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Research Reports |
GG Simoneau, PT, PhD, ATC, is Associate Professor, Department of Physical Therapy, Marquette University, Milwaukee, Wis
RW Marklin, PhD, CPE, is Associate Professor, Department of Mechanical and Industrial Engineering, Marquette University, PO Box 1881, Milwaukee, WI 53201-1881 (USA) (richard.marklin{at}marquette.edu).
JE Berman, PT, MHS, ATC, is Research Assistant, Department of Physical Therapy, Marquette University
Address all correspondence to Dr Marklin
Submitted July 19, 2002;
Accepted May 5, 2003
 | Abstract |
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Key Words: Computer keyboard • Slope angle • Typing • Wrist angle • Wrist extension
| Introduction |
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 TopAbstract Introduction MethodResultsDiscussionConclusionReferences |
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Only a few researchers have investigated the magnitude of forearm muscle electromyographic (EMG) activity during typing on computer keyboards. Electromyography was used to measure the muscle activity of the flexor digitorum superficialis (FDS) and extensor digitorum communis (EDC) muscles while subjects typed on keyboards with keys of varying stiffness.5 These researchers used an amplitude probability distribution function (APDF) for analyzing EMG data, which is a method to quantify the level of EMG activity for a task in which the muscles are changing in length. These researchers found that the 50th percentile of the EMG signals for the FDS muscle was approximately 7% of maximum voluntary contraction (MVC), which means that 50% of the root-mean-square (RMS) EMG data points were under 7% MVC. The 10th percentile (considered a measure of the baseline of muscle activity) was about 2% MVC, which means that 10% of the RMS EMG data points were under 2% MVC during typing trials or that muscle activity level was greater than 2% MVC for 90% of the typing trial. The corresponding 50th and 10th percentiles for the EDC muscle (11.5% and 6.5% MVC, respectively) were much greater than for the FDS muscle. These results5 are similar to findings from other researchers6 who measured median (50th percentile) EMG activity from the EDC muscle that ranged from 6.5% to 14% MVC while subjects were typing on various keyboards. The greater activity of the dorsal forearm musculature was most likely due to the postural requirement of holding the hand and fingers above the keyboard while fingers were typing keys.5
Modifications to the design of the keyboard could possibly reduce the magnitude of forearm muscle activity. A change to the keyboard that might decrease muscle activity of extensor forearm muscles is sloping the keyboard downward. As illustrated in Figure 1, keyboard slope is the angle of the plane of keytops to a horizontal. A typical conventional keyboard has a built-in slope of about 6 degrees. We have shown that changing the slope of the keyboard in a downward direction can change wrist extension angle and not impair typing speed and accuracy.7 Wrist extension angle decreased 1 degree for every 2-degree decrease in keyboard slope angle as the keyboard was positioned at 15 to –15 degrees of slope. Mean wrist extension angle decreased to less than 15 degrees when the keyboard was positioned with a slope of 0 degrees or lower. In theory, wrist extension angles close to the anatomical neutral position (compared with large wrist extension angles) result in less risk of distal upper-extremity MSDs because the pressure in the carpal tunnel is lower and forces pressing against the median nerve and flexor tendons are less.8–10
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The objective of this study was to determine the effect of computer keyboard slope angle on forearm musculature EMG activity in individuals without any upper-extremity symptoms of MSDs. All subjects were 10-digit "touch" typists. We hypothesized that percentage of MVC of the extensor carpi ulnaris (ECU) muscle would change as keyboard slope decreased. If there is a difference, changing the slope of the keyboard in a downward direction may be an intervention that could be used in the design of new computer workstations and modifications to existing workstations.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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All subjects typed at least 40 words per minute (wpm) using the 10-digit "touch" method (capable of typing accurately without looking at the keys) and worked in jobs requiring typing at least 2 hours per workday. The subjects were clerical workers recruited from service-oriented businesses within the metropolitan Milwaukee area. Typing speed was confirmed by Typing Tutor 6.0 software* during a short typing practice session before the commencement of data collection. The subjects' mean shoulder width was 38.1 cm (SD=2.2, range=33.7–42.9). The mean length of the right forearm and hand was 44.3 cm (SD=2.5, range=39.9–48.4), and that of the left forearm and hand was 44.3 cm (SD=2.7, range=39.6–48). A person's shoulder width and the length of the forearm and hand are important because they can affect the ulnar deviation angle of the wrist while the person types.
