Can a more neutral position of the forearm when operating a computer mouse reduce the pain level for VDU operators?

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1 Industrial Ergonomics Can a more neutral position of the forearm when operating a computer mouse reduce the pain level for VDU operators? Arne Aanis a,*, Marvin Dainoff', Ola Roc, Magne Thoresen d a Alcatel Telecom Norway AS, P.o. Box 310, 0kern, 0511 Oslo, Norway b Department of Psychology, Center for Ergonomic Research, Miami University, Oxfort. OH 45056, USA C Premed AS, P.o. Box 275, 0kern, 0511 Oslo, Norway dsection of Medical Statistics, University of Oslo, P.o. Box 1122, BUndern, 0317 Oslo, Norway A prospective epidemiological field study covering a I-year period has been published (Int. J. Hum.-Comput. Interaction, 1999; 2001). The aim of the study was to investigate if subjects with existing musculoskeletal pain experience a reduction in pain development when using a new mouse design (Anir(Renaissance) which allows a more neutral position of the wrist, compared with a traditional mouse. The study population consisted of 67 participants with average intensity of pain during the last 6 months of approximately 50mm on a 100mm visual analog scale (VAS). The total group was randomly divided into one intervention group and one control group. The study was performed as a prospective parallel group design. VAS was used to assess the average level of pain in the musculoskeletal system during the last 6 months. This was a three-part study; the first two parts of which have already been reported. In part I, it was found that after using Anir mouse for 6 months, there were significant reductions in pain for neck, shoulder, forearm, wrist and hand (lnt. J. Hum.-Comput. Interaction 11(2) (1999) 79). The control group, using the traditional mouse reported no significant changes in the pain level. In part II, The Anir mouse was given to the control group 6 months after the start of the study. After 6 months of using the Anir mouse, the former control group reported a significant reduction in pain for neck, shoulder, forearm, wrist and hand. The group getting the initial intervention reported still significant reductions of pain in the upper part of the body, 12 months after the start of the study (lnt. J. Hum.- Comput. Interaction 13(1) (2001) 13). The present paper covers the period from 1 to 3 years of the study. By inspection of the mean values of pain over time, no relevant changes seem to have appeared in the period from 12 to 36 months. No significant changes in confounding factors were found during the period from 12 to 36 months. The results from this study indicate the importance of using a more neutral position of the forearm when using a computer mouse. Relevance to industry This paper presents indications that a more neutral position of the forearm when operating a computer mouse is an important source of pain reduction in the upper part of the body. Several researchers have reported a relationship between the overall duration of mouse use and pain development. Further, the time using a computer mouse is steadily increasing due to the widespread application of graphical user interfaces Elsevier Science B.V. All rights reserved. Keywords: VDU work; Mouse use; Musculoskeletal illness *Corresponding author. Tel.: ; fax: address:arne.aaraas@alcatel.no (A. Aanis) /02/$ - see front matter 2002 Elsevier Science B.V. All rights reserved. PH: SO (02)

2 The number of workers and the duration of use of visual display units (VDU) are increasing. In Sweden, 45% of males and 55% of females of the total VDU workforce, used VDU more than 50% of the work time (Arbetarskyddsstyrelsen, Statistiska centralbyran, 1999). The use of graphical user interfaces has increased the use of computer pointing devices such as mice. Wiagaeus Tornqvist et ai. (2000) found in a study of 1555 VDU operators that 96% used a traditional computer mouse. Further, musculoskeletal discomfort is reported to increase due to mouse usage (Fogleman and Brogmus, 1995). A relationship between overall duration of mouse use and pain in the upper extremity is reported (Karlqvist et ai., 1996; Aaras et ai., 1998). Mouse use is documented to create static load in the forearm extensor with very IJ few gaps in the EMG level (Jensen and Laursen, I 2000). The presence of lower number of gaps during work has been shown to be a risk factor for development of musculoskeletal symptoms in the trapezius muscle (Veiersted et ai., 1993). Supporting the forearm is important in reducing the muscle load on the extensors of the forearm when operating a mouse compared to no support or only wrist support (Aaras et ai., 1997; Wahlstf0m et ai., 2000). The underlying mechanism that causes discomfort and pain in the musculoskeletal system is not 1/'_) well known. According to Hagg (1991), long lasting activity of single muscle fibers may overload their capacity and start a damaging process, with pain as an end result. This is supported by Thorn et ai. (2001). They found single motor units to be active min or more. Laubli et ai. (1999) hypothesize that interactions between individual muscle activation patterns and certain repetitive tasks may cause overloading of the involved muscles. The background for this study was based on the following findings in a laboratory study. The workload of the extensors of the forearm was significantly less when using a mouse (Anir, now called Renaissance) giving a more neutral position of the forearm (Fig. 1) compared with a traditional mouse requiring a more pronated forearm. This was true for the extensor digitorum i communis. The same clear tendency was also found regarding extensor carpi ulnaris (Aaras and Ro, 1997). The basic aim of this study was to ask if the significant reduction in pain level of the upper part of the body reported after 1 year will still be present after 3 years, when using a mouse allowing an almost neutral position of the forearm and wrist. The study started as a prospective parallel group design with two groups of VDU workers with reported upper extremity pain. Sixty-seven participants were randomly divided into one intervention group (Anir mouse) and one control group (traditional mouse). More details regarding the design of the study, the study population and their work tasks, the inclusion criteria as well as the intervention are given by Aaras et ai. (1999). All participants were able to support their forearms on the tabletop. After 6 months all participants used the mouse with a more neutral position of the

