Musculoskeletal symptoms and duration of computer and mouse use

International Journal of Industrial Ergonomics 30 (2002) 265–275

Musculoskeletal symptoms and durationof computer and mouse use

Chris Jensen*, Lotte Finsen, Karen S�gaard, Hanne ChristensenDepartment of Research in Computer Work, National Institute of Occupational Health,

Lers� Parkall!e 105, DK-2100 Copenhagen, Denmark

Received 15 July 2001; received in revised form 1 February 2002; accepted 20 April 2002

Abstract

The primary aim was to study associations between duration of computer and mouse use and musculoskeletal

symptoms among computer users. A questionnaire was delivered to 5033 employees in 11 Danish companies and

institutions and 3475 subjects responded (69%). Logistic regression analyses on 2579 full-time working employees

showed that working almost the whole working day with a computer was associated with neck symptoms (OR=1.92,

CI: 1.21–3.02) and shoulder symptoms (OR=1.83, CI: 1.13–2.95) among women and hand symptoms (OR=2.76, CI:

1.51–5.06) among men. These odds ratios were adjusted for psychosocial factors. Among respondents working almost

all of their work time with a computer the gender and age-adjusted odds ratio for mouse use more than half of the work

time was 1.68 (CI: 1.22–2.31) for hand/wrist symptoms. Call center and data entry workers experienced the lowest

possibilities for development at work. All work tasks involving computer use, except computer maintenance, were

characterised by a higher frequency of movements than desk work without using a computer. Call center work was

characterised by the highest level of repetitiveness as both work tasks and movements were perceived as repetitive.

Relevance to industry

Computer users with a long daily duration of computer use and mouse use experienced more musculoskeletal

symptoms than those with a short duration of computer use. Computer work in general seemed characterised by

repetitive movements, which may be a risk factor for musculoskeletal symptoms. r 2002 Elsevier Science B.V. All

rights reserved.

Keywords: VDU; Musculoskeletal symptoms; Computer work; Computer mouse; Repetitiveness

1. Introduction

The aims of the present paper were to reportassociations between duration of computer use,computer mouse use and symptoms in the neck,shoulders and hands and to investigate inherent

differences between specific computer-based worktasks with respect to physical and psychosocialexposures. These exposure factors were quantita-tive job demands (e.g. time pressure), possibilitiesfor development of skills and repetitiveness ofmovements and tasks, which all were associatedwith symptoms as reported in an accompanyingpaper (Jensen et al. in press). If the exposure torisk factors depends in a predictable way on the

*Corresponding author.

E-mail address: [email protected] (C. Jensen).

0169-8141/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 9 - 8 1 4 1 ( 0 2 ) 0 0 1 3 0 - 0

type of work task performed at the compu-ter, recommendations may be given regardingwork organisational changes at the work place,which could reduce the risk of developing symp-toms.In Denmark 23% of all employees reported to

use a computer as part of their work in 1990,whereas this number has increased to 60% in 1999(Burr, 2000). During the same period the percen-tage of employees using a computer at work for atleast 3/4 of the work time has increased from 4%to 19%. Punnett and Bergqvist (1997) andTittiranonda et al. (1999) reviewed the literatureon studies of computer work and its associationwith musculoskeletal symptoms and found someevidence that continuous computer use for theentire work day and repetitive computer worksuch as data entry were associated with anincreased risk of developing symptoms. Bernardet al. (1994) found a significant associationbetween hours of computer use and hand/wristdisorders among newspaper employees with anodds ratio of 3.3 for those working at least 8 h perday compared to those using a computer for lessthan 2 h. Karlqvist et al. (1996) reported anincreased prevalence of symptoms among compu-ter-aided design (CAD) operators using a compu-ter for more than 5.6 h per day compared to thoseusing a computer for less than 5.6 h. In an earlierstudy we also found a higher frequency of CADoperators (using a computer for at least 5 h perday) reporting symptoms in the neck, shouldersand hands than among the general population ofemployees in Denmark (Jensen et al., 1998).However, a detailed analysis of the relationshipbetween computer work and the risk of developingsymptoms in a more general population ofcomputer users is needed in order to baseinterventions on solid scientific knowledge. Forinstance, while data entry work is primarilyperformed with a keyboard, computer mice areused intensively for CAD work and in Scandinaviamuch focus has been directed towards the possibleharmful effects of mouse use. Furthermore,potential psychosocial risk factors such as highdemands, low influence or low possibilities fordevelopment at work may be present for somework tasks, but not for others.