At the time subjects were recruited, the subjects were asked if they were free of pain or discomfort related to typing. Based on answers to questions regarding various body segments, subjects indicated they were free of symptoms related to musculoskeletal injury, pain, and discomfort that could interfere with typing. Immediately prior to testing, the subjects were asked questions about pain, tingling, and numbness in their upper extremities. This was done to further ensure that the subjects were free of medical problems that could interfere with typing and to confirm that they did not have symptoms in their distal upper extremities that were related to typing. Furthermore, all subjects tested negatively for Phalen and Tinel tests for carpal tunnel syndrome. Phalen and Tinel tests have reported sensitivities of 71% and 44%, respectively, and specificities of 80% and 94%, respectively.13,14 All subjects gave informed consent prior to participation in the study.
Experimental Design
A repeated-measures experimental design was used to determine wrist angle position and percentage of MVC for EMG activity of the forearm musculature from subjects typing on a conventional keyboard with the slope at 4 angles (7.5°, 0°, –7.5°, and –15°). During the testing session, all subjects typed on all 4 slopes. The order of use was presented randomly for each subject.
Dependent Variables
The dependent variables for this study were the following:
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attached to the dorsum of the wrist measured wrist extension and ulnar deviation angles while typing. These electrogoniometers, which are of strain gauge type and are lightweight and unobtrusive to the typist, measured wrist angles in the flexion/extension and radial/ulnar planes simultaneously. The accuracy of the goniometers in each plane was 2 degrees. As assessed by intraclass correlation coefficient (ICC)18,19 on our sample of 16 subjects, intertrial reliability for the measurement of wrist extension and ulnar deviation angles was in excess of .90. These reliability values are of similar magnitude to those previously reported.7
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The pairs of surface electrodes contained circuitry for preamplification with a gain of 35 and minimized artifact.20 Raw bipolar EMG data from the electrodes were processed utilizing the RMS method, which produced a linear envelope or average EMG voltage over the data collection period.21 The time-constant window over which the RMS data were calculated was 55 milliseconds. Online wrist joint position from the electrogoniometers and EMG RMS data were sampled at 300 Hz and fed into a 12-bit analog-to-digital converter
and stored on a personal computer operated with custom-written LabVIEW software. Based on 5 samples of 30 seconds of EMG data collected over a 6-minute typing session, intertrial ICCs of the 50th percentile of the APDF of the RMS EMG data were .99, .94, and .98 for the left ECU, FCU, and FCR muscles, respectively. These ICCs were calculated from all 16 subjects when the keyboard was positioned at a slope of 7.5 degrees. The ICC data were similar for muscles of the right forearm. Fifteen seconds of normalized EMG data for the right ECU muscle expressed as percentage of MVC is shown in Figure 4. Data were collected with the keyboard positioned at a slope of 7.5 degrees.
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Data Processing and Statistical Analysis
Wrist angle data.
After the wrist extension and ulnar deviation voltage data were converted to angular measurements, the angular data were filtered by a second-order, double-pass Butterworth filter with a cutoff frequency of 7 Hz. For each slope, the mean, maximum, and minimum wrist extension and ulnar deviation angles of the subjects were computed for each of the five 30-second trials. The summary statistics for the 5 trials within each slope of the keyboard were then averaged for each subject. The mean, minimum, and maximum wrist angle data for wrist extension and ulnar deviation were analyzed with a 2-way analysis of variance (ANOVA) for repeated measures followed by a Tukey honestly significant difference (HSD) multiple-comparison post hoc test when main effects and any interactions were present. The 2 independent variables were hand (2 levels: right and left) and keyboard slope (4 levels: 7.5°, 0°, –7.5°, and –15°). Statistical significance was set at P<.05.