3 forearm. Therefore, after 12 months both groups were considered as one group, representing 46 participants. Questionnaires regarding demographic data, sick leave, visual conditions and discomfort, headache, musculoskeletal pain and organizational and psychosocial factors that were used in the study are described in detail by Aaras et al. (1999). Confounding factors such as visual and ergonomic conditions as well as organizational and psychosocial factors were collected in order to keep track of any changes during the study period. These questionnaires were used at commencement, and after 6, 12,24 and 36 months. Responses to all questions were made on a 100mm visual analog scale (VAS). The end points of the VAS for all questions related to pain were: "None-Unbearable" These questions asked for: "Average intensity of pain the last month, or the last 6 months." The end points for question regarding how often pain was experienced last month was: "Never-Very often". The end points for questions regarding lighting, glare and visual discomfort were: Lighting "Very bad-very good"; Glare problems "Very much glare-no glare"; Visual discomfort "None-Very much". The end points for Organizational and psychosocial factors are shown in Table 1. Sick leave due to musculoskeletal illness during the last 6 months was reported in number of days. For estimation of the location parameters in assumed continuously distributed variables, means values are used. As an index of dispersion, 95% confidence intervals for the mean are used. For calculation of the confidence intervals for the means, the Student procedure is used. Changes within each group are analyzed by paired t-tests. All tests are performed two-tailed, and p-values less than 0.05 are considered significant. A separate statistical analysis was carried out to investigate possible systematic influence on the results due to subjects who dropped out during the study period. Of particular interest are informative drop-outs, i.e. those subjects whose reasons for dropping out are associated with the use of the particular mouse. To investigate the effect of these drop-outs during the period from 6 to 12 months, we carried forward their worst registration of pain for each of the body areas (neck, shoulder, forearm and writs/hand) to 12 months, and repeated the original analysis (Aaras et ai., 2001). In the present paper, the effects of drop-outs were assessed by comparing the results of the completers (the participants who completed the whole study period) with those of the total group at each time point. If the results are similar, the drop-outs are not likely to have much influence The health outcomes from part I and part II of the study The results from part I and part II of the study regarding average pain intensity during the last 6 months are described in detail by Aaras et al. (1999, 2001). Part I reported that, after using the Anir mouse for 6 months, a significant reductions in group mean VAS values were reported regarding pain in the neck ( ); shoulder ( ); forearm ( ) and wrist/hand ( ) (Aaras et ai., 1999). The control group, using the traditional mouse reported no significant changes in the pain level. In part II, an identical intervention (use of Anir mouse) was given to the control group 6 months after the start of the study. After 6 months of using the Anir mouse, the former control group reported a significant reduction regarding average pain the last 6 months for the following body areas: Shoulder ( ); forearm ( ); wrist/ hand ( ). The group getting the initial intervention reported still significant reduction of pain in the upper part of the body 12 months after the start of the study (Aaras et ai., 2001).

