Background and Purpose. Positioning a computer keyboard with a downward slope reduces wrist extension needed to use the keyboard and has been shown to decrease pressure in the carpal tunnel. However, whether a downward slope of the keyboard reduces electromyographic (EMG) activity of the forearm muscles, in particular the wrist extensors, is not known. Subjects and Methods. Sixteen experienced typists participated in this study and typed on a conventional keyboard that was placed on slopes at angles of 7.5, 0, -7.5, and -15 degrees. Electromyographic activity of the extensor carpi ulnaris (ECU), flexor carpi ulnaris (ECU), and flexor carpi radialis (FCR) muscles was measured with surface electrodes, while the extension and ulnar deviation angles of the right and left wrists were measured with electrogoniometers. Results. Wrist extension angle decreased from approximately 12 degrees of extension while typing on a keyboard with a 7.5-degree slope to 3 degrees of flexion with the keyboard at a slope of-15 degrees. Although the differences were in the range of 1% to 3% of maximum voluntary contraction (MVC), amplitude probability distribution function (APDF) of root-mean-square EMG data points from the ECU, FCU, and FCR muscles varied across keyboard slopes. Discussion and Conclusion. Wrist extension decreased as the keyboard slope decreased. Furthermore, a slight decrease in percentage of MVC of the ECU muscle was noted as the keyboard slope decreased. Based on biomechanical modeling and published work on carpal tunnel pressure, both of these findings appear to be positive with respect to comfort and fatigue, but the exact consequences of these findings on the reduction or prevention of injuries have yet to be determined. The results may aid physical therapists and ergonomists in their evaluations of computer keyboard workstations and in making recommendations for interventions with regard to keyboard slope angle. [Simoneau GG, Marklin RW, Berman JE. Effect of computer keyboard slope on wrist position and forearm electromyography of typists without musculoskeletal disorders. Phys Ther. 2003;83:816-830.]
Key Words: Computer keyboard, Slope angle, Typing, Wrist angle, Wrist extension.
Computers are ubiquitous in our society, with 100 million computers estimated to be in use in the United States in 2000. 1 Except for rare cases, very computer has a keyboard for text and data entry. The occupational risk factors of musculoskeletal disorders (MSDs), such as carpal tunnel syndrome and tenosynovitis, are thought to be due to excessive use (up to 100,000 keystrokes per day2) and due to the use of deviated wrist postures.3,4 An extensive review1 of the literature on the association between keyboard usage and prevalence of MSDs showed that the prevalence of keyboard-related MSDs among computer users, based on symptoms or physical examination findings, ranged from 9% to 50%, as compared with 4.5% to 17% among reference groups who were exposed to low levels of or no keyboard work. Compared with findings for a reference group, the odds ratios for keyboard-related MSDs among computer users were from 0.5 to 9.9 for the neck and shoulders and from 0.7 to 10.1 for the hand, wrist, or elbow.1 Although most of the studies reviewed1 had limitations of comparisons across groups at a single time period and were based on self-reported health measures, the odds ratios and prevalence of upper-extremity MSDs of computer users, as compared with findings for a reference group, suggest that computer keyboards may contribute to MSDs affecting the upper extremities.
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 (MVG), 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
We know of no published studies in which the effect of computer keyboard slope on EMG activity levels of the forearm musculature was examined. There are a number of physiological and biomechanical factors that could affect the amount of activity of the forearm extensors and flexors as a keyboard slope is changed. As the wrist flexion/extension angle changes with the change in keyboard slope, some of those factors include: a change in muscle length,11 a change in the muscles' moment arm at the wrist joint,12 and a change in the location of the hand's center of mass (which exerts a passive moment at the wrist). The EMG activity of the forearm musculature could provide some insight into how keyboard slope affects the recruitment level of the wrist extensors and flexors.
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.
Method
Subjects
Fifteen women and 1 man (mean age = 42.5 years, SD = 8.7, range = 27-53) participated in the study. The number of subjects was determined a priori based on statistical power analysis to ensure type I error did not exceed 0.05 and type II error did not exceed 0.20. This analysis indicated that a minimum of 15 subjects was necessary in a repeated-measures design of one independent variable (keyboard slope angle) with 4 levels (slope angles) to detect a difference of 5% MVC for 50th percentile APDF of forearm RMS EMG activity between pairs of keyboard slopes. We speculated a priori that 5% MVC is clearly a meaningful effect size. A standard deviation of 4% MVC was assumed for each slope angle. A similar power analysis, which resulted in a minimum of 15 subjects, was performed for detecting a 5-degree difference in wrist extension angle between pairs of keyboard slopes and assuming a standard deviation of 4 degrees within each keyboard slope.