EMG data.
The EMG data during typing sessions were normalized to a percentage of MVC, which is the widely accepted method for analyzing EMG data.8,9 In post hoc processing of each of the 5 trials of EMG data per slope, the 10th, 50th, and 90th percentile APDFs of the RMS EMG data were calculated with LabVIEW software. For each muscle, the APDF summary statistics were then averaged over the 5 trials for each keyboard slope. Similar to wrist position data, the 10th, 50th, and 90th percentile APDFs of the RMS EMG data for the ECU, FCU, and FCR muscles were analyzed individually with a 2-way ANOVA for repeated measures, followed by a Tukey HSD multiple-comparison post hoc test when main effects and any interactions were present. The 2 independent variables were hand (2 levels: right and left) and keyboard slope (4 levels: 7.5°, 0°, –7.5°, and –15°). Statistical significance was set at P<.05.
Typing performance data.
Typing speed and accuracy were averaged in the same manner as wrist angle and EMG data and were analyzed with a one-way ANOVA for repeated measures (one independent variable: keyboard slope; 4 levels: 7.5°, 0°, –7.5°, and –15°). Statistical significance was set at P<.05.
Psychometric data.
Ease-of-use and comfort data collected after subjects typed on each keyboard slope angle were analyzed using a Friedman ANOVA by ranks to determine if there was a difference between groups overall and a Wilcoxon signed rank test to determine differences between pairs of keyboard slopes. Statistical significance was set at P<.05.
| Results |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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–5). For all 3 APDF levels, the ECU muscle's percentage of MVC decreased when the keyboard was moved downward in slope. As shown in Table 4 and depicted in Figure 7, the mean 50th percentile APDF for the ECU muscle decreased from approximately 13.5% MVC to 11.5% MVC as the keyboard slope was changed from 7.5 to –15 degrees. Mean 10th and 90th percentile APDFs for the ECU muscles decreased in a similar manner as the 50th percentile APDF (Tabs. 3 and 5). There were no interactions between hand and keyboard slope. The differences across the 4 slopes of the keyboard are summarized in Tables. 3 through 5.
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For the 10th percentile APDF for the FCU and FCR muscles, no main effects were found between hands and across slopes. In addition, no interactions were found. For the 50th percentile APDF for the FCU muscle, a small (1%) increase in muscle activity was seen as the angle of the slope decreased. No difference existed between hands, and no interactions existed. For the 90th percentile APDF for the FCU muscle, a main effect was found between hands, and a main effect was also found for the angle of the slope. No interaction was found. For the 50th and 90th percentile APDFs for the FCR muscle, small differences (about 2% and 4%, respectively) existed between hands. There were no differences across slopes of the keyboards, and no interaction was found between the 2 factors. The differences across the 4 slope conditions are summarized in Tables 3 through 5.
Typing Performance
There were no differences in typing speed and accuracy among the 4 slopes of the keyboard. As shown in Table 6, mean typing speed ranged less than 1 wpm across the slopes of the keyboards (range=66.1–66.9 wpm). In addition, mean typing accuracy was 100% among the 4 slopes.
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| Discussion |
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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With a wrist rest in the same plane as the keyboard, the pivot point for combined keyboard and wrist rest structure is near the user's wrist, as opposed to near the metacarpophalangeal joints when a horizontal, detached wrist rest is used with the keyboard. The location of the pivot point is important, because we believe that when the wrist rest is attached to the keyboard and follows the slope angle of the keyboard, the hands tend to follow the wrist rest, therefore resulting in less wrist extension than if the wrist rest remains horizontal. The difference in the orientation of the wrist rest to the keyboard may explain the approximately 12-degree difference of wrist extension we measured at the lowest negative slope relative to findings from our earlier study.7 In the present study, we found an average of 3 degrees of wrist flexion when the subjects were typing on a wrist rest in the same plane with the keyboard and with a slope of –15 degrees. Previously, we found an average of 9 degrees of wrist extension when the keyboard (and not the wrist rest) was positioned at a slope of –15 degrees. The design of the wrist rest on the same plane as the keyboard that we used in the present study appears to us to be more representative of typical computer workstations than use of a fixed horizontal wrist rest.