4 Table I End points for organizational and psychosocial factors at VAS Determine which tasks you shall undertake from day to day Determine the amount of work from day to day Opportunity to take unscheduled short breaks Opportunity to make job contacts Opportunity to learn something new Opportunity to increase job skills Opportunity of full utilization of ability Opportunity to contact your job immediate superior How much of the day you feel genuinely satisfied with your job Does the VDU increase stimulation at work Other tasks more physically demanding Other tasks more stressful Other tasks more mentally stimulating Sole provider of the household or share burden with others Person(s) you can rely on and get help from Person in your home needing extra physical effort Person in your home needing extra psychological support The housework from day to day Stressful daily journey to work Time to own disposal before and after work Subjective feeling of tenseness Sports/physical activity Nearly all my work tasks are the same No, not at all No Never Scarcely at all Increases stimulation More physically demanding More stressful More mentally stimulating Sole None No Easy Not acceptable (stressful, tiring, take too long time) None Never None Almost none of my.work tasks are the same Completely Complete Always Very much Reduces stimulation Less physically demanding Less stressful More tedious A lot of support Yes several Yes, to a high extent Difficult Quite satisfying A lot of time Very often Very much This paper will focus on the results of the study period from 12 to 36 months. After 6 months both groups had got the same intervention. Therefore, in the period from 12 months to 36 months, the whole study population is considered as one group. Figs. 2-6 present the results for the entire study period. These figures depict the average pain levels for each of the two groups at commencement, after 6 and 12 months of the study period. The pain level at 24 and 36 months are presented for the whole population still participating. The ratings of average intensity of pain during the last month were very similar to those for average intensity of pain during the last 6 months, therefore only the latter variable is discussed Neck, shoulder, forearm and wrist/hand pain as well as headache at 12, 24 and 36 months of the study period There are no significant changes when comparing the average intensity of pain after 12 months with the average pain level after 24 months or after 36 months for those participants who continued in the study. More important is the fact that the mean values are remaining fairly constant over the period. Sample size, mean VAS values and confidence intervals are presented in Table 2. In this table, the former control group and the intervention group data are combined. See also Figs. 2-6 in which the two groups are depicted separately at 12 months. The

5 Neck pain last 6 months in ~ ~ Intervention Control ~ Total Fig. 2. The intensity of average neck pain during the last 6 months. The values are given as group mean with 95% confidence interval on VAS. Shoulder pain last 6 months 70 G:I ~ 40 E Intervention Control ~ Total Fig. 3. The intensity of average shoulder pain during the last 6 months. The values are given as group mean with 95% confidence interval on VAS. same pattern of results was found for frequency (how often) of pain experienced during the past month. These results are contained in Table 3. For the 32 participants who continued through the whole study period, the pain intensity was significantly lower at 36 months than the pain level before the intervention. Mean VAS difference for

6 Forearm pain last 6 months 70 ~ 40 E E g Intervention Control ~ Total Fig. 4. The intensity of average forearm pain during the last 6 months. The values are given as group mean with 95% confidence interval on VAS. WristlHand pain last 6 months 60 Control ~ Total Fig. 5. The intensity of average wrist/hand pain during the last 6 months. The values are given as group mean with 95% confidence interval on VAS. Baseline vs. 36 months and standard deviations are as follows: neck 23.1 (30.4), p<o.ool; shoulder 32.3 (34.6), p<o.ool; forearm 35.8 (29.9), p<o.ooland wrist/hand 25.4 (25.4), p<o.ool. The above values are for average pain intensity during the last 6 months. For headache no such reduction was found, 6.6 (33.8), p = Also here the mean values were remaining fairly constant during the period from 12 to 36 months, with an exception for pain in the neck.

7 Headache last 6 months 60 Intervention Control III Total Fig. 6. The intensity of average headache during the last 6 months. The values are given as group means with 95% confidence interval on VAS. Table 2 Intensity of pain during the last 6 months. The number of Table 3 observations and group means with 95% confidence interval Frequency of pain during the last month. The number of are given. P-values indicated for the comparison of pain observations and group means with 95% confidence interval between 12 and 24 months (pi) and 12 and 36 months (P2) are given. P-values indicated for the comparison of pain respectively between 12 and 24 months (pi) and 12 and 36 months (p2) respectively 12 months 24 months 36 months pi p2 12 months 24 months 36 months pi p2 Head Head Neck Neck Shoulder Shoulder Forearm Forearm Wrist/hand Wrist/hand

8 5.3. Musculoskeletal sick leave 5.4. Confounding factors The sick leave due to musculoskeletal illness has Analyses of potential confounding factors which been very low during the whole study period. may have influenced outcomes on the primary During the period from 12 to 36 months, two dependent measures throughout the study period persons reported sick leave due to musculoskeletal are presented in Tables 4-9. Each factor represents illness. This variable seemed to be unrelated to use the responses to a questionnaire item presented on of mouse. a 100mm VAS. Each table js divided into two Table 4 The lighting and glare conditions at work are given by the number of observations and means (mm VAS) with 95% confidence intervals. The corresponding values for intensity and frequency of visual discomfort are shown Baseline 6 months 12 months 24 months 36 months (a) Lighting condition Glare condition ~, ( Compared with at home Visual discomfort. Intensity Visual discomfort. Frequency (b) Lighting condition Glare condition Compared with at home Visual discomfort. Intensity Visual discomfort. Frequency

