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IN-DEPTH REVIEW

Ergonomics, musculoskeletal disorders and

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computer work

Jens Wahlstrom1,2

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Abstract This review summarizes the knowledge regarding ergonomics and musculoskeletal disorders and the

association with computer work. Amodel of musculoskeletal disorders and computer work is proposed

and the evidence and implications of the model together with issues for future research is discussed.

The model emphasizes the associations between work organization, psychosocial factors and mental

stress on the one hand and physical demands and physical load on the other. It is hypothesized that

perceived muscular tension is an early sign of musculoskeletal disorder, which arises as a result of work

organizational and psychosocial factors as well as from physical load and individual factors. It is further

hypothesized that perceptions of exertion and comfort are other possible early signs of musculoskeletal

disorders in computer work. Interventions aimed at reducing musculoskeletal disorders due to

computer work should be directed at both physical/ergonomic factors and work organizational and

psychosocial factors. Interventions should be carried out with management support and active

involvement of the individual workers.

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Key words Physical load; psychosocial factors; VDU; VDT.. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Introduction

During the last two decades, the number of workers with

visual display units (VDU) has increased dramatically. In

2001, approximately 65% of the Swedish workforce used

a VDU in their occupation, compared to 30% in 1989

[1]. Since the late 1980s, the use of non-keyboard input

devices has increased rapidly and today the market is

filled with a large number of different non-keyboard input

devices, although the most widely used is still the

computer mouse [1,2].

There are only a few published papers that report the

incidence of musculoskeletal symptoms and disorders

among VDU users. A Finnish study [3] reported the

annual incidence of neck pain among VDU users to be

34%. A prospective cohort study from the USA reported

the annual incidence of neck/shoulder musculoskeletal

symptoms to be 58 cases/100 person-years [4]. Cross-

sectional studies of VDU users have reported a preva-

lence of 10–62% of musculoskeletal symptoms in the

neck/shoulder region among VDU users [5–8].

Musculoskeletal symptoms of VDU users are believed

to have a multi-factorial aetiology. Non-neutral wrist,

arm and neck postures, the work station design and the

duration of VDU work as well as psychological and social

factors, such as time pressure and high-perceived work-

load, are believed to interact in the development of these

symptoms [9–13]. Several studies have suggested that an

increased prevalence of upper extremity musculoskeletal

symptoms may be associated with increased computer

mouse use [6,14,15].

The aim of this review is to give a summary of the

knowledge regarding ergonomics, musculoskeletal dis-

orders and computer work and to present a model that

could be used in future research.

Physical load factors

Physical load is defined as factors relating to biomecha-

nical forces generated in the body. In the literature, this

has also been defined as ‘mechanical exposure’ to

indicate that other factors in the environment are not

considered (i.e. lighting, noise, the thermal environment,

work organization, psychosocial factors, etc.) [16].

Muscular load

The most common method to assess muscular load

within this field has been electromyography (EMG). In

the early 1950s, Lundervold [17] used EMG to

investigate muscle activity in patients with so-called

‘occupation myalgia’. ‘Occupation myalgia’ was defined

as pain in muscles overstrained as a result of unvaried

work (i.e. static work). Most of the patients studied had

Occupational Medicine 2005;55:168–176doi:10.1093/occmed/kqi083

q The Author 2005. Published by Oxford University Press on behalf of the Society of Occupational Medicine.All rights reserved. For Permissions, please e-mail: [email protected]

Correspondence to: Jens Wahlstrom, Department of Occupational and

Environmental Medicine, Sundsvall Hospital, SE-851 86 Sundsvall, Sweden.

e-mail: [email protected]

1Department of Occupational Medicine at The Sahlgrenska Academy at

Goteborg University, Sweden.

2Occupational and Environmental Medicine, Sundsvall Hospital, Sweden.

typewriting as one of their main job tasks [17]. To

quantify EMG in relation to risk, load limits have been

proposed based on the 10th, 50th and 90th percentiles of

the amplitude distribution of the muscle activity [18].