We initiated the BIT study (Behaviour inInformation Technology) to analyse relationshipsbetween a wide range of self-reported work-relatedexposure factors and musculoskeletal symptomsamong a population of computer users withdifferent types of computer work. The respondentswill later be asked to answer an identical set ofquestions on musculoskeletal symptoms to studyrisk factors for the development of symptoms.However, data from the baseline survey arereported in the present paper, as some clearassociations between symptoms and duration ofcomputer work emerged from these analyses whichmay form a basis for prevention and interventionstrategies in many different jobs, where computerwork is performed intensively. The study is basedon a population of employees who have beenexposed to computer work for more than 10 yearson average.

2. Methods

2.1. Subjects

A questionnaire was delivered to 5033 employ-ees in 11 Danish companies and institutions with awide range of work tasks. The response rate was69% (n ¼ 3475). Both public administration, aresearch institution and private companies parti-cipated. All employees working in the entirecompany or in specific departments were includedexcept cleaning and kitchen personnel. Names ofemployees were delivered by the company beforethe questionnaire was delivered at the workplaceor at the home address. The questionnaires werereturned by mail or returned in sealed envelopes attheir workplace and later collected by a represen-tative from the Danish National Institute ofOccupational Health. Employees, who did notrespond to the first questionnaire, were given asecond identical questionnaire and two writtenreminders by company representatives.

2.2. Questionnaire

The diagram in Fig. 1 shows the factors thatwere investigated in the full study. The choice of

C. Jensen et al. / International Journal of Industrial Ergonomics 30 (2002) 265–275266

factors was based on existing scientific literatureand discussions with a group of union representa-tives from unions with a particular interest inpresumed computer work-related health problems.The major focus of the study was directed towardsthe duration of computer use and its possiblerelationship with symptoms. However, also thetype and duration of input device use and specificwork tasks performed at the computer (data entry,word processing, data processing, call center tasks,programming, graphics, multimedia editing, com-puter maintenance, information retrieval) couldinfluence the development of musculoskeletalsymptoms. The relationship between musculoske-letal symptoms and the duration of computer use,input device use and work tasks could beinfluenced by more general exposure and indivi-dual factors, of which some are known to be riskfactors for the development of symptoms. Some ofthese exposure factors would be more likely to bereflected by the type of work task performed ortype of input device used. For instance, data entrymay be more repetitive and be characterised by

less possibilities for development at work thansoftware programming. Here, three general workenvironment factors were specifically addressed toinvestigate their association with specific worktasks: repetitiveness, quantitative demands andpossibilities for development at work. Otherfactors, which were not of primary interest in thepresent study, were assumed to have influence onthe development of symptoms, but they were notsuggested to be associated with specific work tasksor input devices. These factors included (1) socialsupport and managerial responsibilities, (2) workstation design (e.g. possibility for arm support anddisturbances by light reflections on the computerscreen) and (3) individual factors (age, gender,computer training and skills). The associationsbetween these latter factors and symptoms werereported in the accompanying paper (Jensen et al.,in press).The present paper specifically addresses expo-

sures related to computer work: duration ofcomputer use and duration of computer mouseuse, which were quantified as a fraction of totalwork time (never, seldom, 1/4, 1/2, 3/4 and almostall of the work time). As the main focus wasdirected towards the association between muscu-loskeletal symptoms and the duration of computerwork, several models are reported. Firstly, theanalyses were stratified by gender. Secondly, boththe crude associations and associations adjustedfor age and psychosocial factors are reported. Thepsychosocial factors were: Quantitative demands(work pace, distribution of work loads, timepressure, ability to keep up with deadlines andthe need to do overtime work), sensorial demands(visual demands, precision, attentiveness), cogni-tive demands (decision making, memory demands,creativity, responsibilities), influence at work(influence at what, how and when you do yourwork, influence on quality of work and who youwork with), possibilities for development (skilldemands, development of skills, demands oftaking initiatives, lack of monotony) and socialsupport at the workplace (from colleagues andsuperiors). These factors were constructed from anumber of different questions, each with fiveresponse categories (never, seldom, sometimes,often, always), and combined to indices from