9 A. Aartis et al. I International Journal of Industrial Ergonomics 30 (2002) Table 5 Total time using the VDU and mouse at work and at home are given by the number of observations and means with 95% confidence intervals. The corresponding values for continuous VDU work before a break are also shown Baseline 6 months 12 months 24 months 36 months (a) Working hours per week VDU use at work (h/day) VDU use at home (h/day) fr fr Mouse use at work (h/day) Mouse use at home (h/day) How is your workplace at home vs. work (mm VAS) Continuos VDU work before break (min) (b) Working hours per week VDU use at work (h/day) VDU use at home (h/day) fr Mouse use at work (h/day) fr Mouse use at home (h/day) {) {) How is your workplace at home vs. work (mm VAS) Continuos VDU work before break (min) parts. Part (a) consists of factor averages for each participant in the study measured at each of the 5 stages of the study (Baseline, 6, 12,24,36 months). Part (b) consists of comparable factor averages for only those 32 participants who remained in the study for the entire 36 months. Table 4 contains

10 316 A. Aaras et al. I International Journal of Industrial Ergonomics 30 (2002) Table 6 Organizational and psychosocial factors at work are given by the number of observations and means with 95% confidence intervals (mm VAS) Baseline 6 months 12 months 24 months 36 months (a) The same work tasks every day Determine which tasks you shall undertake from day to day Determine the amount of work from day to day How far in advance do you know the amount of work ~ Opportunity to take unscheduled short breaks Opportunity to make job contacts Opportunity to learn something new Opportunity to increase job skills Opportunity of full utilization of ability Opportunity to contact your immediate job superior How much of the day do you feel genuinely satisfied with the job Does the VDU increase stimulation at work (b) The same work tasks every day Determine which tasks you shall undertake from day to day

11 A. Aaras et al. / International Journal of Industrial Ergonomics 30 (2002) Table 6 (continued) Baseline 6 months 12 months 24 months 36 months Determine the amount of work from day to day How far in advance do you know the amount of work Opportunity to take unscheduled short breaks Opportunity to make job contacts Opportunity to learn something new Opportunity to increase job skills Opportunity of full utilization of ability Opportunity to contact your immediate job superior How much of the day do you feel genuinely satisfied with the job Does the VDU increase stimulation at work factors related to visual discomfort, general lighting conditions and glare problems. Table 5 contains factors related to duration use of VDU and mouse and to assessment of the workplace at work and at home. Table 6 contains factors related to organizational and psychosocial factors at work, including control and decision latitude. Table 7 includes questions regarding other tasks in addition to VDU work. Table 8 contains factors related to the impact of general home and life situational variables on work. Table 8 contains factors related to feelings of tenseness and extent to which sport and physical activity was practiced. With respect to the focus of the present paper, none of the above confounders showed any significant differences across the period of months. This was true for analyses of all subjects who participated (part (a» and only those subjects who remained in the study (part (b». With regard to the total period of the study, there were two confounder factors which yielded significant results. Feelings of tenseness (Table 9(a) and (b» were significant lower at 36 months than at commencement of the study (65.4 vs. 52.7, p = 0.02). Also, there was a significant reduction in visual discomfort between 6 and 12 months, but

12 318 A. Aaras et al. / International Journal of Industrial Ergonomics 30 (2002) Table 7 Other tasks in addition to VDU work are given by the number of observations and means (mm VAS) with 95% confidence intervals Baseline 6 months 12 months 24 months 36 months (a) Other tasks more physically demanding Other tasks more stressful Other tasks more mentally stimulating Q-47.6 (b) Other tasks more physically demanding Other tasks more stressful Other tasks more mentally stimulating Q not between 36 months and the commencement of the study. This finding is discussed in Aanis et al. (2001) and will not be further considered here. In the study period from 6 to 12 months, 16 participants dropped out of the former control group whereas 5 participants dropped out of the initial intervention group. The reason for these drop-outs is depicted in the first two columns of Table 10. From 12 to 24 months, 12 participants left the study. These were considered non-informative drop-outs: nine subjects got a job in other companies outside the Alcatel company, two women gave birth and one subject died. From 24 to 36 months only two subjects left the study. Both reported that they preferred a traditional mouse and were, therefore, informative drop-outs. The number and type of drop-outs during the period months is summarized in column three of Table 10, and the total number of drop-outs over the entire period of the study is summarized in the last column of Table 10. It will be of interest to investigate differences between the drop-outs and those who completed the study (the completers), even though most of the drop-outs are non-informative. Table 11 gives pain levels for those participants who dropped out during the period from 12 to 36 months, compared with the completers. A clearly higher pain level was reported by those who dropped out, for all body areas. However, if we looked at changes from commencement to 12 months among the 14 participants who dropped out during this period (from 12 to 36 months), they also reported a substantial reduction in pain level for several body parts, although somewhat less than the

