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Alterations in shoulder muscle activity due to changes in data entry organisation
International Journal of Industrial Ergonomics 23 (1999) 231 — 240
Alterations in shoulder muscle activity due to changes in data entry organisation Elisabeth A.C. Fernstro¨m *, Carl M. A> borg Futura, Box 16408, S-10327 Stockholm, Sweden Department of Environmental Technology and Work Science, Royal Institute of Technology, Stockholm, Sweden Futura, S.t Olofsgatan 9, S-753 21 Uppsala, Sweden Department of Human Work Science, Lulea University, Lulea , Sweden Received 29 January 1997; accepted 18 April 1997
Abstract The aim of the present study was to investigate how changed work organisation and different work tasks influence shoulder muscular load and to quantify the magnitude, duration and frequency of rest periods from shoulder muscular load during a working day. Shoulder muscular load was measured in 22 females working at their data entry workplaces, during a whole working day. The activity from both trapezius muscles was measured with EMG before (1991) and after (1992) a reorganisation programme intended to redistribute repetitive work and provide new work tasks. The change in work organisation did not change the magnitude of muscular load or the duration and frequency of rest periods, but decreased musculoskeletal problems. The subjects’ increased desk work involved greater muscular load than the data entry did, but also allowed more movement. The changes in work tasks seemed to be important, although small. In repetitive work, organisational changes aimed at reducing musculoskeletal disorders should focus on providing employees with tasks that afford variation in muscular load. Relevance to industry The paper discusses the need of physical work task variation in repetitive work in order to minimise the risk of musculoskeletal disorders. It seems more important to vary the tasks than to minimise the shoulder muscular load. 1999 Elsevier Science B.V. All rights reserved. Keywords: VDU; Data entry; Electromyography; Muscular load; Neck; Disorder
1. Introduction Office work often involves sedentary VDU (video display unit) work, which increases the risk of * Correspondence address: Futura, Box 16 408, S-103 27 Stockholm, Sweden. Tel.: #46 10 683 3568; fax: #46 821 1391.
developing musculoskeletal complaints and diseases. A VDU operator’s movements are restricted, attention is on the screen or the form (Grandjean, 1987). The occupational “low exposure” risk factors for shoulder—neck complaints are: monotonous work tasks, poor psychosocial work environment, high work/production intensity, lack
0169-8141/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 7 ) 0 0 0 5 8 - 9
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of breaks during working hours apart from ordinary lunch breaks and two coffee breaks, and lack of alternative tasks offering a different exposure pattern during working hours (Winkel and Westgaard, 1992). Mathiassen (1993) stresses that the time for which a constrained posture is maintained is important for the risk of overloading muscles, and that pauses in working life, though their effects are not sufficiently documented, are probably important for minimising the harm of static work. Important factors are working time at the VDU, type of work and how the work is organised (A> borg et al., 1992). In repetitive work a variation in muscle activity is also important for preventing fatigue. This variation could be better achieved by work organisational changes through alteration of work task during a day than by introducing pause activities, which often do not fit into the work (Sundelin, 1992). When improving work organisation the new tasks should provide changes in movements and pace as well as in muscular load to prevent musculoskeletal disorders or decrease the level of symptoms. Small differences alone are not sufficient and the selection of tasks is important (Jonsson, 1988). Upper-arm elevation seems to be one important parameter influencing shoulder muscle load (Sigholm et al., 1984; Schu¨ldt, 1988) and is well correlated to electromyographic (EMG) amplitudes (Hagberg, 1981a) and also to shoulder disorders (Kilbom, 1986). The level of long-term, isometric and harmless muscular activity has been discussed for many years. In 1960, Rohmert (1960) and Rohmert and Laurig (1975) introduced a model for how static muscular work below 15% MVC (maximum voluntary contraction) could be kept up for a long time without harm. In 1977 Bjo¨rkste´n and Jonsson stated that the upper limit for 1 h of contraction is approximately 8% MVC and an acceptable upper limit of force for a “continuous” (4—8 h) contraction should be as low as a few percent of the MVC. Westgaard showed (1988) that a continuous load pattern with a static load component of about 1% MVC “is largely tolerated throughout a normal working day, even when including shorter periods at a higher load”. At the same time, muscular endurance is clearly greater while working with periodic loads than
with static (O®rtengren et al., 1978). Local muscular fatigue is strongly related to endurance time and is slower to develop in intermittent isometric exercise than in sustained (Hagberg, 1981b). Thus, work movements should vary in frequency and load in order to avoid muscular fatigue and this can be achieved by well organised work. The present study was designed to study the physiological and clinical effects of an ergonomically based reorganisation of data entry work at a public authority in Sweden. The reorganisation was planned and effected by management, and was well supported financially. The aims were to redistribute the repetitive data entry work, to provide the staff with new work tasks and less data entry and to increase work variety. The hypothesis was that this would imply less prolonged static work and thus reduce musculoskeletal symptoms (Ericson et al., 1995). The subjects’ shoulder muscular load was measured at their workplaces during an ordinary working day, before the reorganisation and again 18 months later. The following specific questions were analysed: E did the shoulder muscular load change due to the changed organisation or mix of work tasks? E did neck-shoulder-arm disorders change due to the changed work organisation or mix of work tasks? E what was the magnitude of shoulder muscular activity during an office working day? E what were the duration and frequency of shoulder muscular rest periods during an office working day? E what was the magnitude of the shoulder muscular activity associated with different work tasks?
2. Material and methods 2.1. Subjects Twenty-two female office workers volunteered to participate in 1991. At the 1992 follow-up, 18 of them were still employed in the same organisation. One did not want to participate and one could not for medical reasons. Thus, there were 16 subjects in 1992. Ages, general anthropometric data and length of employment in 1991 are presented in
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Table 1 Subjects’ mean age, height, weight and length of employment in 1991 Subject
Age (yr)
Height (m)
Weight (kg)
Employment period (yr)
Mean S.D. n
39 16.2 22
1.66 0.07 21
64 7.6 21
9.4 8.7 22
1.68 0.06 15
65.3 8 15
9.8 8.8 16
Subjects participating both in 1991 and in 1992 Mean 39.6 S.D. 17 n 16
Height 50 percentile, Swedish women "1.66 m. Weight 50 percentile, Swedish women "63 kg.