Based on carpal tunnel pressure studies, wrist extension angles closer to neutral are believed to be beneficial with respect to etiology of nerve conduction injuries affecting the wrist.8,10 With wrist extension angles greater than 15 degrees, pressure in the carpal tunnel could result in more pressure against the median nerve, and this could contribute to the development or perpetuation of carpal tunnel syndrome. Therefore, typing with a wrist position close to an anatomically neutral position could minimize pressure in the carpal tunnel, and this theoretically could benefit individuals with a diagnosis of carpal tunnel syndrome. Data from our study show that mean wrist extension angles for all positions of the keyboard (slopes of 7.5° to –15°) were 15 degrees or less. However, maximum wrist extension angles exceeded 15 degrees and approached 25 degrees at the 7.5-degree slope. Thus, keyboards positioned at a slope of 7.5 degrees may increase pressure in the carpal tunnel because, based on our data, the wrist is often extended beyond 15 degrees from neutral during the normal activity of typing.
Wrist extension angles closer to a neutral position, theoretically, could decrease the probability of developing other MSDs affecting the hand and wrist. Modeling of the tendons in the wrist with a free-body diagram showed that the reaction force exerted against the tendons from the carpal bones and flexor retinaculum increased as the wrist was extended.26,27 Dynamics applied to the static model showed that acceleration of the wrist in the flexion/extension plane increased the reaction forces on the tendons even more than in the static analysis.9 Greater reaction forces against the tendons and their sheaths that pass through the wrist, theoretically, could increase the risk of tendinitis or tenosynovitis.
Ulnar deviation.
The ulnar deviation angles of approximately 13 and 9 degrees for the left and right wrists for the 7.5-degree slope, which are similar to the built-in slopes of many commercially available keyboards, are similar to ulnar deviation angles measured on 90 subjects who typed on a conventional keyboard in a previous study.7 As we found in our earlier study with keyboards positioned at negative slopes, ulnar deviation tended to increase 3 to 5 degrees when the keyboard slope was changed from 7.5 degrees to –15 degrees.7 The reason for this increase in ulnar deviation is not clear at this time, but a change in forearm pronation (which was not measured in this study) could play a role. Typically, typists pronate their forearms approximately 65 degrees when they type on a conventional keyboard,4 which means the wrists are not parallel to the keyboard surface. We found that subjects typing on tilted (also called "tented" or "vertically inclined") keyboards reduced forearm pronation by approximately 20 to 25 degrees compared with subjects using a conventional keyboard, and they reduced ulnar deviation by approximately 12 to 14 degrees in the left and right wrists, respectively.3 Giving the keyboard a downward slope may have an effect of increasing forearm pronation and thereby increasing ulnar deviation at the wrist. Because we did not measure forearm pronation, we cannot determine whether this hypothesis is correct. Regardless of the reason for an increase in ulnar deviation, the increase may partially counteract any benefits of positioning a keyboard with a downward slope because increased ulnar deviation, in theory, increases the net reaction forces and friction on the tendons passing through the wrist.26,27
Forearm Muscle EMG Activity
Surface EMG activity, calibration, and APDF.
Although more than a score of relatively small muscles pass through the forearm and thus present opportunities for cross talk during EMG recording, researchers have shown that surface EMG is a reliable and accurate method to assess electrical activity of the ECU, FCR, and FCU muscles.15–17 In our study, the EMG activity of these muscles was measured for normalization with the wrists extended 8 degrees and ulnarly deviated 10 degrees on the right wrist and 15 degrees on the left wrist, which is the average of wrist positions required for typing on conventional keyboards.4 Therefore, errors in EMG measurement due to change in length of muscles, in our opinion, were minimized.
