into two groups: those subjectswith a workload below 10% and those with a workload above10% when the increasing fatigue with workload was notobserved (p5.5).

caused negligible soreness, pain, or discomfort in the forearm.Hence, the physiology of the muscle was changing as the repetitivetask was completed during the day without causing any pain tothe worker.

For these submaximal workloads there was no significant evi-dence that the fatigue remained into the next workday. However,for both the 20 and 25 RPM conditions the ratio at the beginningof the second day was lower (Figure 3). Low frequency musclefatigue has also been referred to as fatigue of long duration, be-cause it has been observed to last for more than 24 hours andeven after submaximal exertions.(23) Therefore, these self-selectedworkloads at the specific repetition rates were low enough so thatthe muscles recovered for the most part from the LFF resultingfrom the previous days work. However, this exposure was overonly 2 days, and it is possible that for exposure lasting longer than2 days fatigue may start to carry over to the following days. Fur-ther investigations examining fatigue over multiple days aremerited.

The workloads determined in this protocol were within therange of previous studies presented by Ciriello et al.,(11) and theadditional fatigue measure provided further physiological data forthese psychophysical methods. The study participants judged andselected a workload that resulted in LFF, but for the most partthe fatigue and resultant cumulative physiological change in mus-cle did not carry over into the next day. As a result, these levelsof workloads selected by the subjects may help reduce the workerschances of developing long-term muscle fatigue and possiblychronic injury. Fatigue is thought to be a protective response ofthe muscle to work and to be acceptable when the recovery timesare short and allow enough time for damaged tissue to repair it-self.(2729)

Electrical muscle stimulation as used in these experiments canbe a tool to explore the relationship between exposure in termsof force, repetitions, work, or force-time integral, and muscle/tissue response. As demonstrated in Figure 5, for the subjects

whose work load exceeded 10% of their maximum voluntary con-traction (MVC) values, the LFF ratio was lower compared withthat of subjects whose work load was less than 10% MVC. Thefatigue measured at the end of the exposure showed a slight re-lationship (p5.02) with absolute workload (work expressed in N-m) and a stronger (p5.001) relationship with relative workload(work expressed as a %MVC), suggesting that fatigue may be morea measure of relative exposure rather than absolute exposure.However, within the group with workloads above 10% the increas-ing fatigue with workload was not observed (p5.5). Measurementof objective physiological changes in muscle behavior can provideinformation on how to better design the work environment and/or the tools used to evaluate tasks that are repetitious and/ormonotonous in nature. However, the question remains whetherthe fatigue that accumulates over time creates a situation for cu-mulative damage to the soft tissues of the musculoskeletal system.

The results indicated that fatigue occurred when subjects wereexposed to the repetitive work; however, there were no observeddifferences in the level of fatigue at the two different exposures.One explanation is that there may not have been the statisticalpower to detect these differences. The other is that the exposurebetween the two conditions, 20 RPM and 25 RPM were not thatdifferent. Others have suggested, especially for isometric types oftasks, a force-time integral as the exposure factor for fatigue.(23)The force-time integral can be estimated from the product of thetorque and the repetition rate, assuming that the time to turn thehandle remained constant between conditions. Indeed, when boththe self-selected force levels and the repetition rate were accountedfor, there was little difference between the protocols (Table II).During the psychophysical protocol used here, the subjects se-lected a nearly constant force-time workload over the two repe-tition rates. Therefore, it is not possible to determine the individ-ual roles of force or repetition in the onset and development oflow frequency fatigue.


APPL

IED

STU

DIE

S

A large source of variation within the results of this study wasthe exposure of the participants to different workloads in terms ofboth absolute crank torques and a torque normalized to the par-ticipants strength. The relative torques range from 2 to 41%MVC, with an average of 19% for the 20 RPM and 15% for the25 RPM tasks. This lack of experimental control could have re-duced the power of procedures testing for differences between theexposures and the accumulation of fatigue to the next day. None-theless, significant fatigue was observed in the exposure conditionseven though the workloads were self-selected.