Table 1. One subject was left-handed (subject no. 19) and another was ambidextrous (subject no. 16). 2.2. Work organisation The subjects were employed in the same organisation, but in different research departments. Their main work was computer entry of statistical data. The organisation compiles and analyses statistical material, mainly for Swedish state authorities. Before the organisational change (1991) the subjects worked in groups, all within each group dealing with the same tasks. The subjects’ opportunities for influencing the work were very limited. Management focused on work environment changes at that time. However, during the following period the organisation’s structural and financial situation changed dramatically, which was beyond the local managers’ control. Work environment had now lower priority, although the extensive changes presented an opportunity for good ergonomic solutions. In 1992 the subjects still had data entry as their main work, although most of them had more scope for other tasks. 2.3. Work tasks In 1991, all subjects (n"22) had statistical data entry as their main job. Two of them (nos. 15 and
18) had no other tasks, and another five (nos. 5, 9, 11, 12 and 13) were the same except for sorting company post (see below). Twelve got data entry breaks by doing desk work or telephone work and by leaving their work places. Three subjects (nos. 8, 10 and 17) had no data entry on the day of the measurements. Sixteen had a short daily break for physical exercise, pause gymnastics, conducted in a corridor. The 5—10 min exercise was carried out standing with extensive movements to music. One group sorted the department’s post, a job they did standing up. In 1992, the participating subjects (n"16) had less VDU work and more desk tasks such as telephone interviews, sorting of mail and checking forms. None of them joined in the 5 min physical exercise at this time. While entering data all the subjects sat, performing repetitive work, mostly with the right hand on the keyboard and the left hand following the data and turning the form. However, they took short breaks to collect new forms and discuss with passing colleagues. 2.4. Prevalence of neck/shoulder/arm pain At the time of the studies all the subjects were at work and all were interviewed about disorders in the musculoskeletal system. Necks, shoulders and arms were clinically examined in 1991 by a physiotherapist (EF) regarding pain, muscular strength and joint mobility. The examination was carried out in a separate room with palpation, testing
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of joint mobility and strength, and pain analysis. The diagnoses were based on a screening method (Wolgast, 1989) using functional tests for joint mobility, muscular strength and tenderness. The method was tested for inter- and intra-reliability by Wolgast and colleagues. The tests opened with a short medical history in which particulars of pain, stiffness, fatigue and numbness in the neck, shoulders and arms were noted down. In the method, four diagnosis criteria are given for the neck, four for the shoulders, two for the elbows, and three for the hands and fingers. Diagnoses were based on at least three cooperative findings. The examination was followed up 1.5 yr later after the changed organisation. The subjects’ overall neck, shoulder and arm musculoskeletal disorders were compared from 1991 to 1992. The comparison concerned whether subjects’ neck, shoulder and arm disorders were improved, unchanged or worse in 1992. Thus, those who were without problems both times were included in the analysis. Subjects reporting “stiff and/or tired neck” with no other symptoms were considered to be pain-free for the analysis. However, three subjects were diagnosed as having diseases not expected to be influenced by changed work tasks and they were excluded from the analysis. In 1992 the subjects were followed up by the same physiotherapist with interviews and examinations regarding their present and former problems and other relevant functions. The 1991 medical diagnoses based on the screening method are given in Table 2.
Table 2 Diagnosis of neck—shoulder, arm and hand pain in 1991 n Neck and shoulder Tension-neck syndrome Cervical syndrome Other
10 2 1
Shoulder joint Tendinitis/frozen shoulder Degeneration acromioclavicular joint Degeneration glenohumeral joint Other
6 1 2 2
Elbow Epicondylitis
2
Hand Tenosynovitis
3
Other " sport injury, post-operation, muscular disease.
2.5. Equipment and measurement Shoulder muscular activity was measured electromyographically (EMG) from trapezius pars descendens, sinister and dexter. An ME3000 EMG equipment (Mega Electronics Ltd, Finland) was used. The raw EMG signal was RMS detected at a time constant of 100 ms. Forty RMS amplitude values were averaged and stored at a frequency of 0.25 Hz, i.e. one mean EMG amplitude value was registered and digitally stored every 4 s. The preamplifier was placed close to the electrodes, which minimises the risk of signal artefacts. The measuring equipment allowed continuous recording for up
Fig. 1. Placing of the EMG surface electrodes.
to 8 h. The subjects wore the small, light (0.345 kg) ME3000 data collection box in a waistbelt. Surface electrodes (Medicotest A-10-S) were placed with 30 mm centre distance in pairs upon carefully prepared skin overlying trapezius at a measured 1/3 of the distance from the prominent bone of the seventh cervical vertebra to acromion (Veiersted, 1991) (Fig. 1).
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To document the subjects’ work and to investigate how the change might affect shoulder muscular load, the subjects were videotaped throughout the test days both in 1991 and in 1992. 2.6. Procedure The subjects were allowed to move freely in their work and other activities without risk of disturbing the measurement. They wore the equipment for the whole working day including lunch and breaks, i.e. 4—8 h. Before work started in the morning each subject performed standardised maximum reference voluntary contractions (RVC) of the trapezius muscles at 90° elevation in the scapulae plane (Schu¨ldt and Harms-Ringdahl, 1988), to manual resistance. To minimise the risk of misleading results the RVCs were supervised by the same person (ME) every time (Veiersted, 1991). Also, to set EMG baselines, the subjects relaxed for 30 s at the beginning of the recording, following instructions (EF) and practice on how to relax. 2.7. Analysis Mean trapezius muscle activity was analysed during the whole working day as well as for different work tasks. Mean trapezius activity below the levels of 1, 2 and 5% RVC and exceeding 4 s, was also detected and analysed, as was the number of times per hour this occurred. The same analysis was done for muscular activity time exceeding 32 s
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below the RVC levels described above. These RVC levels are called “rest periods” in this paper. For each subject participating in both years, the differences between the two measurements were calculated. The 4 s rest period (P4) was chosen as a “micropause” (Kilbom and Persson, 1987). The 32 s rest period (P32) was designated a “short muscular rest period” (Sundelin, 1992). The levels of 1, 2 and 5% RVC were chosen to correspond approximately to the various proposed load levels for static work (Bjo¨rkste´n and Jonsson, 1977; Westgaard, 1988). When analysed, the mean duration (in s) of the muscular activity exceeding the 4 s rest period (mean P4) for each RVC level was calculated for each subject (Fig. 2). 2.8. Statistics The material was small and the scores under analysis could not be assumed to be drawn from a normally distributed population. For these reasons a non-parametric technique was used for statistical analysis. The analysis concerned differences between the two measurement occasions for each subject. Thus, all individuals were intraindividually analysed in the studies. To detect specific pairwise differences in the analysis of each individual regarding the 1991 and 1992 whole-day mean muscular activity, and between muscular activity below 1, 2 or 5% RVC, the Wilcoxon matched-pairs signed-ranks test (two-tailed) was used for statistical analysis. The same method was used to detect differences in muscular activity from different work
Fig. 2. Characteristic of variation in muscular load and rest periods exceeding 4 s below the levels of 1, 2 and 5%RVC.