VDU work is characterized by low levels of muscle

activity, especially in the trapezius muscles, and no safe

lower limit of muscle activity may exist [16]. Two other

EMG measures used are gap frequency (i.e. number of

periods with muscle activity below a predefined threshold

level per time unit) and muscular rest (i.e. the total time

with muscle activity below the predefined threshold level

relative to the total duration of the recording time) [19,

20]. Some studies have observed that muscular rest and

EMG gaps (short periods of muscle rest) are associated

with a higher risk of musculoskeletal symptoms in the

neck/shoulder region [21–23]; other studies have failed

to show this association [24–26].

Several hypotheses have been proposed for the

pathogenesis of work-related musculoskeletal symptoms

and pain [27–31]. One suggests that low static contrac-

tion during work may result in a recruiting pattern or

motor programme, in which only type I muscle fibres are

used, and this may lead to selective motor unit fatigue

and damage [32]. A similar hypothesis known as the

‘Cinderella hypothesis’ has been proposed by Hagg [28].

In line with these hypotheses is the overload of type I

muscle fibres during prolonged static contractions. The

theory about an overload of type I muscle fibres has been

supported in morphological studies in which ragged-red

fibres (injured type I muscle fibres) were observed to be

associated with occupational static loads [33,34].

Recent results from experimental studies using intra-

muscular EMG support the hypothesis of a recruiting

pattern with type I muscle fibres that are continuously

active during static as well as dynamic arm movements,

VDU work and mental stress [35–39]. Observations of

motor unit substitution have been reported, that is, when

one motor unit is de-recruited and another motor unit

with a higher threshold is recruited [39,40]. Inter-

individual differences in motor unit activity patterns

have been observed, with some but not all subjects

showing continuously active motor units [39,41].

Mathiassen and Aminoff [42] observed inter-individual

differences in the motor response of the trapezius muscle

when subjects performed isometric (static) shoulder

contractions. They suggested that the different motor

responses may explain why individuals with the same

exposure do not contract the same type of symptoms, or

why some individuals remain healthy.

It has been discussed whether the low levels of muscle

activity recorded during light manual and/or VDU work

are associated with increased risk of musculoskeletal

symptoms, since the levels are probably hard to differen-

tiate from levels during inactive living [26]. A recently

proposed hypothesis is the blood vessel–nociceptor

interaction hypothesis by Knardahl [29]. The hypothesis

pertains to work situations with cognitive tasks and low-

level muscle contractions and suggests that blood vessel–

nociceptor interactions are of central importance in

generating pain. Knardahl’s hypothesis raises the need

to rethink the methods and concepts used for studies of

myalgia and musculoskeletal symptoms and pain associ-

ated with VDU use.

Supporting the forearms and wrists during keyboard

and input device work has been proposed as a preventive

measure. This has mainly been based on experimental

studies observing decreased muscle activity in the

trapezius muscle when supporting the forearms

[43–45]. Recently published studies with a longitudinal

design have also observed that supporting the arms during

VDUwork is associated with decreased risk of developing

neck/shoulder symptoms and disorders [46,47].

Postures and movements

The results from a prospective study by Ariens et al. [48]

showed a positive association between sitting at work for

more than 95% of the working time and neck pain; a

trend was also observed for a positive relation between

neck flexion and neck pain. However, another prospec-

tive study observed a greater downward tilt to be

associated with lower risk of neck/shoulder symptoms

and disorders [47].

Non-neutral postures of the shoulder (i.e. flexion and

abduction) have been found to be associated with

musculoskeletal symptoms of the neck and upper limbs

in previous reviews [9,12]. A recently published pro-

spective study found that non-neutral postures of the

shoulder were not associated with neck/shoulder or hand/

arm symptoms or disorders [47]. In the study by Marcus

and colleagues [47], the only posture variable that

showed an increased risk for musculoskeletal symptoms

and disorders was if the inner elbow angle was ,1218.

