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Workload When Using a Mouse as anInput DeviceArne Aaras & Ola RoPublished online: 13 Nov 2009.

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INTERNAllONAL JOURNAL OF HUMANXOMPUTER INTERACTION. 92). 105-118 Copyrbht 0 1997. Lawrence Erlbaum Associates, Inc.

Workload When Using a Mouse as an Input Device

Arne Aarhs Alcatel STK AS, Oslo, Norway

Ola Ro Premed AS, Oslo, Norway

A newly developed mouse, which seemed to give the operator a more neutral forearm position, was compared with a traditional mouse. The muscle load was recorded by electromyography from the extensor digitorum communis, extensor carpi ulnaris, and trapezius muscles. The muscle load of the forearm was significantly less when using the new mouse compared with the traditional one. This was true for the extensor digitorum communis regarding the static (p = ,0005) and the median ( p = .001) values of the amplitude distribution function (ADF) and the number of periods per minute when the muscle load was below 1% of maximum voluntaly contraction ( p = .03). The sameclear tendency was also found regarding the static muscle load from theextensor carpi ulnaris (p = .06). Theseresultsindicate theneed for reducing the pronation of the forearm when working with a mouse.

1. INTRODUCTION

Beginning in the 1970s, visual display units (VDUs) began to be introduced for ordinary office work. In the last decade, the computer has been very widespread for routine use in almost all workplaces in offices and production units. Wilkins (1983) anticipated that 60% of the workforce in North America will be VDU users before the next century.

In fact, most of the work tasks in offices are connected to VDU work. Develop- ment of software programs like windowso and the integrated computer network have given the operator the opportunity to obtain almost all information at the computer. Thus, the time per day spent using the computer and particularly the time spent using the mouse as an input device has been increasing.

Johnson, Dropkin, Hewes, and Rempel(1993) found that mouse use can account for up to two thiids of computer operation time depending on the task performed and the software used. Bergqvist (1993) compared VDU work with non-VDU work. He found a cumulative incidence of 2.8 for discomfort in the forearm and hand

Requests lor wprints should be sent to Arne Aaras, Alcatel STK AS, P. 0. Bax 60. Okem, N-0508Oslo, Norway.

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106 AarAs and Ro

compared with non-VDU workers. Furthermore, a dose-response relation between VDU work and hand-wrist problems was indicated (Bergqvist, Knave, Voss, & Wibom, 1992).

Karlqvist and Hagberg (1994) compared work posture and movements of the arm and hand for workers who used a mouse against workers who did not use a mouse. Mouse operators spent 34% of the analyzed working time in the internal 15" to 30" ulnar deviation of the wrist. The corresponding percentage for those workers not using a mouse was 2%. Mouse operators had a more outward rotated position of the upper arm in the shoulder joint compared with nonmouse users. The more exkeme working posture for mouse operators can cause a risk for discomfort in shoulders, elbows, and wrists. Twisting of the hands toward theulnar side is related to an increase in muscle fatigue of the arms (Laubli, 1986).

Karlqvist and Hagberg (1994) found that the prevalence rate ratio for thecategory of highest intensity symptoms of shoulder and upper arms was 6.3 for those who had nonoptimal placement of the computer mouse. Their calculation of expo sureeffect relations supported the hypothesis that long durations in nonoptimal positions of the computer mouse on the table gave higher prevalence rate ratios of symptoms in the upper limb.

Postural angle of the lower arm and hand, as well as pain in these areas, are correlated toulnar abduction of the hand in the wrist (Hunting, Laubli, &Grandjean, 1981). Hunting et al. (1981) recommended that the ulnar deviation of the hand in the wrist should be less than 20'. Preferably, the hand should be held in the most neutral position possible in the wrist joint.

Posture at the extreme of joint range of motion may require high muscle forces as seen in maintaining the forearm in full pronation. This is the case in keyboard work that requires high muscle activity (Zipp, Haider, Halpern, & Rohmert, 1983). Pronation of the forearm is considered to be a potential risk factor for musculoskele- tal illness in theelbow and forearm. Thismay be due toa steep increasein theactivity of pronator teres and pronator quadriceps over 60" pronation (Zipp et al., 1983). It is also stressful to flex the elbow and keep the forearm pronated because biceps brachii is a prime supinator of the forearm. Muscles crossing many joints need greater exertion to produce a given force than biceps brachii.