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tasks. The differences from 1991 to 1992 in prevalence of neck, shoulder and arm disorders were analysed with the sign test. The significant level chosen was p)0.05.
3. Result 3.1. Video recordings The analysis of the video recordings showed that the accumulated mean time spent actually entering data during the filmed working day was 120 min in 1991 and 106 min in 1992. The time spent on “desk work”, i.e. non-computer work, increased significantly from 50 min in 1991 to 93 min in 1992. The rest of the time was spent on telephone work, sorting of mail (1991), pause gymnastics (1991), visits by colleagues and “out” (meetings and collecting new forms). None of these tasks lasted longer than 45 min except “post” 1991 (56 min) and “out” 1992 (75 min). The analysed mean filmed day’s working time for the group was 6 h 15 min in 1991 and 6 h 37 min in 1992. 3.2. Prevalence of neck and shoulder disorders In 1991, 20 of 22 subjects (91%) complained of tenderness in the neck and shoulders. Sixteen subjects (73%) complained of pain in neck, shoulder and/or arm (Fig. 3). In 1992 (n"16) one of those with pain in 1991 (n"13) reported no pain, seven less pain, three no change of pain, and two more pain. For the whole group there was no significant difference (sign test) in pain from 1991 to 1992 at the significance level chosen. However, when the three subjects diagnosed as having diseases not expected to be improved were excluded, there was a significant improvement among the subjects. 3.3. Electromyographic muscular activity measurements The “whole-day” mean trapezius muscular activity expressed in %RVC for each subject in 1991 (n"22) and in 1992 (n"16), and the mean values for the whole group (“all subjects”) are presented in Table 3. Mean 1991 values for the subjects particip-
Fig. 3. Prevalence of neck—shoulder and arm pain, 1991 and 1992.
ating in both years are also given. In 1991 there are two measurements missing on the left trapezius muscle due to technical problems. One of these two subjects (no. 6) also participated in 1992. This makes 15 pairs on the left trapezius muscle and 16 pairs on the right for both years. The difference in %RVC levels for each subject from 1991 to 1992 was calculated. The intraindividual differences were not statistically significant (Wilcoxon matchedpairs signed-ranks test). The mean trapezius muscular activity (%RVC) measured during different work tasks in 1991 is presented in Table 4. Desk work required higher muscular activity in the right trapezius muscle than did data entry work, 9.0 and 6.0% RVC, respectively (Wilcoxon matched-pairs signed-ranks test). The mean time for each task was analysed separately and is also presented in Table 4. The mean duration of rest periods exceeding 4 s (mean P4) below 1, 2 and 5 %RVC for the subjects participating in both years is presented in Table 5, as are the number of rest periods per hour (nP4/h). The table also shows the numbers of rest periods longer than 32 s per hour below the levels of 1, 2 and 5% RVC (nP32/h). The table is based on whole-day measurements. Due to technical problems, data from subject no. 4 is missing. None of the differences in the number or duration of rest periods was statistically significant (Wilcoxon matched-pairs signed-ranks test).
4. Discussion The work changes were insufficient to influence the shoulder muscular load levels, or the duration
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Table 3 Mean individual trapezius activity (%RVC) in 1991 and 1992 based on whole-day measurements and differences between the two measuring occasions. Group mean values are also presented Subject
Trapezius sinister
Trapezius dexter
1991
1992
Difference
1991
1992
Difference
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
8.8 15.8 6.2 17.7 4.4 — 15.5 — 8.1 4.7 4.1 8.1 2.9 3.7 9.8 6.0 8.9 2.6 6.3 12.6 4.8 6.0
14.9 — — 21.9 6.0 5.7 11.7 — — 8.9 — — 2.0 4.5 8.1 3.2 27.2 5.1 3.0 5.5 5.5 4.9
6.1 — — 4.2 1.6 — !3.5 — — 4.2 — — !0.9 0.8 !1.7 !2.8 18.3 2.5 !3.3 !7.1 0.7 !1.1
8.7 10.9 5.4 12.7 5.9 9.9 8.2 10.5 7.3 5.9 4.8 7.3 2.6 4.6 9.9 5.0 11.7 2.6 6.7 5.0 6.5 10.6
7.8 — — 7.0 4.7 5.8 7.2 — — 11.9 — — 2.2 4.7 9.2 4.3 19.9 5.7 3.6 3.6 5.3 5.6
!0.9 — — !5.7 !1.2 !4.1 !1.0 — — 6.0 — — !0.4 0.1 !0.7 !0.7 8.2 3.1 !3.1 !1.4 !1.2 !5.0
All subjects Mean S.D. n
7.8 4.4 20
8.6 7.1 16
7.4 2.9 22
6.8 4.2 16
Pairs 1991—1992 Mean S.D. n
7.6 4.5 15
8.8 7.3
7.3 3.1 16
6.8 4.2
1.20 5.9
and frequency of rest periods. The trapezius EMG measurements showed no significant difference between recordings before and after the changes. However, the subjects had significant, although moderate, improvement in musculoskeletal disorders after the changed organisation. When excluding the subjects not expected to be improved, there was an improvement in the subjects’ neck, shoulder and arm complaints from 1991 to 1992. As the material is small and the criteria for the diagnoses wide, it is difficult to draw firm conclusions. The subjects claimed that with less data entry they
!0.5 3.7
had less pain, while the pain returned when they had much data entry work. For the whole-day mechanical factors, there were no differences in muscular load between the two measuring occasions. However, differences were found in work task distribution and muscular load for different work tasks. Thus, desk work required higher mean muscular activity than data entry work. The subjects had more desk work in 1992 than in 1991 and less neck—shoulder—arm pain. The mean level of muscular activity does not seem to be as important for the risk of getting
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Table 4 Mean trapezius activity (%RVC) during different work tasks and mean analysed time (min) for these tasks in 1991 Data entry