The limitations of this laboratory-based study include gener-alizing these conclusions to the actual work settings, nonrandom-ization of conditions, limitation associated with electrical stimu-lation of the muscle, and the small levels of fatigue observed. Areal job may have a natural variability in terms of force and work-load. Exposure assessment of repetitive work in the actual work-place is needed for designing real work-like laboratory experi-ments. Characterizing the workplace exposures and measuring theresultant fatigue in the lab would help extend the results of thesetypes of studies to the real work force. In this study, force was thevariable the participants were able to self-select; often it is not theforce, but the frequency that workers can select when controllingworkload. Furthermore, the order of the conditions was not ran-domized, allowing the effects of training and strengthening topotentially introduce a systematic bias into the results. However,although this bias would be toward the null, fatigue was still ob-served. The exclusively female study population also limits theability, to a certain degree, to generalize conclusions.

During the administration of the electrical stimulation, thestimulus intensity was limited by the subjects ability to toleratethe discomfort associated with the electrical stimulation. The re-searchers were not able to adequately recruit the muscle in 3 ofthe 13 subjects who participated in the full protocol. Higher stim-ulus intensities typically result in a greater force output from themuscle. If the force response of the muscle increases while sourcesof measurement error remain constant (i.e., measurement errorsassociated with passive contributions from muscle and tendons),then the overall experimental variability could be reduced. Also,only one muscle was examined that creates ulnar deviation at thewrist; therefore, conclusions drawn are limited to this muscle.Other muscles may be acting in synergy, sharing the load andreducing the effect on the ECU muscle.

Muscle fiber recruitment of and induced fatigue by electricalstimulation may be of concern. In this context the goal of theelectrical stimulation was to repeatedly recruit the same portionof a muscle within the day to measure changes within and overthat cross-section of the muscle. The changes measured are thenrepresentative of the condition of the fibers contained in that crosssection and may be representative of the other fibers comprisingthe rest of the muscle. Another concern is that electrical stimula-tion may cause fatigue. The electrical stimulation protocol in thisstudy was very brief, four 2-sec trains. The exposure resulting fromthe repetitive work was much greater than from the brief bouts ofelectrical stimulation administered to the muscle. In addition,there was not a consistent decrease in the ratios over the day forthe control conditions, indicating any systematic fatigue affectsresulting from the electrical stimulation.

In conclusion, even though the study participants did not re-port any substantial levels of perceived forearm fatigue, low fre-quency muscle fatigue was observed in the extensor carpi ulnarismuscle after exposure to a repetitive wrist task designed to simu-late 2 full workdays. The torque levels of the repetitive task, whichwere determined as acceptable through a psychophysical protocol,

did create a temporary state of fatigue, indicating that psycho-physical acceptable levels may not prevent the onset and devel-opment of LFF.

ACKNOWLEDGMENTS

The authors wish to thank the following individuals for theirassistance from data collection to statistical analysis: MaryDionne, Joanne Gouin, Peter Teare, Eric Jones, and Rick Holihanof Liberty Mutual Research Institute for Safety; Steve Lehman andTom Hruska of the University of California, Berkeley; and LouiseRyan at the Harvard School of Public Health.

REFERENCES

1. U.S. Department of Labor: Bureau of Labor Statistics. [On-line]Available at http://www.bls.gov/iif/home.html (Accessed 2001).

2. Armstrong, T.J., J. Foulke, and S. Goldstein: An investigation ofcumulative trauma disorders in a poultry processing plant. Am. Ind.Hyg. Assoc. J. 43:103116 (1982).

3. Silverstein, B., E. Welp, N. Nelson, and J. Kalat: Claims Incidenceof work-related disorders of the upper extremities: Washington State,1987 through 1995. Am. J. Public Health 8:18271833 (1998).

4. Hansson, G.A., I. Balogh, K. Ohlsson, B. Palsson, L. Rylander,and S. Skerfving: Impact of physical exposure on neck and upperlimb disorders in female workers. Appl. Ergonom. 31(3):301310(2000).

5. Stoy, D.W., and J. Aspen: Force and repetition measurement of hamboning. Relationship to musculoskeletal symptoms. AAOHN J. 47:254260 (1999).

6. Stal, M., G.A. Hansson, and U. Moritz: Wrist positions and move-ments as possible risk factors during machine milking. Appl. Ergonom.30:527533 (1999).

7. Masmajean, E.H., H. Chavane, A. Chantegret, J.J. Issermann,and J.Y. Alnot: The wrist of the Formula 1 driver. Br. J. Sports Med.33:270273 (1999).

8. Snook, S.H., D.R. Valliancourt, V.M. Ciriello, and B.S. Webster:Psychophysical studies of repetitive wrist flexion and extension. Ergo-nomics 38:14881507 (1995).

