Mouse versus keyboard use: A comparison of shoulder muscle load

International Journal of Industrial Ergonomics 22 (1998) 351—357

Mouse versus keyboard use:A comparison of shoulder muscle load

Annabel Cooper!, Leon Straker",*! Suite 3, 647 Wellington St, Perth, Western Australia 6000, Australia

" School of Physiotherapy, Curtin University of Technology, Selby St, Shenton Park, Western Australia 6008, Australia

Abstract

Use of the computer mouse as an input device at visual display units is increasing, yet few studies could be found thatdirectly addressed related musculoskeletal problems. Tasks similar to mousing, such as keyboarding, have been shown toresult in static muscle loading of the shoulder, therefore the potential may also exist for increased neck and upper limbdisorder with mouse use. The major aim of this pilot study was to compare the dominant shoulder muscle load fromupper trapezius and anterior deltoid, gross postures and discomfort during mousing and keyboarding. This wasperformed in an effort to investigate whether use of the computer mouse is likely to be a risk factor for neck and upperlimb disorders and the extent of this risk compared to keyboarding.

Eight subjects performed a 10 min computer task with both mouse and keyboard input. Electromyographs of anteriordeltoid and upper trapezius muscles, observations of posture and ratings of discomfort were collected. Group differencessuggested increased anterior deltoid loads with mouse use and decreased trapezius loads. Also evident was theconsiderable inter-individual variation in muscle loads. This may help explain the pattern of work-related neck andupper limb disorders.

Relevance to industry

Investigation of shoulder muscle loading during mousing may help identify risk factors contributing to the develop-ment of potentially associated neck and upper limb disorders. Costly negative effects of these disorders to industry caninclude workers compensation claims, decreased productivity and reduced user satisfaction and well being. Once possiblerisk factors such as shoulder load are identified, approaches to their minimisation in industry may then be ad-dressed. ( 1998 Elsevier Science B.V. All rights reserved.

Keywords: Work-related neck and upper limb disorders; Electromyography; Anterior deltoid; Upper trapezius;Computer mouse; Keyboarding

*Corresponding author. Tel.: 61 8 9266 3634; fax: 61 8 92663636; e-mail: [email protected].

0169-8141/98/$19.00 Copyright ( 1998 Elsevier Science B.V. All rights reservedPII S 0 1 6 9 - 8 1 4 1 ( 9 7 ) 0 0 0 8 8 - 7

1. Introduction

Use of the computer mouse as an input device atvisual display units is increasing (Hagberg, 1994).Since its invention in 1975 (McIntosh, 1994) themouse has either been used in combination with thekeyboard (Armstrong et al., 1994) or replaced it(Karlqvist et al., 1994). Moreover, software appearsto be becoming more mouse-oriented with theevolution towards graphical computing environ-ments (Rempel et al., 1994). Thus, it seems thatcomputers are becoming not only more numerousbut more reliant upon mouse use.

Despite the prevalence of mice, few studies couldbe found that directly addressed related musculo-skeletal problems in the upper limb (Hagberg, 1994;Karlqvist et al., 1994). Several studies have indirect-ly discovered subjective complaints associated withmouse use (Attwood, 1989; Franzblau et al., 1993;Karlqvist and Hagberg, 1994), with Karlqvist andHagberg (1994) finding these to be more prominentin the proximal rather than distal upper limb.Others have suggested an increased incidence ofupper limb injuries associated with mouse use (Ar-mstrong et al., 1994; Rempel et al., 1994). Karlqvistet al. (1994) performed a study to investigate differ-ences in upper limb posture and movement duringword-processing with and without mouse use. Theyfound the mouse users worked in shoulder flexionand external rotation whereas the non-mouse usersworked in shoulder extension and internal rotation.Unfortunately, their “observation method was notvalidated or tested for reliability” (p. 1267). Theirapproach also appeared to be somewhat flawed, ascalculations and conclusions about pure wrist devi-ation and pure shoulder rotation were based onmeasurements of combined movements. Therefore,their results should probably be viewed cautiously.