Extreme positions of the wrist have been considered to

be a risk factor for musculoskeletal symptoms of the hand

and wrist [49–51]. A recently published study suggested

wrist extension of .208 increased the risk of carpal

tunnel syndrome (CTS) [52], though a prospective study

concluded that VDU work does not impose an occu-

pational hazard for CTS [53]. Repetitive work has been

associated with an increased risk of musculoskeletal

symptoms of the wrist and forearm [51,54–57]. With

exposure to both extreme postures and repetitive tasks it

has been suggested that the risk increases, compared with

exposure to only one risk factor [51].

Force

The forces applied to the computer mouse and keyboard

may be a risk factor for musculoskeletal symptoms

[58,59]. It has been observed that 3–4 h of computer

J. WAHLSTROM: ERGONOMICS, MUSCULOSKELETAL DISORDERS AND COMPUTERWORK 169

mouse work could lead to fatigue in the muscles of the

forearm [59]. It is not known if the forces applied to the

sides and button of the computer mouse is associated

with increased risk for developing musculoskeletal

symptom. It has been observed that subjects with more

severe musculoskeletal symptoms apply higher force

while keyboarding [58].

Visual demands

Eye symptoms and visual discomfort has been associated

with VDU work [9,46,60]. Positive results from

improved visual conditions and optometric corrections

have been demonstrated in a 6-year follow-up study [46].

Current guidelines regarding monitor placement at

VDUs suggest that the top of the screen should be at or

slightly below eye level. In recent years, lower monitor

placements have been proposed [61]; however, there is

not enough scientific evidence available to change the

current guidelines.

Work organization, psychosocial factorsand mental stress

Since the early 1990, the role of work organization and

psychosocial factors within the work environment has

gained more focus in the study of work-related musculo-

skeletal disorders. The work organization or work system

has been suggested to consist of five important com-

ponents: organizational structure, people or personnel

sub-system, technology or technological sub-system,

work tasks, and the relevant external environment [30].

The different elements in the work system are thought to

affect psychosocial factors, for example, job demands,

decision latitude and social support from managers and

colleagues.

Duration and patterns of work

The duration of VDU work has in several studies been

identified as a risk factor for musculoskeletal symptoms

of the neck and upper limbs [5,9,12,62]. When reviewing

the literature, there seems to be more evidence for an

association between the duration of VDU work and

musculoskeletal disorders in the forearm and hand

compared to associations between the duration of VDU

work and symptoms/disorders in the neck/shoulder

region [47,63,64].

Blangsted et al. [65] studied VDU users in a

department at a municipal administration and reported

the mean hourly number of mouse clicks to be 230 and

the mean hourly number of keystrokes to be 1960. In a

subgroup of the NUDATA-cohort, the keying speed in

the 75th percentile was measured to be from 8000

to 22 000 keystrokes/h [53]. This indicates a large

between-subject variability in keying speed and number

of keystrokes performed. It is also reasonable to believe

that the number of mouse clicks varies to a large extent,

depending on occupation, work task and the software

used to carry out the work task. The combination of high

repetitiveness in the fingers and wrist, the static loading

imposed on the thumb to grip the mouse, the prolonged

extension and ulnar deviation of the wrist and the long

duration may all be contributing to the development of

musculoskeletal symptoms in the forearm and hand/

wrist. Several studies have also found an increased risk for

hand/wrist symptoms among individuals with long daily

duration of VDU and computer mouse use [5,9,47,63].

Psychosocial factors and mental stress

A wide range of different instruments have been used to

assess psychosocial factors in the work environment, one

of the most widely used being the demand–control model

developed by Karasek and Theorell [66]. The most

common way of assessing psychosocial factors has been

through use of questionnaires (i.e. self-judgements). A

number of different psychosocial factors have been

proposed as risk factors for musculoskeletal symptoms

in the neck/shoulder region, for example: high job

demands, low decision latitude, time pressure, mental

stress, job dissatisfaction, high workload and lack of

social support from colleagues and superiors [9–12,

67–78]. Several theoretical models of how psychosocial

factors are associated with musculoskeletal symptoms

and disorders have been proposed [10,29,31,71,79–82];

for an overview, see Huang et al. [83]. Several of the

models suggest that adverse psychosocial factors cause

mental stress, which is hypothesized to increase the risk of

musculoskeletal symptoms.