Musculoskeletal symptoms

Repetitiveness

Demands: Quantitative Sensorial Cognitive

Influence

Possibilities for development

Input device

Work station design

Work tasks

Social support

Duration of computerwork

Individual factors

Fig. 1. Diagram showing the possible relationship between

exposure factors and musculoskeletal symptoms. The focus is

towards the duration of computer use and its relationship with

symptoms, which may be affected by several other factors. It

was hypothesised that the specific work tasks performed at the

computer (data entry, word processing, data processing, call

center tasks, programming, graphics, multimedia editing,

computer maintenance, information retrieval) influenced the

development of symptoms due to more general exposure factors

which differ systematically between some of the tasks.

C. Jensen et al. / International Journal of Industrial Ergonomics 30 (2002) 265–275 267

0 to 100 for each dimension (Kristensen et al., inpress). Cronbachs alpha for the different indiceswere higher than 0.70 (Kristensen et al., in press).Furthermore, the association between specific

work tasks and levels of ‘‘repetitiveness’’, ‘‘quan-titative demands’’ and ‘‘possibilities for develop-ment’’ was reported. These factors were chosen onthe basis of results reported in the accompanyingpaper (Jensen et al. in press).‘‘Repetitiveness’’ was defined as follows: respon-

dents with similar arm/hand/finger movementsseveral times per minute for at least 3/4 of thework day were defined to have repetitive move-ments. Respondents reporting that they performthe same task several times per hour for at least 3/4of the work day and in addition that their workseldom or never is varied were defined as havingrepetitive work tasks. Questions on musculoskele-tal symptoms were given according to a modifiedversion of the Nordic questionnaire for the neck,shoulder and hand/wrist regions (Kuorinka et al.,1987).

2.3. Statistics

Only employees working full time (32–41 h perweek) were included in the analyses of associationsbetween exposure factors and symptoms(N ¼ 2579). Health outcome in the present ana-lyses was defined as musculoskeletal symptoms forat least 8 days within the last year.Initially, associations between symptoms and

the duration of computer use were studied. Inlogistic regression analyses for each of three bodyregions: neck, one or both shoulders and one orboth hands/wrists, the factor ‘‘duration of com-puter work’’ was initially included in the models.In a second set of models the association betweenduration of computer work and symptoms wasadjusted for age and the psychosocial factors.As duration of mouse use is highly correlated

with duration of computer use, the associationbetween mouse use and symptoms was studied inthe subgroup of respondents who worked all oralmost all of their work time at a computer. Agroup of mainly female respondents employed at acall center used a computer for the entire worktime, but they only used a keyboard as input

device and constituted a large part of the subjectsin the reference group in these analyses. Thus, theassociations between mouse use and symptomscould be biased by the call center employees andthe analyses were repeated with this particulargroup excluded from the reference group.Specific work tasks were combined with re-

ported exposure to ‘‘quantitative job demands’’,‘‘possibilities for development’’ and ‘‘repetitivenessof movements and tasks’’. The work tasks weredata entry, word processing, data processing, callcenter tasks, programming, graphics, multimediaediting, computer maintenance, information re-trieval and desk work without a computer. Thus,for each task the mean index and confidenceinterval of the mean index of ‘‘quantitativedemands’’ and ‘‘possibilities for development’’were calculated among those respondents perform-ing the specific work task for at least 50% of thework time. For the same group of respondents thepercentage reporting repetitive movements andtasks was calculated for each work task.Symptom prevalences of female respondents

performing call center work for at least half ofthe work time were compared to female computerusers performing any other specific work tasks atthe computer for at least half of the work time.The procedures FREQ, MEAN and GENMOD

in SAS (version 6.02) were used. The likelihoodratio test probability was calculated for each factorin the logistic regression models with a significancelevel of po0:05:

3. Results

Two-thirds of the respondents were women(2310 women and 1165 men). About 75% of therespondents worked full time (32–41 h per week)and most of them had used a computer at work forseveral years (Table 1). The percentages of full-time working respondents, who used a computerfor 0–1/4, 1/2, 3/4 and 1/1 of the work time were12%, 17%, 24% and 47%, respectively. Themouse was used never or seldom, 1/4 and 1/2–1/1 of the work time by 40%, 32% and 28% of therespondents, respectively. To reveal any trend ofselective response from employees with symptoms,


the respondents who returned the questionnaireafter having received it the first time werecompared to those, who returned the question-naire after one or two reminders. The reportedprevalence of symptoms for more than 7 dayswithin the last year was about the same for those,who responded quickly, as for those respondinglater, e.g. for neck symptoms the prevalencesamong fast responders were 53% and 26% forwomen and men, respectively, whereas for slowresponders the prevalences were 51% and 31%,respectively.Women reported musculoskeletal symptoms

twice as frequently as men for all regions (Table2). The duration of computer use predicted necksymptoms among women (Table 3), whereas formen this factor was not significantly associatedwith neck symptoms (Table 4). For the shoulderthe duration of computer use was significantlyassociated with symptoms for women. For menthere was a similar (non-significant) trend of ahigher prevalence of symptoms among respon-dents with a long duration as compared to a shortduration of computer use. For the hand/wristthe duration of computer use was significantly

associated with symptoms for men, but not forwomen.Analyses on the association between mouse use

and symptoms among 1137 respondents workingalmost all of their work time at a computer showedthat mouse use for at least half of the work time ascompared to intensive computer use without amouse was significantly associated with symptomsin the hand/wrist (Table 5). This was not the casefor the neck and shoulder regions.Among the intensive computer users a large

group of mainly female respondents with callcenter work tasks used only a keyboard as inputdevice. Respondents with other work tasks in mostcases used a computer mouse in addition to thekeyboard. The association between mouse use andsymptoms could be biased by a high prevalence ofsymptoms among the call center employees com-pared to other computer users, who seldom use amouse, and further analyses were undertakenexcluding the call center respondents. Among theother 513 respondents with computer use for theentire work time the age-adjusted odds ratios forhand/wrist symptoms were much higher thanthose shown in Table 5 (OR was 2.38 (1.24–4.58)for mouse use 1/4 of the work time and OR was3.09 (1.67–5.74) for mouse use for at least 1/2 ofthe work time). The odds ratio for females versusmales was 1.38 (0.90–2.14) in this model. Theassociation between shoulder symptoms andmouse use was significant (p ¼ 0:019) in the modelexcluding call center employees. The age-adjustedodds ratios for shoulder symptoms of intensivecomputer users (excluding call center employees)with mouse use for 1/4 of the work time and for at

Table 1

Characteristics of respondents

Variable Categories Women Men

N (%) N (%)

Age 18–29 yr 318 14 149 13

30–39 yr 610 27 279 25

40–49 yr 633 29 341 31

>49 yr 655 30 348 31

Seniority o1 yr 164 7 64 6

1–3 yr 536 24 284 25

4–7 yr 255 12 175 15

8–12 yr 497 22 214 19

>12 yr 781 35 398 35

Weekly work hours 0–31 h 355 16 83 7

32–41 h 1745 77 834 73

>41h 158 7 227 20

Years using a computer 0–3 yr 286 13 151 13

4–7 yr 374 17 190 17

8–12 yr 765 35 380 34

>12 yr 769 35 400 36

Table 2

Prevalence of musculoskeletal symptoms for respondents

working full-time (32–41 h per week)

Variable Women Men

N (%) N (%)

Neck symptoms >7 days last

year (%)

906 53 221 27

Shoulder symptoms >7 days

last year (%)

716 42 185 23

Hand/wrist symptoms >7 days

last year (%)

522 30 151 19


least 1/2 of the work time were 1.69 (0.96–2.97)and 2.10 (1.24–3.56). The odds ratio for femalesversus males was 2.58 (1.69–3.95) in this model.For neck symptoms the associations with duration

of mouse use differed markedly between gendersand logistic regression models were thereforeperformed separately for men and women. Forwomen (n ¼ 333) the association was almost