Tasks similar to mousing, such as keyboarding,have been shown to result in static muscle loadingof the shoulder (Aaras, 1994a); therefore, the poten-tial may also exist for increased neck and upperlimb disorders with mouse use. Research into pos-tural shoulder load associated with keyboard andlight assembly tasks has been fairly extensive.

Hagberg and Sundelin (1986) and Aaras andWestgaard (1987) found that increased shoulderflexion resulted in increased static postural load as

measured from upper trapezius. Furthermore,Aaras (1994b) found that “postural load clearlyinfluenced the development of musculoskeletalsick-leave” (p. 1687).

If tasks performed using a mouse also contributeto neck and upper limb disorders, work at visualdisplay units could have costly ramifications tothe growing number of computer users in the com-munity. Negative effects could include financialexpenses as a result of escalating workers compen-sation claims, diminished productivity due to losttime at the workplace, and reduced job quality, aswell as decreased user satisfaction and well-being.

The major aim of this pilot study was to comparethe dominant shoulder muscle load measuredfrom upper trapezius and anterior deltoid, grosspostures and discomfort during mousing and key-boarding. This was performed in an effort to inves-tigate whether use of the computer mouse is likelyto be a risk factor for neck and upper limb dis-orders and the extent of this risk compared tokeyboarding.

2. Methods

This research involved two main factors, namelythe type of computer input device and time. Thecomputer input device was either keyboarding ormousing. Subjects therefore participated in a 2]5repeated measures study with electromyographic(EMG) muscle activity measured from both an-terior deltoid and the descending part of uppertrapezius. It was hypothesised that mean EMGactivity levels for mousing would be greater thanthose for keyboarding in both muscles.

Eight post-graduate students volunteered as sub-jects for this pilot study. To be included, subjectshad to be 18 to 35 years of age inclusive in an effortto reduce age-related musculoskeletal disorders.Subjects were excluded if they failed to give writteninformed consent, had a pre-existing neck and up-per limb disorder, a history of skin reactions, anyvisual inadequacy not adequately corrected, anyneurological or systemic illness which may haveimpaired performance, or were pregnant. Subjectswere free to withdraw from the study withoutprejudice in the event of withdrawal of consent,

352 A. Cooper, L. Straker / International Journal of Industrial Ergonomics 22 (1998) 351—357

presence of new extraneous variables, or failure toparticipate in both tasks.

The independent variables included the type ofinput device used for the computer task and time.Both the keyboard and mouse were of stan-dard issue with the Macintosh IIsi computer. EMGsamples were taken at 6, 7, 8, 9 and 10 min into thetask.

Three dependent variables were collected: electro-myography, observations and discomfort. Ampli-tude of EMG muscle activity was collected via10 mm bipolar surface electrodes placed parallel tothe muscle fibres. For anterior deltoid, the elec-trodes were centred over the muscle bulk with18 mm interelectrode distance (Mangharam, 1992).For upper trapezius, the electrodes were placed atthe anterolateral margin of the descending portionof the muscle at approximately mid distance be-tween occiput and acromion with 30 mm interelec-trode distance (Lannersten and Harms-Ringdahl,1990). Raw EMG data was sampled during five 10 stime periods while using each input device. Thefour channels of EMG signals were conductedthrough electrode leads via a Medelec pre-amplifierto a Medelec MS6 oscilloscope. Once there, theEMG was passed through the amplifier incorpor-ated into the oscilloscope. The high-pass filter wasset at 8 Hz and the low pass at 320 Hz (Hagbergand Sundelin, 1986). The EMG activity was thendigitally converted via Superscope II on an AppleMacII computer. A sampling frequency of 1000 Hzwas used. A mean value was calculated for each 10 ssample.

Observations were made regarding the amountof upper limb resting on the worksurface during thetask just before starting each EMG collection.