9. Snook, S.H., D.R. Valliancourt, C.M. Ciriello, and B.S. Webster:Maximum acceptable forces for repetitive ulnar deviation of the wrist.Am. Ind. Hyg. Assoc. J. 59:509517 (1997).

10. Snook, S.H., V.M. Ciriello, and B.S. Webster: Maximum accept-able forces for repetitive wrist extension with a pinch grip. Int. J. Ind.Ergonom. 24:579590 (1999).

11. Ciriello, V.M., K.J. Bennie, P.W. Johnson, and J.T. Dennerlein:Comparison of three psychophysical techniques to establish maximumacceptable torques of repetitive ulnar deviation. Theor. Issues Ergonom.Sci. 3:274284 (2002).

12. Ciriello, V.M., S.H. Snook, B.S. Webster, and P. Dempsey: Psy-chophysical study of six hand movements. Ergonomics 44:922936(2001).

13. Ciriello, V.M., B.S. Webster, and P.G. Dempsey: Maximal accept-able torques of highly repetitive screw driving, ulnar deviation, andhandgrip tasks for 7-hour workdays. Am. Ind. Hyg. Assoc. J. 63:594604 (2002).

14. Valencia, F.: Local muscle fatigue. A precursor to RSI? Med. J. Aust.145:327330 (1986).

15. Takala, E.P.: Static muscular load, an increasing hazard in moderninformation technology. Scand. J. Work Environ. Health 28:211213(2002).

16. Rempel, D.M., R.J. Harrison, and S. Barnhart: Work-related cu-mulative trauma disorders of the upper extremity. J. Am. Med. Assoc.267:838842 (1992).

17. Hagg, G.M.: Static work load and occupational myalgiaa new ex-planation model. In P. Anderson, D. Hobart, and J. Danoff, editors,


APPLIED

STU

DIES

Electromyographical Kinesiology, pp 141144. Amsterdam: Elsevier,1991.

18. Hagg, G.M.: Human muscle fiber abnormalities related to occupa-tional load. Eur. J. Appl. Physiol. 83:159165 (2000).

19. Enoka, R.M., and D.G. Stuart: Neurobiology of muscle fatigue. J.Appl. Physiol. 72:16311648 (1992).

20. Westerblad, H., J.D. Bruton, D.G. Allen, and J. Lannegren: Func-tional significance of Ca 21 in long-lasting fatigue. Eur. J. Appl.Physiol. 83:166174 (2000).

21. Edwards, R.H.T.: Fatigue of long duration in human skeletal muscleafter exercise. J. Physiol. 272:769778 (1977).

22. Bystrom, S., and A. Kilbom: Electrical stimulation of human fore-arm extensor muscles as an indicator of handgrip fatigue and recovery.Eur. J. Appl. Physiol. Occup. Physiol. 62:363368 (1991).

23. Bystrom, S., and C. Fransson-Hall: Acceptability of intermittenthandgrip contractions based on physiological response. Hum. Factors36:158171 (1994).

24. Bennie, K.B., V.M. Ciriello, P.W. Johnson, and J.T. Dennerlein:Electromyographic activity of the ECU muscle changes with exposureto repetitive ulnar deviation. Eur. J. Appl. Physiol. 88:512 (2002).

25. Buchannan, T.S., M.J. Moniz, J.P.A. Dewald, and W.Z. Rymer:Estimation of muscle forces about the wrist joint during isometrictasks using an EMG coefficient method. J. Biomech. 26:547560(1993).

26. Perotto, A.: Anatomical Guide for the Electromyographer, 3rd ed.Springfield, Ill.: Charles C. Thomas, 1994.

27. Bigland-Ritchie, B., D.A. Jones, and J.J. Woods: Excitation fre-quency and muscle fatigue: Electrical responses during human vol-untary and stimulated contractions. Exp. Neurol. 6:414427 (1979).

28. Garner, S.H., A.L. Hicks, and A.J. McComas: Prolongation oftwitch potentiating mechanism throughout muscle fatigue and recov-ery. Exp. Neurol. 103:277281 (1989).

29. Green, H.J., and S.R. Jones: Does post-tetanic potentiation com-pensate for low frequency fatigue? Clin. Physiol. 9:499514 (1989).

dennerlein,forearmlff,aiha2003

Documents