Musculoskeletal discomfort for VDU workers is reported at a high prevalence in many studies. The reasons for this high prevalence seem to be multifactorial and not fully understood. Several studies have documented a relation between trapez- ius load (particularly the static part of the load) and development of musculoskele- tal discomfort in the upper part of the body (AarAs, 1994; Erdelyi, Sivonen,Helin, & Hanninen, 1988; Hagberg, 1981; Jensen, Nilsen, Hansen, & Westgaard, 1993; Veiersted, Westgaard, & Andersen, 1990, 1993). It has also been found that an increase in the number of micropauses (periods below 1% MVC) seems to reduce the incidence of such discomfort (AarAs, 1994; Veiersted et al., 1990, 1993). Aaris, Fostervold, Thoresen, and Larsen (1995) reported that supporting the whole fore- arm on the tabletop in front of the operator seems to be of fundamental importance for reducing the static trapezius load when using the keyboard and mouse. Sup- porting only the wrist during such work has been documented to increase the trapezius load (Bendix & Jessen, 1986).

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Workload When Using a Mouse as an Input Device 107

The newly developed mouse seemed to give theoperator a moreneutral forearm and wrist joint position compared with a traditional mouse design. According to a priori knowledge, this may reduce the muscle load in the forearm.

2. AIM OF THE STUDY

The aim of this study was to compare the load on the musculoskeletal system when operating two different types of mice (a newly developed mouse and a traditional mouse design) both with and without support of the forearm. The muscle load was assessed by measuring the electromyography (EMG) from the extensors of the forearm and the trapezius muscles.

3. MATERIAL AND METHODS

3.1. Types of Mice Investigated

One type of mouse was a newly developed one (International Patent Application No. KT-N096-00197), referred to as Mouse A (Animax International AS, Norway) in this article (see Figure 1). The second type was the MicrosoftTM serial mouseType 2.OA, referred to as Mouse Bin this article (see Figure 2). Mouse A may be operated

FIGURE 1 Mouse A

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Aarhs and Ro

- . . . .

FIGURE 2 Mouse B

with the hand and forearm in a more neutral position compared with Mouse B. Mouse B normally requires more pronation of the forearm, which creates more muscle load of the extensors (Zipp eta]., 1983).

3.2. Study Population

The study population consisted of 13 healthy male VDUoperators (see Table 1). All operators had experience using a mouse. The participants had 2 days of training with Mouse A before the test.

3.3. Design of the Study

The study was a parallel randomized block design. Seven participants were allo- cated tostart withMouseA, whereas theother six wereallocated to start withMouse B. The work consisted of "painting" small squares on the screen by using the Paintbrush program. This task was chosen because it requires very precise motor control of the movements of the mouse. It may be representative for the mouse usage in many design tasks. The participants worked for approximately 30 min, and after a 10-min break they continued for another period of approximately 30 min with the other mouse. They worked in a sitting position with the hands at elbow height and

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Wofiload When Using a Mouse as an Input Device 109

Table 1: Participant Population

M Mdtt Range

Age (years) 47.6 47.0 3342 Height (cm) 182.4 182.0 174494 We~ght (kg) 81.8 76.0 7 M 1 0

supported the whole forearm on the tabletop. This was true when using bothMouse A and Mouse 8.

After this study, acorrespondingstudy was performed, the only difference being that the participants worked with no support of the forearms.

3.4. EMG

Selection of muscles that represent the load of the body area of interest are impor- tant. In order to assess the load on the forearm, the musculus extensor carpi ulnaris and the musculus extensor digitorum communis were selected. The static load and the fatigue effects are generally larger on the extensor compared with the flexor of the forearm (Hagg & Milerad, 1996). Therefore, extensor muscles were selected as the risk muscles for high static load due to theirstabilizingfunctions of these muscles when using the mouse as an input device. The EMG procedure was carried out by using the Physiometer (Premed AS, Oslo) c o ~ e c t e d to a personalcomputer (AarAs, Veierad, Larsen, Orentgren, & Ro, 1996; see Figure 3). The EMG was also recorded from the upper part of trapezius muscle in order to assess the postural load of neck and shoulder. The upper part of the trapezius is static loaded during movements of the upper arm. Surface electrodes were used. The electrical signal in terms of the root mean square (RMS) value was calibrated to muscle force. Simultaneous values of force and (EMG,,,) were recorded up to the maximum force level of the actual work. Then, the EMGforce relation was calculated by h e a r regression. The EMG,, is normalized to the force at MVC, giving a percentage (AarBs et al., 1996). The electrical signal may vary due to many factors, including the electrode position. In order to have comparable quantities, the signal was converted to percentage MVC (Aariis eta]., 1996). Several methods for calibration of EMG exist. One method that has been shown to bereproduciblewas described by AarBset al. (1996). In this study, the comparison between the muscleload recorded when using the two typesof mice could also use microvolt instead of MVC because the electodes were not removed during the period of the study. However, MVC was used in order to compare the muscle load with results from other studies.