Discomfort was collected after the performanceof each task according to a 101-point NumericRating Scale (Jensen et al., 1986).

Workstation set-up was controlled across sub-jects. Equipment included a fully adjustable chairwith lumbar support, a height-adjustable desk(PayCo, Perth, Australia), a personal computer(Macintosh IIsi, Macintosh, Australia) with stan-dard screen, keyboard and mouse, a fully-adjustable monitor arm (VMA007GCL, Armdec,Australia) for the screen and a footstool (Z-Rest,Ridge Enterprises, Australia). This equipment was

adjusted for each subject according to the Austra-lian Standard for screen-based workstations (Stan-dards Australia, 1990).

All subjects preferred the right hand for mouseuse. The task was consistent across input devicesand entailed playing a computer game called‘Blobbo’ for 10 min. Screen objects were manipu-lated using ‘d’, ‘k’, ‘t’ and ‘space’ keys with left andright hand during keyboard use and were manipu-lated by the mouse in the right hand during mouseuse. Ethical approval was obtained, and each sub-ject was required to read an information documentand sign a consent form before being allowed toparticipate in the study.

3. Results

To assess group differences between mousingand keyboarding, subjects’ mean EMG values fromeach time period for each task were analysed usingtwo repeated measures 2]5 ANOVAs, one foranterior deltoid and one for upper trapezius. Forboth muscles, the statistics package, Super-ANOVA, was used to calculate a mean value andstandard error for each task. The alpha criterionlevel was set at 0.05. The statistics package, SPSS,was used to calculate the power of the statisticaltests.

Group mean (standard error) EMG activity levelfor anterior deltoid was 183 mV (129 mV) duringmousing and 104 mV (67 mV) during keyboarding(see Fig. 1). Statistical testing revealed F

1,7"1.027

(p"0.3446) and power of 0.145 for the effect ofinput device. The effect of time and the time-inputdevice interaction were not significant.

Group mean (standard error) EMG activity levelfor upper trapezius was 76 mV (22 mV) duringmousing and 137 mV (56 mV) during keyboarding(see Fig. 2). Statistical testing revealed F

1,7"1.071

(p"0.3351) and power of 0.148 for the effect ofinput device. The effect of time and the time-inputdevice interaction were not significant.

While EMG data shown above appear to suggestone-way trends for differences between mousingand keyboarding, individual differences were vari-able. Subjects’ mean EMG values from each timeperiod were averaged for each task. Then, for each

A. Cooper, L. Straker / International Journal of Industrial Ergonomics 22 (1998) 351—357 353

subject, the mean value for keyboarding was sub-tracted from the mean value for mousing. Thisproduced values for the individual differences inEMG activity between mousing and keyboarding.Individual differences for anterior deltoid rangedfrom #617 to !56 mV (see Fig. 3). Individualdifferences for upper trapezius ranged from #66to !239 mV (see Fig. 4). During keyboarding,seven of the eight subjects rested their wrists on theworksurface while one of them rested the base ofthe palm of the hands. During mousing, six of theeight subjects rested the entire forearm while the

Fig. 1. Group mean EMG activity for anterior deltoid duringmousing and keyboarding.

Fig. 2. Group mean EMG activity for upper trapezius duringmousing and keyboarding.

other two rested either the distal half of the forearmor the wrist.

Two of the eight subjects complained of discom-fort in the neck or upper limb (see Table 1).

4. Discussion

Although not statistically significant, this pilotstudy appears to demonstrate group trends to-wards higher muscular load for anterior deltoidand lower muscular load for upper trapezius duringmousing compared to keyboarding.