Table 3

Duration of computer use in logistic regression models for women working full-time (32–41h per week). Models without (model 1) and

with adjustment for psychosocial factors (model 2) are shown

Region Categories Model 1 Model 2

Factor OR (95% CI) p OR (95% CI) p

Neck

Work time at computer 0–1/4 1.00 o0.001 1.00 0.028

1/2 1.55 (0.97–2.47) 1.40 (0.86–2.28)

3/4 1.76 (1.13–2.74) 1.54 (0.97–2.45)

1/1 2.17 (1.43–3.29) 1.92 (1.21–3.02)

Shoulder

Work time at computer 0–1/4 1.00 o0.001 1.00 0.051

1/2 1.43 (0.88–2.32) 1.40 (0.84–2.34)

3/4 1.47 (0.92–2.35) 1.45 (0.88–2.36)

1/1 2.00 (1.29–3.11) 1.83 (1.13–2.95)

Hand/wrist

Work time at computer 0–1/4 1.00 0.127 1.00 0.244

1/2 1.76 (1.04–2.99) 1.56 (0.89–2.73)

3/4 1.37 (0.82–2.30) 1.26 (0.73–2.17)

1/1 1.58 (0.97–2.56) 1.53 (0.90–2.60)

Table 4

Duration of computer use in logistic regression models for men working full-time (32–41 h per week). Models without (model 1) and

with adjustment for psychosocial factors (model 2) are shown

Region Categories Model 1 Model 2

Factor OR (95% CI) p OR (95% CI) p

Neck


1/2 1.50 (0.94–2.40) 1.50 (0.91–2.49)

3/4 1.50 (0.95–2.37) 1.28 (0.79–2.49)

1/1 1.61 (1.02–2.52) 1.18 (0.70–2.00)

Shoulder


1/2 1.10 (0.66–1.84) 1.12 (0.65–1.92)

3/4 1.42 (0.88–2.29) 1.40 (0.85–2.32)

1/1 1.62 (1.01–2.60) 1.67 (0.98–2.87)

Hand/wrist


1/2 1.41 (0.79–2.52) 1.58 (0.85–2.92)

3/4 1.81 (1.05–3.11) 2.09 (1.17–3.72)

1/1 2.29 (1.35–3.88) 2.76 (1.51–5.06)


significant (p ¼ 0:098), and the odds ratios forneck symptoms were 1.70 (0.93–3.13) and 1.82(1.04–3.21) for mouse use for 1/4 of the work time

and for at least 1/2 of the work time, respectively.For men the odds ratios for neck symptoms wereclose to 1 for all durations of mouse use.Indices of quantitative job demands, possibili-

ties for development at work and the distributionof respondents with repetitive movements andwork tasks were compared between subjects withdifferent work tasks for at least half of their worktime. Subjects performing desk work without acomputer for at least half of the work time werealso included (Figs. 2–4). Call center work andgraphical work were characterised by lowerquantitative demands than the other work tasksat the computer, whereas call center workers anddata entry workers experienced the lowest possi-bilities for development at work. All work tasks atthe computer, except computer maintenance, werecharacterised by more repetitive movements thandesk work without using a computer. Call centerwork was characterised by the highest level ofrepetitiveness as both work tasks and movementswere perceived as repetitive by a large fraction ofthe respondents performing this work task.Age-adjusted odds ratios were calculated for

symptoms among female respondents performingcall center work for at least half of the work timecompared to female computer users performing

Table 5

Association between mouse use and symptoms among full-time

working respondents, who use a computer almost all their

worktime (n=1137)