The terminology regarding the word ‘stress’ has not

always been used consistently by the different research

traditions within the field of ergonomics (e.g. psychology

and biomechanics). Here, a ‘stressor’ is a factor or

condition causing a physiological or psychological

response. The definition of ‘stress’ is, therefore, that it

is a non-specific response to a stressor, physical or

psychological/ mental, consisting of several physiological

and/or psychological reactions.

Individual factors

Sex

In almost all scientific studies of work-related musculo-

skeletal disorders, women are found to be at higher risk

than men, regardless of the kind of work or occupation

involved. The same difference exists between women and

men regarding VDU users [3,5,6,9,12,62,84]. In the

study by Ekman et al. [84], in which the aim was to

investigate possible differences between women and men

170 OCCUPATIONAL MEDICINE

in the reporting of musculoskeletal symptoms among

VDU users in the Swedish workforce, the estimated odds

ratio for sex (women/men) was 11.9 (95% confidence

interval [95% CI] 2.9–50.0). Two explanations for this

increased risk for women discussed by the authors were

that sex could be a confounder of non work-related

factors and that there could be a difference in the

occupational exposure among men and women [84]. In

a review of epidemiological findings on VDU work and

musculoskeletal symptoms, Punnett and Bergqvist [9]

stated that women appear to consistently report more

neck and upper extremity symptoms than men. No

definite explanations were found in the reviewed studies,

but differences in household work and childcare, work

situation differences and constitutional differences

were mentioned as possibilities. In a recent review,

Tittiranonda and colleagues [12] suggested that differ-

ences in anthropometrics may cause women to work in

more extreme postures or using higher relative muscle

forces than men. In a cross-sectional study of Swedish

VDU users, women reported more symptoms in all body

regions than men and were more often exposed to

physical and psychosocial conditions that have been

considered harmful [62].

Working technique

Differences in working technique when performing VDU

work have been observed, both in experimental studies

[43–45] and in field studies [85]. It has been suggested

that individuals with a poor working technique (assessed

with an observational checklist) during VDU work, work

with higher muscle activity in the forearm and shoulder

(trapezius) and their wrist more extended [85].

A concept somewhat similar to working technique is

workstyle, which has been conceptualized as a multi-

dimensional (i.e. behavioural, cognitive and physio-

logical) stress response to work [86]. Wrist postures,

finger movements, speed/jerkiness of movements and

force applied while keying are examples of variables

included in this construct. Previous research on workstyle

has indicated that various dimensions of the construct are

associated with pain, symptom severity and functional

limitations [58,87].

A model of musculoskeletal disorders andcomputer work

Amodel of musculoskeletal disorders and computer work

is proposed (Figure 1), the model is modified from Sauter

and Swanson [81].

Work technology (‘VDU/Office technology’) has a

direct path to physical demands, as defined by the

physical coupling between the worker and the tool (i.e.

workstation ergonomics). There is also a direct path from

work technology to work organization. The path from

work organization to physical demands suggests that the

physical demands from work can be influenced by work

organization. Increased time pressure leads to an

increased number of keystrokes or implementation of

new software leads to increased computer mouse use,

which in turn may increase the physical load [88,89].

Individual factors are hypothesized to modify the

association between physical demands and physical

load. It has been observed that individual factors such

as working technique and sex may affect the physical load

[43–45,85]. Individual factors are also hypothesized to

modify the association between work organization and

mental stress. The model also shows a path from work

organization to mental stress, which in turn influences

musculoskeletal outcomes. Mental stress may increase

muscle activity [38,89–94], but also the forces applied to

the computer mouse [89], which compounds physical

load induced by physical demands. Mental stress has also

been hypothesized to moderate the relationship between

physical load and musculoskeletal outcomes. A difference

between the present model and the original model

proposed by Sauter and Swanson [81] is the direct path

from mental stress to perceived muscular tension. This

association is supported by recent research that has

observed mental stress and psychosocial factors to be

independent risk factors for neck pain [68,78]. The

reason for having a direct path from mental stress to

musculoskeletal outcomes, not mediated through physi-

cal load, is that the mechanisms behind unspecific

musculoskeletal symptoms are not well known. Physical

load has a direct path to perceived muscular tension, and

this association is also supported by recent findings [95].