Desk work
Telephone work
¹rapezius sinister (%RVC) Mean 7.2 S.D. 5.1 n 19
9.1 4.5 16
6.3 3.3 10
¹rapezius dexter (%RVC) Mean 6.0 S.D. 3.2 n 20
9.0 4.9 18
6.1 3.0 11
¹ime analysed (min) Mean 11.8 S.D. 8.0 n 20
11.1 10.2 18
6.5 7.4 11
Sorting of post
Coffee break
Pause gymnastics
7.5 4.6 6
5.8 3.8 19
11.4 10.9 13
8.0 2.7 9
5.3 2.2 21
11.2 8.1 15
29.5 11.5 21
6.8 2.6 15
60 23 9
Table 5 Mean rest period (s) and number of rest periods (times) in the trapezius muscles, based on whole-day measurements 1991 Mean P4 (s)
1992 nP4/h (n)
nP32/h (n)
Mean P4(s)
nP4/h (n)
nP32/h (n)
¸eft (14 subjects) (1%RVC 16.1$ 6.0 (2%RVC 23.0$ 7.9 (5%RVC 33.4$17.4
33.6$22.6 44.0$23.2 58.6$21.4
4.7$3.4 8.7$4.9 14.0$9.0
15.9$ 9.4 25.6$10.8 35.9$20.8
25.1$24.0 40.1$21.5 51.3$21.4
4.3$5.4 8.5$4.0 12.7$6.4
Right (15 subjects) (1%RVC 17.0$ 5.7 (2%RVC 20.6$ 4.6 (5%RVC 30.9$18.1
31.9$24.2 47.4$25.0 62.7$17.8
3.7$3.1 7.9$4.4 14.5$6.4
17.2$ 6.1 22.6$ 6.3 31.9$19.9
27.9$22.6 42.1$21.6 63.8$25.3
4.0$3.3 7.9$4.4 13.1$4.9
Note: Mean P4" mean time in seconds (1, 2 or 5%RVC exceeding 4 s. nP4/h" no. occasions per hour (1, 2 or 5%RVC exceeding 4 s. nP32/h" no. occasions per hour (1, 2 or 5%RVC exceeding 32 s.
muscular disorders as repetitiveness, duration and the muscular variation in load level (Mathiassen, 1993; Winkel and Oxenburgh, 1990). The subjects seemed to be better off while having more loaded but perhaps also more varied desk-work, than constrained data entry work. In the desk work they were more mobile with tasks demanding bigger movements. The greater the variation in frequency and duration, e.g. work technique, the less the
neck—and—shoulder pain (Kilbom, 1986). Mathiassen (1993) states that the duration of static muscular activity is important for the risk of overloading muscles. He also concludes that the time needed for recovering metabolic balance to resting homeostasis depends on the kind and the duration of the load. A protracted low load needs more time for recovery. Kilbom and Persson (1987) found that periods of shoulder rest lasting more than 2 s were
E.A.C. Fernstrom, C.M. Aborg / International Journal of Industrial Ergonomics 23 (1999) 231—240
negatively correlated to complaint. Consequently, interventions should aim at optimising muscular activation pattern and the work should allow dynamic patterns of movement (Kilbom and Persson, 1987). Therefore, work content and mix of tasks seem to be important factors for minimising the risk of musculoskeletal disorders. During the one-and-a-half-year period, the data-processing unit was closed down and most of the subjects were transferred to groups dealing with more varied and qualified tasks but mostly with the same main job content. The reorganisation gave opportunities to improve the working conditions of the personnel from the former unit. However, these opportunities were only partly exploited and the work tasks were not distributed within the groups as planned. The subjects still had repetitive and monotonous tasks with too little demanding work and personal control (Ericson et al., 1995). Comparing EMG measurements on different occasions might be a source of error. However, EMG measurements of muscular activity expressed in %RVC, though made at different times, are sufficiently reliable (Veiersted, 1991), although it is important that RVC tests and relaxation tests are standardised and performed by the same person (Mathiassen, personal communication, 1993). Also the placement of the electrodes is of great importance. To minimise the risk of misleading results the same person (ME) gave all the RVC tests and the same person (EF) placed all the electrodes at the distance measured. In 1991, 55% of our subjects reported neck—and—shoulder disorders, which corresponds well to a study on female workers performing light manual work (Veiersted and Westgaard, 1993). Whether these disorders influence the reference contraction or the way people work we do not know. At the times of the EMG measurements (in 1991 and 1992) only two of the subjects had pain estimated to inhibit a maximal reference contraction. For these two, and for two who had more pain in 1992, there are no obvious differences within the pairs of reference contractions compared to the other subjects. Thus, while we consider the risk of misleading results to be small, it is difficult to say whether the pain influences the work movements.
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5. Conclusion From the study presented and the questions asked, the following conclusions may be drawn: Whole-day data entry work mixed with other work tasks including breaks involved E mean shoulder-muscular activity at 7—8% percentage of maximum reference contraction. Changes of work tasks and work task distribution comprised E more non-computerised desk work time, E changes in mean shoulder-muscular activity from different work task, E higher shoulder-muscle activity from non-computerised desk work than data entry work. Results, difficult to associate directly with the changed work task distribution after 1.5 yr, are E no change in whole-day shoulder muscle activity, E slight decrease in neck, shoulder and arm disorders. It is suggested that changes must be extensive to influence mean muscle activity. Alternation between different office work tasks implies variations in shoulder muscular activity. It is suggested that non-computerised desk work, consisting of mixed tasks, supplies muscular load variation needed. Decrease in neck-and-shoulder pain might depend on increased desk work, but also on other factors.
Acknowledgements This work was supported by a grant from the Swedish Work Environment fund. Special thanks to Acting Professor Mats O. Ericson, and to those who participated in the study.