Region Category Odds ratio 95% conf. int. p

Neck

Gender Male 1.00 o0.001Female 3.37 2.38–4.77

Mouse use Seldom 1.00 0.300

1/4 1.32 0.92–1.88

1/2–1/1 1.12 0.83–1.52

Shoulder

Gender Male 1.00 o0.001Female 2.79 1.95–4.01


1/4 1.22 0.85–1.74

1/2–1/1 1.36 1.00–1.84

Hand/wrist

Gender Male 1.00 0.011

Female 1.61 1.11–2.34


1/4 1.30 0.88–1.90

1/2–1/1 1.68 1.22–2.31

0

10

20

30

40

50

60

70

80

90

100

call-center, n=629

data-entry, n=448

word-processing, n=394

data-processing, n=137

graphics, n=125

information

retrieval, n=95

programm

ing, n=68

multim

edia, n=21

maintenance, n=19

desk work without computer, n=214

Ind

ex-

qu

anti

tati

ved

eman

ds

Fig. 2. Mean level of ‘‘quantitative demands’’ for all computer users reporting to perform a specific task for at least half of their work

time. Error bars show confidence interval of the mean. The scale for the index ranged from 0 to 100 and was calculated on the basis of

responses to a set of specific questions regarding time pressure in relation to work load.


0

10

20

30

40

50

60

70

80

90

100

call-center, n=629

data-entry, n=448

word-processing, n=394

data-processing, n=137

graphics, n=125

information

retrieval, n=95

programm

ing, n=68

multim

edia, n=21

maintenance, n=19

desk work without computer, n=214

Ind

ex-

po

ssib

iliti

esfo

rd

evel

op

men

t

Fig. 3. Mean level of ‘‘possibilities for development’’ for all computer users reporting to perform a specific task for at least half of their

work time. Error bars show confidence interval of the mean. The scale for the index ranged from 0 to 100 and was calculated on the

basis of responses to a set of specific questions on their possibilities of using and developing their skills at work.

0

20

40

60

80

100

call center, n = 629

data entry, n = 446

word-processing, n = 395

data-processing, n = 137

graphics, n = 127

information retrieval, n = 95

programm

ing, n = 68

multim

edia, n = 21

maintenance, n = 20

desk work without computer, n = 213

Res

pond

ents

(%

)

Repetitive movements, no repetitive tasksRepetitive movements and tasks Varied work

Fig. 4. Percentage of respondents with repetitive movements and tasks among those computer users who reported to perform a specific

task for at least half of their work time. ‘‘Repetitiveness’’ was defined as follows: respondents with similar arm/hand/finger movements

several times per minute for at least 3/4 of the work day were defined to have repetitive movements. Respondents reporting that they

perform the same task several times per hour for at least 3/4 of the work day and in addition that their work seldom or never is varied

were defined as having repetitive work tasks.


any of the other specific work tasks at thecomputer for at least half of the work time. Thus,the comparison group was formed from all of thecomputer users shown in Fig. 2 except the callcenter workers. The analyses were restricted tothose subjects, who used a computer for at least3/4 of the work time and seldom or never used amouse (n ¼ 368). For call center employees theodds ratio for neck symptoms was 1.59 (0.98–2.60), for shoulder symptoms the odds ratio was2.06 (1.19–3.56) and for hand/wrist symptoms theodds ratio was 1.95 (1.06–3.61). Due to the smallnumber of male call center employees a similaranalysis was not performed for men.

4. Discussion

The duration of computer work was positivelyassociated with symptoms in the neck amongwomen, for shoulder symptoms the associationwas strongest for women, but for hand/wristsymptoms there was an association among menonly. The associations persisted after adjusting forthe effect of psychosocial factors. This indicatedthat differences in psychosocial exposures, such associal support or possibilities for development,between respondents using a computer for a smallor a large fraction of their work time could notexplain the higher prevalence of symptoms amongthe intensive computer users. Mouse use amongintensive computer users was associated withsymptoms in the hand/wrist and shoulder regions.The lack of association between computer use andhand/wrist symptoms among the women maypartly be explained by the group of call centeremployees working full time at the computerwithout using a mouse. When mouse use influencesthe prevalence of hand/wrist symptoms, but isunevenly distributed in relation to the distributionof computer use, the association between compu-ter use and hand/wrist symptoms may have beendiluted. Thus, the duration of computer useappears critical for the reporting of musculoskele-tal symptoms, but a further increase in hand/wristand shoulder symptom prevalences may be due tointensive mouse use.