Quantification of the muscle load was done by ranking the interval estimates (0.1 sec) to produce an amplitude distribution function (ADF) according to Jonsson (1982). Static, median, and peak load are defied as the ADF levels 0.1,0.5, and 0.9, respectively. That means that the load is equal or higher than the given level in 90% (static), 50% (median), and 10% (peak) of the recording time.

For development of musculoskeletal illness, both the muscle load, quantified in terms of ADF, as well as the number and distributionof time periods with low levels of muscular activity may be important factors (AarBs, 1994; Veiersted et al., 1993).

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I

FIGURE 3 Physiorneter with four channels of EMG and six channels for angle r~ordings.

Therefore, an analysis of low levels of muscle activity was performed. MVC levels of 1% were chosen and the number and total duration of periods below these levels were calculated from the EMG,,, values.

EMG recordings were performed with an optimal adjustment of the workplace for each participant.

3.5. Statistical Methods

This was a simple crossover designed study with two periods (one with Mouse A and the other with Mouse B). Possiblecarryover effects wereinvestigated by adding

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Workload When Using a Mouse as an Input Device 111

the responses in the two periods and comparing the group that started with Mouse B to the group that started with Mouse A. No carryover effects were found. The two types of mice were then compared by using the Wilcoxon signed-rank test. All tests were two tailed with a significance level of 5%. The results are presented as median with 95% confidence intervals.

4. RESULTS

4.1. Supporting the Forearm on the Tabletop

There was a significantly smaller muscle load on the right forearm in terms of extensor digitorurn communis when using Mouse A compared with Mouse B. The right extensor carpi ulnaris also showed a clear tendency to reduce static load when using Mouse A. Furthermore, the number of periods below 1% MVC (micropauses) was significantly higher regarding the extensor digitorum communis and musculus trapezius when comparing Mouse A with Mouse B.

The static load on the extensor digitorum communis was significantly less when using Mouse A; the group median values with a 95% confidence interval were 4.5 (2.1-7.0)% MVC compared with Mouse B, which was 10.8 (7.2-13.5)% MVC (p = ,0005; see Figure 4). The same clear tendency was found for the extensor carpi

Ttapszms Ext. Cupi U1nui.i E n . Dig. Comm.

flGURE 4 Static muscle load with support of forearm ( p = .I, in MVC) for hapezius, extensor carpi ulnaris, and extensor digitorurn cornmunis as median values with 95% confidence interval for the group using Mouse A (dark columns), and Mouse B (gray columns). An asterisk indicates a significant difference when comparing the column containing a plus sign. An (asterisk) is very close to a significant difference.

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112 AarBs and Ro

ulnaris: 1.7 (1.4-4.2)% MVC for Mouse A versus 2.7 (2.CL5.5)% MVC for Mouse B (p = .06; see Figure 4). There was no significant difference between the static trapezius load when the operators used Mouse A versus Mouse B @ =.96; see Figure 4). The static trapezius load was very low and the group median values were less than 0.5% MVC.

The median muscle load was significantly less on the extensor digitonm com- munis using Mouse A, 10.3 (5.1-15.0)% MVC as compared with Mouse B, 17.0 (11.&22.5)% MVC (p = ,001; see Figure 5). A tendency to lower muscle load on extensor carpi ulnaris was also found: 4.9% MVC versus 5.2% MVC, respectively (p = .11; see Figure 5). No significant difference was found when comparing the two mice for the median trapezius load (p = .53). The median trapezius load was less than 1% MVC for the two mice (see Figure 5).

The number of periods per minute when the load is below 1% MVC was significantly higher for extensor digitorum communis when using Mouse A, 0 (CL3.2) versus 0 (0-0) for Mouse B ( p = .03; see Figure 6). Such micropauses were also significantly higher when recording from trapezius, 37.0 (Mouse A) versus 27.0 (Mouse B; p =.005; see Figure 6). There was also a tendency for a higher number of periods below loh MVC for extensor carpi ulnaris when using Mouse A (4 periods per min) versus Mouse B (0 periods per min; p = .16; see Figure 6). Table 2 summarizes the results.

Trapezius En. Carpi Ulnaris E n . Dig. Comm.

FIGURE5 Median muscleload with supportof forearm(p = .5) for trapezius,extmor carpi ulnaris, and extensor digitorum communis as median values with 95% confi- dence interval for the group using Mouse A (dark columns) and Mouse B (gray columns). An asterisk indicates a significant difference when comparing with the column containing a plus sign.

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Workload When Using a Mouse as an Input Device 113

u 10

I + I I *

0 I

Trapenus En C q l ULnans En Dlg. Comm.

FIGURE 6 Number of periods per minute with support of forearm when the muscle load is below 1% MVC for trapezius, extensor carpi ulnaris, and extensor digitorum communis as median values with 95% confidence interval for the group using Mouse A (dark columns) and Mouse B (gray columns). An asterisk indicates a significant difference when comparing with the column containing a plus sign.