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Schu¨ldt, K., Harms-Ringdahl, K., 1988. Activity levels during isometric test contractions of neck and shoulder muscles. Scandinian Journal of Medicine 20, 117—127. Sigholm, G., Herberts, P., Almstro¨m, C., Kadefors, R., 1984. Electromyographic analysis of shoulder muscle load. Journal of Orthopaedic Research 1, 379—386. Sundelin, G., 1992. Electromyography of shoulder muscles — the effects of pauses, drafts and repetitive work cycles. Arbete och Ha¨lsa, 16, National Institute of Occupational Health, S-171 84 Solna, Sweden. Veiersted, K.B., 1991. The reproducibility of test contractions for calibration of electromyographic measurements. European Journal of Applied Physiology 62, 91—98. Veiersted, K.B., Westgaard, R., 1993. Development of trapezius myalgia among female workers performing light manual work. Scandinian Journal of Work Environment and Health 19, 277—283. Westgaard, R.H., 1988. Measurement and evaluation of postural load in occupational work situations. European Journal of Applied Physiology 57, 291—304. Winkel, J., Oxenburgh, M., 1990. Towards optimising physical activity in VDT/office work. In: Sauter, S., Dainoff, M., Smith, M. (Eds.), Promoting Health and Productivity in the Computerized Office. Taylor and Francis, London, pp. 94—117. Winkel, J., Westgaard, R., 1992. Occupational and individual risk factors for shoulder—neck complaints: Part I - guidelines for the practitioner. International Journal of Industrial Ergonomics 10, 79—83. Wolgast, E., 1989. Screening of musculo-skeletal diseases in the neck, shoulder and arm. Screening av muskel och ledsjukdomar i nacke, skuldra och arm — en metodbeskrivning. Report from National Institute of Occupational Health, S-171 84 Solna, Sweden, (in Swedish).
Alterations in shoulder muscle activity due to changes in data entry organisation Elisabeth A.C. Fernstro¨m *, Carl M. A> borg Futura, Box 16408, S-10327 Stockholm, Sweden Department of Environmental Technology and Work Science, Royal Institute of Technology, Stockholm, Sweden Futura, S.t Olofsgatan 9, S-753 21 Uppsala, Sweden Department of Human Work Science, Lulea University, Lulea , Sweden Received 29 January 1997; accepted 18 April 1997
Abstract The aim of the present study was to investigate how changed work organisation and different work tasks influence shoulder muscular load and to quantify the magnitude, duration and frequency of rest periods from shoulder muscular load during a working day. Shoulder muscular load was measured in 22 females working at their data entry workplaces, during a whole working day. The activity from both trapezius muscles was measured with EMG before (1991) and after (1992) a reorganisation programme intended to redistribute repetitive work and provide new work tasks. The change in work organisation did not change the magnitude of muscular load or the duration and frequency of rest periods, but decreased musculoskeletal problems. The subjects’ increased desk work involved greater muscular load than the data entry did, but also allowed more movement. The changes in work tasks seemed to be important, although small. In repetitive work, organisational changes aimed at reducing musculoskeletal disorders should focus on providing employees with tasks that afford variation in muscular load. Relevance to industry The paper discusses the need of physical work task variation in repetitive work in order to minimise the risk of musculoskeletal disorders. It seems more important to vary the tasks than to minimise the shoulder muscular load. 1999 Elsevier Science B.V. All rights reserved. Keywords: VDU; Data entry; Electromyography; Muscular load; Neck; Disorder
1. Introduction Office work often involves sedentary VDU (video display unit) work, which increases the risk of * Correspondence address: Futura, Box 16 408, S-103 27 Stockholm, Sweden. Tel.: #46 10 683 3568; fax: #46 821 1391.
developing musculoskeletal complaints and diseases. A VDU operator’s movements are restricted, attention is on the screen or the form (Grandjean, 1987). The occupational “low exposure” risk factors for shoulder—neck complaints are: monotonous work tasks, poor psychosocial work environment, high work/production intensity, lack
0169-8141/99/$ — see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 8 1 4 1 ( 9 7 ) 0 0 0 5 8 - 9
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of breaks during working hours apart from ordinary lunch breaks and two coffee breaks, and lack of alternative tasks offering a different exposure pattern during working hours (Winkel and Westgaard, 1992). Mathiassen (1993) stresses that the time for which a constrained posture is maintained is important for the risk of overloading muscles, and that pauses in working life, though their effects are not sufficiently documented, are probably important for minimising the harm of static work. Important factors are working time at the VDU, type of work and how the work is organised (A> borg et al., 1992). In repetitive work a variation in muscle activity is also important for preventing fatigue. This variation could be better achieved by work organisational changes through alteration of work task during a day than by introducing pause activities, which often do not fit into the work (Sundelin, 1992). When improving work organisation the new tasks should provide changes in movements and pace as well as in muscular load to prevent musculoskeletal disorders or decrease the level of symptoms. Small differences alone are not sufficient and the selection of tasks is important (Jonsson, 1988). Upper-arm elevation seems to be one important parameter influencing shoulder muscle load (Sigholm et al., 1984; Schu¨ldt, 1988) and is well correlated to electromyographic (EMG) amplitudes (Hagberg, 1981a) and also to shoulder disorders (Kilbom, 1986). The level of long-term, isometric and harmless muscular activity has been discussed for many years. In 1960, Rohmert (1960) and Rohmert and Laurig (1975) introduced a model for how static muscular work below 15% MVC (maximum voluntary contraction) could be kept up for a long time without harm. In 1977 Bjo¨rkste´n and Jonsson stated that the upper limit for 1 h of contraction is approximately 8% MVC and an acceptable upper limit of force for a “continuous” (4—8 h) contraction should be as low as a few percent of the MVC. Westgaard showed (1988) that a continuous load pattern with a static load component of about 1% MVC “is largely tolerated throughout a normal working day, even when including shorter periods at a higher load”. At the same time, muscular endurance is clearly greater while working with periodic loads than
with static (O®rtengren et al., 1978). Local muscular fatigue is strongly related to endurance time and is slower to develop in intermittent isometric exercise than in sustained (Hagberg, 1981b). Thus, work movements should vary in frequency and load in order to avoid muscular fatigue and this can be achieved by well organised work. The present study was designed to study the physiological and clinical effects of an ergonomically based reorganisation of data entry work at a public authority in Sweden. The reorganisation was planned and effected by management, and was well supported financially. The aims were to redistribute the repetitive data entry work, to provide the staff with new work tasks and less data entry and to increase work variety. The hypothesis was that this would imply less prolonged static work and thus reduce musculoskeletal symptoms (Ericson et al., 1995). The subjects’ shoulder muscular load was measured at their workplaces during an ordinary working day, before the reorganisation and again 18 months later. The following specific questions were analysed: E did the shoulder muscular load change due to the changed organisation or mix of work tasks? E did neck-shoulder-arm disorders change due to the changed work organisation or mix of work tasks? E what was the magnitude of shoulder muscular activity during an office working day? E what were the duration and frequency of shoulder muscular rest periods during an office working day? E what was the magnitude of the shoulder muscular activity associated with different work tasks?