The APDF method of analyzing EMG data recorded during an activity where a limb moves has been used often in investigations of upper-extremity muscle activity.5,6,28 The APDF model, which was described and illustrated by Jonsson,29 states that the static level of muscle activity is the percentage of MVC that is below the lowest 10% of the cumulative RMS EMG signal, whereas the dynamic level (90th percentile APDF) is the percentage of MVC that is above the greatest 10% of the cumulative EMG signal.
Magnitude of EMG activity.
The percentages of MVC of the 10th, 50th, and 90th percentile APDFs for the ECU muscle from this study (6%, 12%, and 24%, respectively) are similar to results reported by Gerard et al,5 who found percentages of 6%, 11%, and 19% MVC for APDFs of the finger extensor muscles. Fernstrom et al6 measured a range of 7% to 12% MVC for the 50th percentile APDF for the extensor digitorum muscle while subjects typed on computer keyboards and typewriters.
As for the ECU muscle, the percentages of MVC for the wrist flexors (FCU and FCR muscles) in our study are comparable to those reported in the literature.5,8 Although Gerard et al5 measured EMG activity from the finger flexors (and not the wrist flexors [ie, FCR and FCU muscles] as in our present study), their results show similar percentage of MVC. Measurements of approximately 2%, 6%, and 19% MVC for the 10th, 50th, and 90th percentile APDFs from the finger flexor muscles8 are in agreement with our findings of approximately 1%, 4%, and 10% MVC for the FCR muscle and 1%, 10%, and 28% MVC for the FCU muscle for the 10th, 50th, and 90th APDFs, respectively.
Overall, the percentage of MVC was greater for the ECU muscle than for the wrist flexors. The percentage of MVC for the ECU muscle was twice as large as the percentage of MVC for the FCR and FCU muscles for the 10th and 50th percentile APDFs. The role of the ECU muscle in typing is to hold the wrists in an extended and ulnarly deviated position above the keyboard so a user can minimize the distance required to reach the keys. The role of the wrist flexors in a typing task is to move the wrist in the radial/ulnar plane to type alphabetic, numeric, and special function keys that are located toward the perimeter of the keyboard. The percentage of MVC was greater for the FCU muscle than for the FCR muscle at the 50th and 90th percentile APDFs, but not at the 10th percentile APDF. Although the 10th percentile APDF for both the FCR and FCU muscles averaged about 1% MVC, the percentage of MVC was 3% greater for the FCU muscle than for the FCR muscle at the 50th percentile APDF and about 10% greater for the 90th percentile APDF. The greater EMG activity of the FCU muscle over the FCR muscle is apparently due to the ulnarly deviated wrist position required when typing. The FCU muscle is used to ulnarly deviate the wrist approximately 10 degrees or more to type on a conventional keyboard4 and also to ulnarly deviate the wrist to type numeric and special function keys that are located to the ulnar side of both wrists. We found in our present study and in our earlier work3,4,24 that the left wrist is ulnarly deviated up to 5 degrees more than the right wrist while typing on conventional and split keyboards. This finding may explain why the percentage of MVC for the FCU muscle was greater for the left wrist than for the right wrist (approximately 2% MVC greater for 50th percentile APDF, as illustrated in Fig. 7). In theory, the FCU muscle is required to exert more tension to ulnarly deviate the wrist 5 degrees more than the right wrist. Whether a small difference of 2% MVC of the FCU muscle is clinically relevant is not known.
Effect of keyboard slope on forearm EMG activity.