Table 2: Muscle Load With Support of the Forearm for Trspedus, Extensor Carpi Ulnaris, and Extensor Digitorurn Communis

Momc A Mouse B

G m a p Group Median 95% CI Median 95% C1 I'

Trapezius Static (?bMVC) 0.2 0.14.5 0.1 0.1-1.17 .96 Median VoMVC) 0.7 0.44-1.28 0.5 0.4Ck2.78 .53 Number of periodslmin 37.0 27.G70.9 27.0 11.H1.5 ,005

Extensor carpi ulnaris Static (%MVC) 1.7 1.44-4.15 2.7 2.0-5.46 .06 Median (%MVC) 4.9 3.0W1.36 5.2 4.Ck9.81 .I1 Number of perioddmin 4.n 0.11-11.21 0.0 0.0-1.21 .I6

Extensor digitorum communis Static (%MVC) 4.5 2.1-7.01 10.8 7.2-13.5 .MH)5 Median (%MVC) 10.3 5.614.95 17.0 11.6-22.5 .001 Number of ~eriods/min 0.0 0.0-321 0.0 0.0-0.0 .03

Nole. ADF = amplitude distribution function; MVC = maximum voluntary contraction

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114 Aarbs and Ro

4.2. No Support of the Forearm

A significantly smaller muscle load on the extensor digitorurn communis and extensor carpi ulnaris were recorded when comparing Mouse A with Mouse B. The group median values with 95% confidence interval of the static load on the extensor digitorurn communis were 3.3 (1.6-6.0)% MVC versus 11.2 (7.3-12.1)"h MVC (p = ,0002; see Figure 7). Corresponding values for the extensor carpi ulnaris were 1.9 (1.1-5.0)% MVC versus 2.9 (2.3-4.7)% MVC (p = .06; see Figure 7).

The median values of muscle load on extensor digitorum comrnunis were s~gnificantly lower using Mouse A compared with Mouse B, 7.6 (5.3-13.6)% MVC versus 17.4 (11.5-20.2)% MVC, respectively (p = .002; see Figwe 8). For the extensor carpi ulnaris, the recordings showed significantly less load when using Mouse A compared with Mouse B, 4.5 (2.9-10.4)% MVC versus 6.1 (4.7-11.0)% MVC (p = .03; see Figure 8).

The number of periods per minute was significantly higher for the operators using Mouse A compared with Mouse B. For the extensor dlgitorum comrnunis, the values were 2 periods versus no periods, respectively (p = .004; see Figure 9). Corresponding values for the extensor carpi ulnaris were 8 periods versus no periods (p = ,005; see Figure 9).

0 1 I I + Tnpczios Ext. Carpi UlnarB E n . Dig. Comm.

FIGURE 7 Static muscle load without support of forearm ( p =.I) for trapezius, extensor carpi ulnaris, and extensor digitorum communis as median values with 95% confidence internal for the group using Mouse A (dark columns) and Mouse B (gray columns). An asterisk indicates a significant difference when comparing with the column containing a plus sign. An (asterisk) is very close to a significant difference.

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Tnpczios Ea. C q i ULnarip Ea. Dig. Comm.

FIGURE 8 Median muscle load without support of forearm ( p = .5) for trapezius, extensor carpi ulnaris, and extensor digitomm communis as median values with 95% confidence interval for the group using Mouse A (dark columns) and Mouse B (gray columns). An asterisk indicates a significant difference when comparing with the column containing a plus sign.

En. Carpi Ulnaris E n . Dig. Comm.

FIGURE 9 Number of periods per minute without support of forearm when the muscle load is below 1% MVC for trapezius, extensor carpi ulnaris, and extensor digitorum communis as median values with 95% confidence interval for the group using Mouse A (dark columns) and Mouse B (gray columns). An asterisk indicates a significant difference when comparing with the column containing a plus sign.

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Workload When Using a Mouse as an Input Device 117

porting the forearm reduces the muscle load on the trapezius. These results are further supported by Aards e t al. (1995).

6. CONCLUSION

The muscle load of the forearm was less when using the newly developed Mouse A compared with the traditional Mouse B.

The following measured variables were significantly different:

Staticand medianvalues of ADFand thenumber of periodsperminute when the muscle load was below 1% MVC for the extensor digitomm comrnunis both with and without support of the forearm. Median value of the ADF and the number of periods per minute when the muscle load was below 1% MVC for the extensor carpi ulnaris when not supporting the forearm. Number of periods per minute when the trapezius load was below 1% MVC when supporting the forearm.

The static value of the ADF for the extensor carpi ulnaris both with and without support of the forearm was very close to a significant difference.

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