13 A. Aaras et al. I International Journal of Industrial Ergonomics 30 (2002) Table 8 Work related conditions of the life situation are given by the number of observations and means (mm VAS) with 95% confidence intervals Baseline 6 months 12 months 24 months 36 months (a) Sole provider of the household or share burden with others Person(s) you can rely on and get help from Stressful daily journey to work The housework from day to day Person in your home needing extra physical efforts Person in your home needing extra psychological support Time to own disposal before and after work (b) Sole provider of the household or share burden with others Person(s) you can rely on and get help from Stressful daily journey to work The housework from day to day Person in your home needing extra physical efforts Person in your home needing extra psychological support Time to own disposal before and after work

14 Table 9 Individual factors are given by the number of observations and means (mm VAS) with 95% confidence intervals Baseline 6 months 12 months 24 months 36 months (a) Subjectively feeling of tenseness Sport/physical activity (b) Subjectively feeling of tenseness Sport/physical activity Table 10 Reason for drop-outs at 12 months for intervention and control groups, drop-outs for the combined (total) group from 12 to 36 months, and summary of drop-outs across 36 months for the combined group Intervention group: Control group: Total group: Total group: 12 months 12 months months 36 months Could not use Anir mouse: left handed mouse not available 3 3 Could not use Anir mouse: wrist fracture I I Could not use Anir mouse: Macintosh computer I I Preferred traditional mouse Total informative drop-outs Left Alcatel STK Work tasks with no mouse I I Other 3 3 Total non-informative drop-outs Grand total drop-outs Table II Pain levels at 12 months among the 14 drop-outs from 12 to 36 months compared with the completers Average intensity of pain last 6 months, means with SD (mm VAS) Drop-outs Completers Neck 29.9 (25.0) 24.0 (22.5) Shoulder 31.9 (23.1) 24.6 (23.7) Forearm 24.5 (21.7) 17.7 (22.3) Wrist/hand 26.2 (23.3) 15.0 (20.2) completers: Neck 18.4 (27.9), p = 0.03; shoulder 11.1 (39.9), p = 0.31; forearm 29.4 (29.5), p = 0.003; wrist/hand 20.8 (33.3), p = 0.04 and headache 2.0 (25.0), p = The values are given as mean reduction with standard deviation for the pain level the last 6 months. Confounding factors such as visual and ergonomic conditions, organizational and psychosocial factors were not significantly different when comparing those participants who are left in