A causal relationship between exposure andhealth effects cannot be established in a cross-sectional study. As longitudinal studies on healthproblems associated with computer work arelacking, the use of cross-sectional data to dealwith the working environment of the rapidlygrowing number of computer users seemed justi-fied. Other methodological weaknesses includeproblems with the validity of exposure assessmentbased on self-reports (Wiktorin et al., 1993;Viikari-Juntura et al. 1996) and also report bias,which could artificially increase the magnitude ofthe relationship between duration and symptoms ifintensive computer users without symptoms wereless likely to participate. The response rate of 69%cannot preclude a significant report bias, but acomparison of fast and slow responders did notreveal such a trend. In contrast, a healthy workereffect due to the allocation of less harmful tasks toworkers with severe symptoms or a complete jobchange would reduce the magnitude of theassociation between exposure and symptoms.While the respondents reported to have manyyears of experience in computer work, it was notknown how the duration of computer use hadchanged during the years preceeding the study.In contrast to the present findings Bergqvist et al.

(1995a) found no simple association betweencomputer use and musculoskeletal symptomswhen comparing computer users with non-com-puter users. Others have reported findings similarto ours. Bernard et al. (1994) reported an adjustedodds ratio of 2.1 for hand or wrist disordersamong computer users with 6–8 h of typing perday and Polanyi et al. (1997) found an odds ratioof 1.6 for musculoskeletal symptoms amongcomputer users spending 5 h keying per daycompared to 1.5 h per day. Only few studies havefocused on mouse use, except Karlqvist et al.(1996) and Jensen et al. (1998), who found anexcessive prevalence of musculoskeletal symptomsamong intensive CAD operators. In a recent studyBlatter and Bongers (submitted) also reported anassociation between musculoskeletal symptomsand duration of computer use, which was strongerfor women than men. Their odds ratios for neckand shoulder symptoms increased significantlywith the duration of computer use for women


but not for men, which is similar to our results.However, for elbow arm or hand/wrist disorderstheir results diverged from ours, as they found noassociation with duration of computer use formen. Thus, both their and our study suggestedclear differences between women and men. Thismay partly be explained by differences in worktasks and work techniques between genders(Karlqvist et al., 1999; Wahlstr .om et al., 2000).It may also be speculated that the weak, non-significant, association between elbow, arm orhand/wrist symptoms and mouse use reported byBlatter and Bongers (submitted) was due todifferent work tasks performed by their popula-tion of computer users as compared to ourpopulation. We showed that the exclusion of callcenter workers led to a considerably strongerassociation between mouse use and hand/wristsymptoms. The increased strength of the associa-tion when excluding call center workers was due tothe relatively high prevalence of symptoms amongthe call center employees compared to otherintensive computer users without mouse use.The higher prevalence of symptoms among call

center employees was probably due to otherpsychosocial and/or physical factors. Their worktask differed markedly from the work tasks ofother computer users, as call center work wasperceived as more repetitive and was characterisedby lower possibilities for development. Among thevery heterogeneous group of workers using acomputer at work special attention should be paidto the growing number of people performing veryspecialised work such as call center work, as theymay be exposed to more adverse working condi-tions than other computer users.Other work tasks, such as multimedia editing

and computer maintenance were characterised byhigh quantitative demands. While this may beassociated with an increased prevalence of muscu-loskeletal symptoms, other factors such as possi-bilities for development at work and job controlwas relatively high for most work tasks performedby computer users. This may enable computerusers to cope with high job demands, even thoughit is not clearly established how the interactionbetween physical and psychosocial exposuresaffects the risk of developing musculoskeletal

disorders among computer users (Jensen et al., inpress; Bergqvist et al., 1995b). Almost all compu-ter tasks were characterised by repetitive move-ments compared to non-computer based deskwork. Therefore, it seems difficult to create largevariations in the exposure to this potential riskfactor for developing musculoskeletal symptomsby performing different computer-based worktasks, such as data entry, word-processing or dataprocessing. However, as the psychosocial settingdiffers considerably from that experienced byindustrial workers performing highly repetitivework, further efforts should by taken to studythe importance of repetitiveness as a risk factoramong computer users.

Acknowledgements

We thank Ebbe Villadsen for assistance on thepreparation of the SAS-datafile and HeleneFeveile and Kim Mikkelsen, Ph.D., for advice onanalysis strategies and data interpretation.

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Documents

Musculoskeletal symptoms and duration of computer and mouse use