These group trends could be explained by thepostures that subjects adopted. Higher EMG activ-ity in anterior deltoid during mousing (see Fig. 1)may be associated with greater shoulder flexion, assuggested by Karlqvist et al. (1994) and anecdotalevidence. DeGroot (1987) found anterior deltoidactivity to increase significantly with increasedshoulder flexion, while Mangharam (1992) founddiscomfort to increase significantly with increasedshoulder flexion during a computer task. If it isaccepted that mousing involves greater shoulderflexion than keyboarding and mousing also resultsin higher EMG activity for anterior deltoid, in-creased muscular load on anterior deltoid may thenbe able to explain discomfort experienced by mouseusers and perhaps even associated neck and upperlimb disorders.

In studies examining activities similar to mouseuse, Aaras and Westgaard (1987) and DeGroot(1987) found static upper trapezius load to be sig-nificantly greater with increased shoulder flexion.Assuming that mousing is also synonymous withgreater shoulder flexion, the group trend for de-creased EMG activity in upper trapezius duringmousing (see Fig. 2) does not appear to concur withthis literature. The observed tendency duringmouse use to support the whole forearm, however,may have acted to unload upper trapezius.Milerand and Ericson (1994), in their study ofsimulated dental work, found that supporting thearm, but not the hand alone, resulted in signifi-cantly decreased load on upper trapezius. Also,Schuldt et al. (1987) found that a reduction in uppertrapezius EMG activity is obtained with either el-bow support or arm suspension while performinglight assembly work.


Fig. 3. Individual differences in mean EMG activity between mousing and keyboarding for anterior deltoid.

Fig. 4. Individual differences in mean EMG activity between mousing and keyboarding for upper trapezius.

Group trends in this study could be used tosuggest that the neck and proximal upper limb maybe particularly at risk during mousing if the useradopts a posture of excessive shoulder flexion anddoes not have the entire forearm supported.

Individual differences between keyboarding andmousing were unexpectedly variable (see Figs. 3and 4). High EMG activity levels were present incertain muscles during certain tasks in only a few of

the subjects. This could be indicative of certainpeople being predisposed to injury despite perform-ing the same task on the same equipment as otherusers. This hypothesis is supported not only byanecdotal evidence provided by ergonomists but bythe scientific literature. For example, Aaras (1991)suggests that stress factors, such as work outsidethe official work environment and psychologicalproblems, influence muscle load. It may therefore


Table 1Discomfort data

Discomfort Area Score/100

Subject 1 keyboard Forearms Left — 20 right — 70Subject 1 mouse Forearms Left — 0 right — 70Subject 5 keyboard Bilateral neck 2Subject 5 mouse Bilateral neck 5

prove beneficial to screen workers exposed to com-puter use and offer intervention to those found tobe at potential increased risk.

Interestingly, subjects in this study who com-plained of discomfort also recorded greater EMGactivity during mouse use than keyboard use forboth anterior deltoid and upper trapezius. The highEMG activity and discomfort scores during mous-ing may be indicative of precursors to the develop-ment of neck and upper limb disorders, as found byAaras (1994b) in light assembly workers.

The power of this pilot study was limited dueto the small sample size and was likely to havecontributed to the failure to detect significant dif-ferences in muscular load between mousing andkeyboarding. Also, although EMG has been foundto be a reliable and valid indicator of postural loadin the neck and shoulder regions (Aaras, 1994a),difficulty was encountered with eliminating rela-tively large electrocardiograph noise from the elec-tromyograph signals. This was likely to havefurther interfered with the detection of any differ-ences between mousing and keyboarding.

5. Conclusions

This pilot study found group trends towardshigher muscular load on anterior deltoid and lowermuscular load on upper trapezius during mousingcompared to keyboarding. These trends could beexplained by anterior deltoid being more activeduring mouse use due to increased shoulder flexion,and upper trapezius being less active due to a tend-ency to rest the forearm on the worksurface. If thetrends are representative of undetected significantdifferences, they could prove to have importantimplications.

This investigation also showed individual differ-ences between muscle activity for mousing and key-boarding to be unexpectedly variable. This couldbe indicative of certain people being predisposed toinjury.

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Mouse versus keyboard use: A comparison of shoulder muscle load