15 the study with the total group at each of the 5 measurements (see Table 3(a) and (b) to Table 8(a) and (b)). This may indicate that the drop-outs were not very much different from the completers with regard to these factors. The average pain level for the neck, shoulder, forearm and wrist/hand did not change much when comparing measurements at 36 months with 12 months. Thus, the positive effects of the earlier intervention, after allowing the operators to work with the mouse in a more neutral position of the forearm, have been maintained. The effects resulted in a significant reduction of pain in the neck, shoulder, forearm and hand/wrist reported after 12 months of the study (Aan'is et ai., 1999, 2001). These results are supported by comparing the pain levels for those participants who are still in the study at 36 months with the commencement. For all body areas (neck, shoulder, forearm and hand/wrist) a significant reduction in average pain level was reported (p<0.001). Further, the findings of reduced pain reported are supported by clinical examination. The number of trigger points, pain during movements of the cervical spine and lateral epicondylitis were reduced by comparing before and after intervention (Aan'is et ai., 2001). Exposure in terms of the amount of time using the mouse did not change significantly during the study period (Table 5(a) and (b)). Compliance regarding intervention with the Anir mouse is also important for exposure. The participants had no other mice at their workplace and used only the Anir mouse. This was checked for each of the participants every time the author visited the workplace for distributing the questionnaires (5 visits) or to replace broken mice with new ones. Questions regarding the functioning of the Anir mouse were also asked when the participants visited the Medical Department. The only persons not using Anir the informative participants who dropped out of the study. The reason for reduction in pain level may be the reduction in pronation of the forearm and reduction in extension of the wrist by using Anir mouse. A traditional mouse requires an almost full pronation of the forearm. Such position of the forearm increases the static load on the extensor muscles of the forearm in terms of extensor digitorum communis and extensor carpi ulnaris, which is assessed as a risk factor for musculoskeletal discomfort (Zipp et ai., 1983; Hagberg et ai., 1995). Lower muscle load in these forearm extensors when using an almost neutral position of the forearm compared with a pronated one, is documented by Aan'is and Ro (1997) and Gustafsson and Hagberg (2000) compared the wrist extension between operating a traditional mouse with a mouse requires a neutral position of the forearm. They found the median (50th percentile C)) angle of extension for a traditional mouse was 23.5 vs. a vertial mouse High muscle activity in the forearm is reported to relate to musculoskeletal illness in the upper extremity (Ranney et ai., 1995). Reduced ulnar deviation by using Anir mouse vs. a traditional one may also contribute to reduced pain. Ulnar deviation is avoided by supporting the hand on the base of the Anir mouse. Increased ulnar deviation from a neutral position increase muscle activity in extensor carpi and flexor carpi ulnaris (Zipp et ai., 1983). Further, a relationship between ulnar deviation and musculoskeletal discomfort is also documented (Hunting et ai., 1981; Sauter et ai., 1991). Operating a traditional mouse is shown to require ulnar deviation of the wrist as well as a more outward rotated position of the upper arm in the gleno-humeral joint compared with nonmouse users (Karlqvist et ai., 1994, 1996). Increased flexion and abduction of the upper arm in the gleno-humeral joint are important risk indicators for musculoskeletal illness in the neck and shoulder (Aan'is, 1994; Kadefors, 1994; Sigholm et ai., 1984). However, other features of the design may also be of crucial importance. During postural load measurements of the Anir mouse, even small modifications of the design of the I mouse had great impact on the muscle load of the - extensor digitorum communis and extensor carpi ulnaris. Studying the persistence of the intervention effects, it is of crucial importance to track the confounding factors and evaluate the effects of the drop-outs regarding the health outcomes. Several studies have documented a relation between the

16 pain in the upper extremity and the time the operator uses the keyboard and mouse (Jensen and Christensen, 2001; Brandt et ai., 2001). No significant differences were found regarding these variables during the study period i.e. the amount of VDU work did not change significantly. Other physical work environmental factors for VDU workers are also documented to have a connection to discomfort and pain in the musculoskeletal system (Punnett and Bergqvist, 1997). These factors did not change significantly during the study. A detail discussion of these confounding factors is found in Table 4(a) and (b) to Table 9(a) and (b). Organizational and psychosocial conditions at work and outside work may influence musculoskeletal discomfort or pain (Smith, 1997; Karasek and Theorell, 1990). These factors could hardly be the reason for the observed reduction in pain level during the study period. The confounding factors did not differ significantly during the study period. Further, no significant differences were found for the confounding factors when comparing those who continue in the study with the total group at each of the five measurements. This was also true for psychosocial factors outside work (Frankenhouser, 1989; Karasek et ai., 1987) (see Table 3(a) and (b) to Table 8(a) and (b)). This means that the effect of these factors may have had approximately the same influence on the dependent variables, i.e. the pain level in different body areas during the study period. No ergonomic changes regarding tables and chairs were carried during the study period. The reason for this was that extensive ergonomic improvements of the work tables and chairs were carried out 5-6 years earlier in the company. The reduction in pain level due to the intervention may be modified by the level of pain of those who dropped out of the study. The best way to evaluate this effect seems to be to compare the pain level of those who dropped out with those who continued in the study. Those who dropped out between 12 and 36 months had higher pain level in all body areas compared with those who continued in the study (see Table 11). Therefore, it could be expected that the reduction of pain level in Figs. 2-5 had been less if the 14 subjects who dropped out had continued in the study. However, even those who dropped out reported significant reduction of pain level in the neck, forearm and wrist/hand, but not shoulder. A complete analysis of the effect of drop-outs is of crucial importance before a conclusion is drawn in an epidemiological study. No significant change in headache was found during the study period. The reason may be that headache has multifactorial etiology. A subjective feeling of tenseness seems to be related to musculoskeletal pain (Vasseljen et ai., 1995). Those participants who continued in the study reported a clear reduction in subjective feelings of tenseness. One explanation for these results may be that when pain level is reduced, the subjects relaxed their muscles more compared with when higher level of pain is experienced. The performance cost of using the Anir mouse in terms of reduced speed and accuracy is small compared with the total range of variability exhibited in mouse use (Aariis et ai., 2001). The complexity of the VDU workplace requires that ergonomic consideration must be taken at the planning stage when establishing the work environment for VDU workers (Aariis et ai., 2000). Recommendations for healthier computer work based on scientific evidence provided under "PROCID", stated that each VDU workplace should be supplied with at least three different mice in order to get variation in muscle use (Christensen et ai., 2001). Primary prevention is t7 of utmost importance, since when musculoskeletal d_ illness has reached a chronic state, it seems very difficult to cure the disease, even if the most J important risk factors are reduced to a minimum (Berg and Torell, 1988). This study has shown that a more neutral position of the forearm and wrist/hand when using Anir mouse reduced significantly the pain in the neck, shoulder, forearm and wrist/hand for VDU workers having pain in these areas. The pain reduction lasted for at least 2.5 years.