As the keyboard slopes increased in a downward direction from a positive angle of 7.5 degrees to a negative angle of 15 degrees, the percentage of MVC for the ECU muscle decreased 2% to 3% MVC across all percentile ranges of APDF. Based on the assumption that percentage of MVC is representative of muscle tension, this decrease in percentage of MVC for the ECU muscle indicates that less EMG activity of the ECU muscle is needed when typing on a negatively sloped keyboard. This simple interpretation is tempered by the dynamic nature of the ECU muscle during typing and by the fact that wrist angle (and therefore muscle length) also changed as the keyboard was adjusted, as did the moment arm of the muscle. The clinical significance of a 2% or 3% change in percentage of MVC for the ECU muscle is difficult to interpret. Although a 2% to 3% difference is seemingly very small, and likely of no clinical importance when typing for short periods of time, we believe that such a difference may be important when sustained for several hours per day.
The results of our study show that positioning the keyboard with a downward slope did not lead to an increase in the muscle activity of the wrist extensors. Furthermore, the overall decrease of the percentage of MVC for the ECU muscle as the keyboard was positioned with a downward slope provides some insight that warrants future study. If future research indicates that there is a clinically relevant decrease in ECU muscle EMG activity with the keyboard positioned with a downward slope, then this keyboard positioning may reduce muscle activity in the forearm muscles. This could reduce the pain and discomfort that some people experience while typing.
Although the magnitude of RMS EMG activity has been shown to increase with muscle fatigue over long durations of contraction,30 fatigue did not likely come into play in our study. The duration of typing trials were short enough (6 minutes), in our view, to minimize effects from fatigue and therefore any confounding effect on EMG activity levels. In addition, random assignment of the slopes of the keyboard should have eliminated any systematic bias during testing.
There was an increase of 1% to 1.5% MVC in the muscle activity of the FCU muscle as the keyboard downward slope increased. This change could be related to the increased ulnar deviation noted with a keyboard with a negative slope. Again, although these changes are small, they may warrant consideration in the design of keyboards. The incorporation of a slant angle—where the keyboard is split into halves and the halves are angled outward—in the design of keyboards with negative slopes would be justified because the slant angle incorporated in split keyboards tends to place the wrists in a relatively neutral anatomical posture.3
Typing Performance
The narrow ranges of mean typing speeds (66.1–66.9 wpm) and 100% accuracy for all the keyboard slopes indicate to us that subjects quickly adapted to new keyboard slope angles, even after only 3 minutes of practice. The typing performance we found is similar to results from our previous study,7 where we showed that keyboards with negative slopes did not impair typing performance. We expected perfect typing accuracy because subjects were allowed to correct errors as they typed.
Ease of Use
The assessments of ease of use and comfort show that the keyboard with a slope of –15 degrees was rated least comfortable and most difficult to use of all the keyboards tested.
Future Work
Our studies, including the current study, on keyboards with negative slopes were conducted in the laboratory and thus may not be representative of what actually occurs with wrist positions in real-world office settings.
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TopAbstractIntroductionMethodResultsDiscussionConclusionReferences |
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Whether positioning a keyboard downward from its built-in slope is helpful in preventing or managing distal upper-extremity MSDs attributed to the use of keyboards is not known and warrants further study. However, data on wrist extension from our study considered in light of the theoretical biomechanical modeling of the wrist suggest that a keyboard with a neutral (horizontal) slope or a keyboard with a downward slope might have beneficial effects to prevent or treat upper-extremity injuries related to the frequent use of computer keyboards.
| Footnotes |
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Testing of human subjects was approved by the Marquette University Office of Research and Sponsored Program's Institutional Review Board (HR-94–292).
This research was presented in partial and condensed form at the International Society of Biomechanics Conference; Zurich, Switzerland; July 8–13, 2001.
* Kriya Systems Inc, Sterling, Va. 
Biometrics Corp, PO Box 340, Ladysmith, VA 22501.
Therapeutics Unlimited, 2835 Friendship St, Iowa City, IA 52240.
National Instruments Corp, 11500 N Mopac Expressway, Austin, TX 78759.
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