2. Material and methods 2.1. Subjects Twenty-two female office workers volunteered to participate in 1991. At the 1992 follow-up, 18 of them were still employed in the same organisation. One did not want to participate and one could not for medical reasons. Thus, there were 16 subjects in 1992. Ages, general anthropometric data and length of employment in 1991 are presented in
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Table 1 Subjects’ mean age, height, weight and length of employment in 1991 Subject
Age (yr)
Height (m)
Weight (kg)
Employment period (yr)
Mean S.D. n
39 16.2 22
1.66 0.07 21
64 7.6 21
9.4 8.7 22
1.68 0.06 15
65.3 8 15
9.8 8.8 16
Subjects participating both in 1991 and in 1992 Mean 39.6 S.D. 17 n 16
Height 50 percentile, Swedish women "1.66 m. Weight 50 percentile, Swedish women "63 kg.
Table 1. One subject was left-handed (subject no. 19) and another was ambidextrous (subject no. 16). 2.2. Work organisation The subjects were employed in the same organisation, but in different research departments. Their main work was computer entry of statistical data. The organisation compiles and analyses statistical material, mainly for Swedish state authorities. Before the organisational change (1991) the subjects worked in groups, all within each group dealing with the same tasks. The subjects’ opportunities for influencing the work were very limited. Management focused on work environment changes at that time. However, during the following period the organisation’s structural and financial situation changed dramatically, which was beyond the local managers’ control. Work environment had now lower priority, although the extensive changes presented an opportunity for good ergonomic solutions. In 1992 the subjects still had data entry as their main work, although most of them had more scope for other tasks. 2.3. Work tasks In 1991, all subjects (n"22) had statistical data entry as their main job. Two of them (nos. 15 and
18) had no other tasks, and another five (nos. 5, 9, 11, 12 and 13) were the same except for sorting company post (see below). Twelve got data entry breaks by doing desk work or telephone work and by leaving their work places. Three subjects (nos. 8, 10 and 17) had no data entry on the day of the measurements. Sixteen had a short daily break for physical exercise, pause gymnastics, conducted in a corridor. The 5—10 min exercise was carried out standing with extensive movements to music. One group sorted the department’s post, a job they did standing up. In 1992, the participating subjects (n"16) had less VDU work and more desk tasks such as telephone interviews, sorting of mail and checking forms. None of them joined in the 5 min physical exercise at this time. While entering data all the subjects sat, performing repetitive work, mostly with the right hand on the keyboard and the left hand following the data and turning the form. However, they took short breaks to collect new forms and discuss with passing colleagues. 2.4. Prevalence of neck/shoulder/arm pain At the time of the studies all the subjects were at work and all were interviewed about disorders in the musculoskeletal system. Necks, shoulders and arms were clinically examined in 1991 by a physiotherapist (EF) regarding pain, muscular strength and joint mobility. The examination was carried out in a separate room with palpation, testing
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of joint mobility and strength, and pain analysis. The diagnoses were based on a screening method (Wolgast, 1989) using functional tests for joint mobility, muscular strength and tenderness. The method was tested for inter- and intra-reliability by Wolgast and colleagues. The tests opened with a short medical history in which particulars of pain, stiffness, fatigue and numbness in the neck, shoulders and arms were noted down. In the method, four diagnosis criteria are given for the neck, four for the shoulders, two for the elbows, and three for the hands and fingers. Diagnoses were based on at least three cooperative findings. The examination was followed up 1.5 yr later after the changed organisation. The subjects’ overall neck, shoulder and arm musculoskeletal disorders were compared from 1991 to 1992. The comparison concerned whether subjects’ neck, shoulder and arm disorders were improved, unchanged or worse in 1992. Thus, those who were without problems both times were included in the analysis. Subjects reporting “stiff and/or tired neck” with no other symptoms were considered to be pain-free for the analysis. However, three subjects were diagnosed as having diseases not expected to be influenced by changed work tasks and they were excluded from the analysis. In 1992 the subjects were followed up by the same physiotherapist with interviews and examinations regarding their present and former problems and other relevant functions. The 1991 medical diagnoses based on the screening method are given in Table 2.
Table 2 Diagnosis of neck—shoulder, arm and hand pain in 1991 n Neck and shoulder Tension-neck syndrome Cervical syndrome Other
10 2 1
Shoulder joint Tendinitis/frozen shoulder Degeneration acromioclavicular joint Degeneration glenohumeral joint Other
6 1 2 2
Elbow Epicondylitis
2
Hand Tenosynovitis
3
Other " sport injury, post-operation, muscular disease.
2.5. Equipment and measurement Shoulder muscular activity was measured electromyographically (EMG) from trapezius pars descendens, sinister and dexter. An ME3000 EMG equipment (Mega Electronics Ltd, Finland) was used. The raw EMG signal was RMS detected at a time constant of 100 ms. Forty RMS amplitude values were averaged and stored at a frequency of 0.25 Hz, i.e. one mean EMG amplitude value was registered and digitally stored every 4 s. The preamplifier was placed close to the electrodes, which minimises the risk of signal artefacts. The measuring equipment allowed continuous recording for up
Fig. 1. Placing of the EMG surface electrodes.
to 8 h. The subjects wore the small, light (0.345 kg) ME3000 data collection box in a waistbelt. Surface electrodes (Medicotest A-10-S) were placed with 30 mm centre distance in pairs upon carefully prepared skin overlying trapezius at a measured 1/3 of the distance from the prominent bone of the seventh cervical vertebra to acromion (Veiersted, 1991) (Fig. 1).