Perceived muscular tension is hypothesized to be an early

sign (cf. ‘detect sensation’ in the original model by Sauter

and Swanson) of musculoskeletal symptoms, which has

also been observed in a prospective study of VDU users

[96]. Other factors hypothesized to be early signs of

musculoskeletal symptoms are perceptions of comfort

and exertion. Finally, it is hypothesized that the

experience of musculoskeletal disorders feeds back to

influence mental stress at work and the work

organization.

Neither the model proposed here nor the original

model by Sauter and Swanson is complete. None of the

models takes environmental factors outside of work into

account, for example home life factors that could modify

perceptions of mental stress. Productivity is an important

outcome that is not accounted for in the model and it is

not certain that the factors associated with musculo-

skeletal symptoms are the same as those associated with

decreased productivity. Decreased productivity due to

musculoskeletal symptoms among VDU users has been

observed [97]. Further research is needed to establish

which factors cause perceived muscular tension and other


possible sensations, such as perceptions of exertion and

comfort. The aetiology behind unspecific musculo-

skeletal symptoms in the neck/shoulder and forearm

region is unknown and there is a need for more

knowledge regarding the specific mechanisms causing

pain in these regions. An excess risk due to the interaction

between physical and psychosocial exposure during VDU

work has recently been indicated [96] and that is an

interesting issue for future research.

New intervention strategies focusing on the individual

may be an alternative approach in occupations charac-

terized by intense VDU work, which would entail low

force requirements but high physical exposure with

regard to precision and repetitive demands. This issue

has been addressed by Holte et al. [98] and Holte and

Westgaard [98,99], who pointed out that there may be a

need to assess factors that focus more on the individual

translation of the exposure into an individual response

than on the more traditional risk factors such as job strain

and physical exposure. Another factor to consider in

intervention strategies is working technique in combi-

nation with information about the importance of work-

station layout and psychosocial risk factors. Ketola et al.

[100] investigated the effect of an intensive ergonomics

approach and education on workstation changes and

musculoskeletal disorders among VDU users. Their

study was a randomized controlled trial and the subjects

were allocated within three different groups, an intensive

ergonomics, an ergonomic education and a reference

group. After 2 months of follow-up, less discomfort was

reported by the intensive ergonomics and the ergonomic

education groups than by the reference group. The

authors concluded that cooperative planning in which

both employees and practitioners are actively involved

(i.e. participatory ergonomics) will achieve the best

results when attempts are made to improve physical

ergonomics of VDU workstations. Other intervention

studies, though not as well designed as the study by

Ketola and colleagues [100], have reported similar results

for ergonomic improvements [46,101,102]. It has also

been suggested that office ergonomics programmes may

be effective in reducing worker compensation costs and

injury rates due to musculoskeletal disorders [103].

Interventions conducted by the occupational health

services could also focus on more than one factor in the

proposed model. For example, an intervention could

comprise optimization of the workplace layout (modify-

ing the physical demands) in combination with a feedback

survey of the psychosocial work environment (modifying

the psychosocial factors) and individual training focusing

on working technique (modifying the individual factors).

This approach would not add any scientific knowledge,

other than knowledge about whether the intervention

could decrease musculoskeletal symptoms or decrease

exposure, but would be a way for the occupational health

service to design effective interventions based upon the

existing scientific knowledge. Following the suggestions

from a recent review [104], this should be carried out by

actively involving the individual worker. Management

support and active involvement have also been pointed

out as important factors to address when designing

interventions [104,105].

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Figure 1. Amodel of musculoskeletal disorders and computer work, modified from Sauter and Swanson [81]. Broken lines denote modifying effects.


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