17 The authors would like to acknowledge great support from management and all participants at Alcatel STK. The authors would like also to thank to Animax International that has financed the statistical analysis. Aaras, A., The impact of ergonomic intervention on individual health and corporate prosperity in a telecommunication environment. Ergonomics 37, Aaras, A., Ro, 0., Workload when using a mouse as an input device. International Journal of Human-Computer Interaction 9, Aaras, A., Horgen, G., Ro, 0., Work with Visual Display Unit: Health Consequences. International Journal of Human-Coumpter Interaction 12 (I), Aaras, A., Fostervold, K.I., Ro, 0., Thoresen, M., Postural load during VDU work: a comparison between various work postures. Ergonomics 40, Aaras, A., Horgen, G., Bj0rset, H.-H., Ro, 0., Thoresen, M., Musculoskeletal, visual and psychosocial stress in VDU operators before and after multidisciplinary ergonomic interventions. Applied Ergonomics 29, Aaras, A., Ro, 0., Thoresen, M., Can a more neutral position of the forearm when operating a computer mouse reduce the pain level for visual display operators? A prospective epidemiological intervention study. International Journal of Human-Computer Interaction II (2), Aaras, A., Dainoff, M., Ro, 0., Thoresen, M., Can a more neutral position of the forearm when operating a computer mouse reduce the pain level for visual display operators? A prospective epidemiological intervention study. Part II. International Journal of Human-Computer Interaction 13 (I), Arbetarskyddsstyrelsen, Statistiska Centralbyran, Statistics Sweden, The working environment (AM 68 SM 0001). Berg, M., Tore11, G., Persistence of musculoskeletal symptoms: a longitudinal study. Ergonomics 31, Brandt, L.P.A., Lassen, C.F., Mikelsen, S., Andersen, J.H., Kryger, A., Overgaard, E., Butcher, I., Thomsen, J.F., Neck and shoulder disorders among technical assistants and machine technicians in Denmark-the Nudata study. Programme and Abstract book of the Fourth International Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders. Nederlands Centrum voor Beroepsziekten Academic Medical Center, Coronel Instituut KO-078, University of Amsterdam Meibergdreef IS, 1105 AZ Amsterdam, P.O. Box 22660, 1100 DD Amsterdam, 130. Christensen, H., S0gaard, K., Sj0gaard, G., the PRocm group, Recommendations for healthier computer work based on scientific evidence provided under "PRO- CID". Programme and Abstract book of the Fourth International Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders. Nederlands Centrum voor Beroepsziekten Academic Medical Center, Coronel Instituut KO-078, University of Amsterdam Meibergdreef 15, 1105 AZ Amsterdam, P.O. Box 22660, 1100 DD Amsterdam, 193. Fogleman, M., Brogmus, G., Computer mouse use and cumulative trauma disorders of the upper extremities. Ergonomics 38 (12), Frankenhouser, M., Stress on and off job as related to sex and occupational status in white-co11arworkers. Journal of Organizational Behaviour 10, Gustafsson, E., Hagberg, M., Evaluation of exposure and acute effects on comfort, perceived excertion and productivity in work with computer mouse in two different hand positions. Proceedings of the NES Arskonferanse Trondheim, Norway, pp Hagberg, M., Silverstein, B., We11s,R., Smith, M.J., Hendricks, H.W., Caryon, P., Perusse, M., Work Related Musculoskeletal Disorders (WMSDs): A Reference Book for Prevention. Taylor & Francis, Ltd., London. Hiigg, G.M., Static work loads and occupational myalgia-a new explanation model. In: Anderson, P.A., Hobart, DJ., Danoff, J.V. (Eds). Electromyographical Kinesiology. Elsevier Science Publishers, Amsterdam, pp Hiinting, W., Laubli, T., Grandjean, E., Postrual and visual loads at VDT workplaces: I constrained postures. Ergonomics 24, Jensen, c., Christensen, H., Duration of computer use and musculoskeletal symptoms. Programme and Abstract Book of the Fourth International Scientific Conference on Prevention of Work-Related Musculoskeletal Disorders. Nederlands Centrum voor Beroepsziekten Academic Medical Center, Coronel Instituut KO-078, University of Amsterdam Meibergdreef IS, 1105 AZ Amsterdam, P.O. Box 22660, 1100 DD Amsterdam, 148. Jensen, B.R., Laursen, B., Musculoskeletal workload during non-linear video editing. Proceedings of the XIV Triennial Congress of the International Ergonomics Association and 44th Annual Meetings of the Human Factors and Ergonomics Society, Ergonomics for the New Millennium, Vol. I, July 29-August 4, 2000, San Diego, CA, USA. Human Factors and Ergonomics Society. P.O. Box 1369 Santa Monica, CA USA, pp Kadefors, R., Welding and musculoskelatal disease: a review. Proceedings of the 12th Triennial Congress of the lea, Vol. 2. Toronto 2, pp Karasek, R., Theore11, T., Healthy Work. Stress, Productivity and the Reconstruction of Working Life. Basic Books, Inc., New York.