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To document the subjects’ work and to investigate how the change might affect shoulder muscular load, the subjects were videotaped throughout the test days both in 1991 and in 1992. 2.6. Procedure The subjects were allowed to move freely in their work and other activities without risk of disturbing the measurement. They wore the equipment for the whole working day including lunch and breaks, i.e. 4—8 h. Before work started in the morning each subject performed standardised maximum reference voluntary contractions (RVC) of the trapezius muscles at 90° elevation in the scapulae plane (Schu¨ldt and Harms-Ringdahl, 1988), to manual resistance. To minimise the risk of misleading results the RVCs were supervised by the same person (ME) every time (Veiersted, 1991). Also, to set EMG baselines, the subjects relaxed for 30 s at the beginning of the recording, following instructions (EF) and practice on how to relax. 2.7. Analysis Mean trapezius muscle activity was analysed during the whole working day as well as for different work tasks. Mean trapezius activity below the levels of 1, 2 and 5% RVC and exceeding 4 s, was also detected and analysed, as was the number of times per hour this occurred. The same analysis was done for muscular activity time exceeding 32 s
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below the RVC levels described above. These RVC levels are called “rest periods” in this paper. For each subject participating in both years, the differences between the two measurements were calculated. The 4 s rest period (P4) was chosen as a “micropause” (Kilbom and Persson, 1987). The 32 s rest period (P32) was designated a “short muscular rest period” (Sundelin, 1992). The levels of 1, 2 and 5% RVC were chosen to correspond approximately to the various proposed load levels for static work (Bjo¨rkste´n and Jonsson, 1977; Westgaard, 1988). When analysed, the mean duration (in s) of the muscular activity exceeding the 4 s rest period (mean P4) for each RVC level was calculated for each subject (Fig. 2). 2.8. Statistics The material was small and the scores under analysis could not be assumed to be drawn from a normally distributed population. For these reasons a non-parametric technique was used for statistical analysis. The analysis concerned differences between the two measurement occasions for each subject. Thus, all individuals were intraindividually analysed in the studies. To detect specific pairwise differences in the analysis of each individual regarding the 1991 and 1992 whole-day mean muscular activity, and between muscular activity below 1, 2 or 5% RVC, the Wilcoxon matched-pairs signed-ranks test (two-tailed) was used for statistical analysis. The same method was used to detect differences in muscular activity from different work
Fig. 2. Characteristic of variation in muscular load and rest periods exceeding 4 s below the levels of 1, 2 and 5%RVC.
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tasks. The differences from 1991 to 1992 in prevalence of neck, shoulder and arm disorders were analysed with the sign test. The significant level chosen was p)0.05.
3. Result 3.1. Video recordings The analysis of the video recordings showed that the accumulated mean time spent actually entering data during the filmed working day was 120 min in 1991 and 106 min in 1992. The time spent on “desk work”, i.e. non-computer work, increased significantly from 50 min in 1991 to 93 min in 1992. The rest of the time was spent on telephone work, sorting of mail (1991), pause gymnastics (1991), visits by colleagues and “out” (meetings and collecting new forms). None of these tasks lasted longer than 45 min except “post” 1991 (56 min) and “out” 1992 (75 min). The analysed mean filmed day’s working time for the group was 6 h 15 min in 1991 and 6 h 37 min in 1992. 3.2. Prevalence of neck and shoulder disorders In 1991, 20 of 22 subjects (91%) complained of tenderness in the neck and shoulders. Sixteen subjects (73%) complained of pain in neck, shoulder and/or arm (Fig. 3). In 1992 (n"16) one of those with pain in 1991 (n"13) reported no pain, seven less pain, three no change of pain, and two more pain. For the whole group there was no significant difference (sign test) in pain from 1991 to 1992 at the significance level chosen. However, when the three subjects diagnosed as having diseases not expected to be improved were excluded, there was a significant improvement among the subjects. 3.3. Electromyographic muscular activity measurements The “whole-day” mean trapezius muscular activity expressed in %RVC for each subject in 1991 (n"22) and in 1992 (n"16), and the mean values for the whole group (“all subjects”) are presented in Table 3. Mean 1991 values for the subjects particip-
Fig. 3. Prevalence of neck—shoulder and arm pain, 1991 and 1992.
ating in both years are also given. In 1991 there are two measurements missing on the left trapezius muscle due to technical problems. One of these two subjects (no. 6) also participated in 1992. This makes 15 pairs on the left trapezius muscle and 16 pairs on the right for both years. The difference in %RVC levels for each subject from 1991 to 1992 was calculated. The intraindividual differences were not statistically significant (Wilcoxon matchedpairs signed-ranks test). The mean trapezius muscular activity (%RVC) measured during different work tasks in 1991 is presented in Table 4. Desk work required higher muscular activity in the right trapezius muscle than did data entry work, 9.0 and 6.0% RVC, respectively (Wilcoxon matched-pairs signed-ranks test). The mean time for each task was analysed separately and is also presented in Table 4. The mean duration of rest periods exceeding 4 s (mean P4) below 1, 2 and 5 %RVC for the subjects participating in both years is presented in Table 5, as are the number of rest periods per hour (nP4/h). The table also shows the numbers of rest periods longer than 32 s per hour below the levels of 1, 2 and 5% RVC (nP32/h). The table is based on whole-day measurements. Due to technical problems, data from subject no. 4 is missing. None of the differences in the number or duration of rest periods was statistically significant (Wilcoxon matched-pairs signed-ranks test).
4. Discussion The work changes were insufficient to influence the shoulder muscular load levels, or the duration
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237
Table 3 Mean individual trapezius activity (%RVC) in 1991 and 1992 based on whole-day measurements and differences between the two measuring occasions. Group mean values are also presented Subject
Trapezius sinister
Trapezius dexter
1991
1992
Difference
1991
1992
Difference
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
8.8 15.8 6.2 17.7 4.4 — 15.5 — 8.1 4.7 4.1 8.1 2.9 3.7 9.8 6.0 8.9 2.6 6.3 12.6 4.8 6.0
14.9 — — 21.9 6.0 5.7 11.7 — — 8.9 — — 2.0 4.5 8.1 3.2 27.2 5.1 3.0 5.5 5.5 4.9
6.1 — — 4.2 1.6 — !3.5 — — 4.2 — — !0.9 0.8 !1.7 !2.8 18.3 2.5 !3.3 !7.1 0.7 !1.1
8.7 10.9 5.4 12.7 5.9 9.9 8.2 10.5 7.3 5.9 4.8 7.3 2.6 4.6 9.9 5.0 11.7 2.6 6.7 5.0 6.5 10.6
7.8 — — 7.0 4.7 5.8 7.2 — — 11.9 — — 2.2 4.7 9.2 4.3 19.9 5.7 3.6 3.6 5.3 5.6