18 Karasek, R.A., Gardell, B., Lindell, J., Work and nonwork correlates of illness and behaviour in male and female Swedish, white-collar workers. Journal of Occupational Behaviour 8, Karlqvist, L., Hagberg, M., Selin, K., Variation in upper limb posture and movement during word processing with and without mouse use. Ergonomics 37, Karlqvist, L., Hagberg, M., K0ster, M., Wenemark, M., Anell, R., Musculoskeletal symptoms among computerassisted design (CAD) operators and evaluation of a self-assessment questionnaire. International Journal of Occupational Environmental Health 2, Liiubli, T., Schnoz, M., Weiss, J., Krueger, H., Fatigue and muscle activity in fast repetitive finger movements. In: Bullinger, H-J., Ziegler, J. (Eds.), Human-Computer Interaction, Vol. 1. Lawrence Erlbaum Associates, Inc., Mahwah, NJ, pp Punnett, L., Bergqvist, U., 1997:16. Visual Display Unit Work and Upper Extremity Musculoskeletal Disorders. A Review of Epidemiological Findings. Ergonomic Expert Committee Document No 1. National Institute of Working Life, Solna, Sweden. ISBN , ISSN Ranney, D., Wells, R., Moore, A., Upper limb musculoskeletal disorders in highly repetitive industries: precise anatomical physical findings. Ergonomics 38, Sauter, SL, Schleiffer, L.M., Knutson, S.1., Work posture, workstation design, and musculoskeletal discomfort in a VDT data entry task. Human Factors 33, Sigholm, G., Herberts, P., Almstmm, C., Kadefors, R., Electromyographic analysis of shoulder muscle load. Journal of Orthopaedic Research I, Smith, M.J., Psychosocial aspects of working with video display terminals (VDTs) and employee physical and mental health. Ergonomics 40, Thorn, S., Forsman, M., Zhang, Q., Taoda, K., Motor unit firing patterns in the trapezius muscle during low-level long-duration static contractions. In: Sandsj0, L., Kadefors, R. (Eds.), The Second PROCID Symposium. Prevention of Muscle Disorders in Computer Users. Scientific Basis and Recommendations, 8-10 March G0teborg, Sweden. National Institute for Working Life/West, G0teborg ISBN: Vasseljen Jr., 0., Johansen, B.M., Westgaard, R.H., The effect of pain reduction on perceived tension and EMGrecorded trapezius muscle activity in workers with shoulder and neck pain. Scandinavian Journal of Rehabilitation Medicine 27, Veiersted, K.B., Westgaard, R.H., Andersen, P., Electromyographic evaluation of muscular work pattern as a predictor of trapezius myalgia. Scandinavian Journal of Work Environmental Health 19, Wahlstmm, J., Svensson, J., Hagberg, M., Johnson, P.W., Working with the computer mouse-differences between work and gender. Scandinavian Journal of Environmental Health 26 (5), Wiagaeus Tornqvist, E., Karlqvist, L., Hagberg, M., Hagman, M., Hansson Risberg, E., Isaksson, A., Toomingas, A., Musculoskeletal disorders and working conditions among male and female computer users. Proceedings of the NES Arskonferanse Trondheim, Norway, pp Zipp, P., Haider, E., Halpern, N., Rohmert, W., Keyboard design through physiological strain measurements. Applied Ergonomics 14,