!0.9 — — !5.7 !1.2 !4.1 !1.0 — — 6.0 — — !0.4 0.1 !0.7 !0.7 8.2 3.1 !3.1 !1.4 !1.2 !5.0
All subjects Mean S.D. n
7.8 4.4 20
8.6 7.1 16
7.4 2.9 22
6.8 4.2 16
Pairs 1991—1992 Mean S.D. n
7.6 4.5 15
8.8 7.3
7.3 3.1 16
6.8 4.2
1.20 5.9
and frequency of rest periods. The trapezius EMG measurements showed no significant difference between recordings before and after the changes. However, the subjects had significant, although moderate, improvement in musculoskeletal disorders after the changed organisation. When excluding the subjects not expected to be improved, there was an improvement in the subjects’ neck, shoulder and arm complaints from 1991 to 1992. As the material is small and the criteria for the diagnoses wide, it is difficult to draw firm conclusions. The subjects claimed that with less data entry they
!0.5 3.7
had less pain, while the pain returned when they had much data entry work. For the whole-day mechanical factors, there were no differences in muscular load between the two measuring occasions. However, differences were found in work task distribution and muscular load for different work tasks. Thus, desk work required higher mean muscular activity than data entry work. The subjects had more desk work in 1992 than in 1991 and less neck—shoulder—arm pain. The mean level of muscular activity does not seem to be as important for the risk of getting
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Table 4 Mean trapezius activity (%RVC) during different work tasks and mean analysed time (min) for these tasks in 1991 Data entry
Desk work
Telephone work
¹rapezius sinister (%RVC) Mean 7.2 S.D. 5.1 n 19
9.1 4.5 16
6.3 3.3 10
¹rapezius dexter (%RVC) Mean 6.0 S.D. 3.2 n 20
9.0 4.9 18
6.1 3.0 11
¹ime analysed (min) Mean 11.8 S.D. 8.0 n 20
11.1 10.2 18
6.5 7.4 11
Sorting of post
Coffee break
Pause gymnastics
7.5 4.6 6
5.8 3.8 19
11.4 10.9 13
8.0 2.7 9
5.3 2.2 21
11.2 8.1 15
29.5 11.5 21
6.8 2.6 15
60 23 9
Table 5 Mean rest period (s) and number of rest periods (times) in the trapezius muscles, based on whole-day measurements 1991 Mean P4 (s)
1992 nP4/h (n)
nP32/h (n)
Mean P4(s)
nP4/h (n)
nP32/h (n)
¸eft (14 subjects) (1%RVC 16.1$ 6.0 (2%RVC 23.0$ 7.9 (5%RVC 33.4$17.4
33.6$22.6 44.0$23.2 58.6$21.4
4.7$3.4 8.7$4.9 14.0$9.0
15.9$ 9.4 25.6$10.8 35.9$20.8
25.1$24.0 40.1$21.5 51.3$21.4
4.3$5.4 8.5$4.0 12.7$6.4
Right (15 subjects) (1%RVC 17.0$ 5.7 (2%RVC 20.6$ 4.6 (5%RVC 30.9$18.1
31.9$24.2 47.4$25.0 62.7$17.8
3.7$3.1 7.9$4.4 14.5$6.4
17.2$ 6.1 22.6$ 6.3 31.9$19.9
27.9$22.6 42.1$21.6 63.8$25.3
4.0$3.3 7.9$4.4 13.1$4.9
Note: Mean P4" mean time in seconds (1, 2 or 5%RVC exceeding 4 s. nP4/h" no. occasions per hour (1, 2 or 5%RVC exceeding 4 s. nP32/h" no. occasions per hour (1, 2 or 5%RVC exceeding 32 s.
muscular disorders as repetitiveness, duration and the muscular variation in load level (Mathiassen, 1993; Winkel and Oxenburgh, 1990). The subjects seemed to be better off while having more loaded but perhaps also more varied desk-work, than constrained data entry work. In the desk work they were more mobile with tasks demanding bigger movements. The greater the variation in frequency and duration, e.g. work technique, the less the
neck—and—shoulder pain (Kilbom, 1986). Mathiassen (1993) states that the duration of static muscular activity is important for the risk of overloading muscles. He also concludes that the time needed for recovering metabolic balance to resting homeostasis depends on the kind and the duration of the load. A protracted low load needs more time for recovery. Kilbom and Persson (1987) found that periods of shoulder rest lasting more than 2 s were
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negatively correlated to complaint. Consequently, interventions should aim at optimising muscular activation pattern and the work should allow dynamic patterns of movement (Kilbom and Persson, 1987). Therefore, work content and mix of tasks seem to be important factors for minimising the risk of musculoskeletal disorders. During the one-and-a-half-year period, the data-processing unit was closed down and most of the subjects were transferred to groups dealing with more varied and qualified tasks but mostly with the same main job content. The reorganisation gave opportunities to improve the working conditions of the personnel from the former unit. However, these opportunities were only partly exploited and the work tasks were not distributed within the groups as planned. The subjects still had repetitive and monotonous tasks with too little demanding work and personal control (Ericson et al., 1995). Comparing EMG measurements on different occasions might be a source of error. However, EMG measurements of muscular activity expressed in %RVC, though made at different times, are sufficiently reliable (Veiersted, 1991), although it is important that RVC tests and relaxation tests are standardised and performed by the same person (Mathiassen, personal communication, 1993). Also the placement of the electrodes is of great importance. To minimise the risk of misleading results the same person (ME) gave all the RVC tests and the same person (EF) placed all the electrodes at the distance measured. In 1991, 55% of our subjects reported neck—and—shoulder disorders, which corresponds well to a study on female workers performing light manual work (Veiersted and Westgaard, 1993). Whether these disorders influence the reference contraction or the way people work we do not know. At the times of the EMG measurements (in 1991 and 1992) only two of the subjects had pain estimated to inhibit a maximal reference contraction. For these two, and for two who had more pain in 1992, there are no obvious differences within the pairs of reference contractions compared to the other subjects. Thus, while we consider the risk of misleading results to be small, it is difficult to say whether the pain influences the work movements.
239
5. Conclusion From the study presented and the questions asked, the following conclusions may be drawn: Whole-day data entry work mixed with other work tasks including breaks involved E mean shoulder-muscular activity at 7—8% percentage of maximum reference contraction. Changes of work tasks and work task distribution comprised E more non-computerised desk work time, E changes in mean shoulder-muscular activity from different work task, E higher shoulder-muscle activity from non-computerised desk work than data entry work. Results, difficult to associate directly with the changed work task distribution after 1.5 yr, are E no change in whole-day shoulder muscle activity, E slight decrease in neck, shoulder and arm disorders. It is suggested that changes must be extensive to influence mean muscle activity. Alternation between different office work tasks implies variations in shoulder muscular activity. It is suggested that non-computerised desk work, consisting of mixed tasks, supplies muscular load variation needed. Decrease in neck-and-shoulder pain might depend on increased desk work, but also on other factors.
Acknowledgements This work was supported by a grant from the Swedish Work Environment fund. Special thanks to Acting Professor Mats O. Ericson, and to those who participated in the study.
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