Academic journal article Human Factors

Effect of Computer Keyboard Slope and Height on Wrist Extension Angle

Academic journal article Human Factors

Effect of Computer Keyboard Slope and Height on Wrist Extension Angle

Article excerpt

The goal of this study was to determine the systematic effect that varying the slope angle of a computer keyboard along with varying keyboard height (relative to elbow height) have on wrist extension angle while typing. Thirty participants typed on a keyboard whose slope was adjusted to +15[degrees], +7.5[degrees], 0[degrees], -7.5[degrees], and -15[degrees]. The height of the keyboard was set up such that participants' wrists were at the same height as their elbows, above their elbows, and four cm below their elbows. Results showed that as keyboard slope angle moved downward from +15[degrees] to -15[degrees], mean wrist extension decreased approximately 13[degrees] (22[degrees] at +15[degrees] slope to 9[degrees] at -15[degrees] slope). Keyboard height had a similar effect with mean wrist extension decreasing from 21.8[degrees] when the keyboard was lower than elbow height, to 7.3[degrees] when the keyboard was higher than elbow height. Potential application of this research includes the downward sloping of computer keyboards, which could possibly be beneficial in the prevention of musculoskeletal disorders affecting the wrist.


Most conventional computer keyboards have a built-in positive slope, which requires the user to extend the wrist approximately 20[degrees] while typing (Simoneau, Marklin, & Monroe, 1999). Raising the legs on the back of the keyboard will increase the positive slope angle and, concomitantly, further increase wrist extension. The positive slope of many conventional keyboards complies with the current voluntary U.S. guidelines for visual display terminal (VDT) workstation layout (ANSI/HFS 100, 1988), which states that keyboard slope should be between 0[degrees] and +25[degrees]. There are a number of positively sloped keyboards on the market, but commercially available keyboards and keyboard support surfaces that allow the user to slope the keyboard downward represent a small portion of the total market. The goal of negatively sloped keyboards is to decrease wrist extension so the wrist is held close to a neutral wrist posture, thereby theoretically lessening the probability of a work-related musculoskeletal di sorder (WMSD) of the hand or wrist. WMSDs at the wrist include carpal tunnel syndrome and tenosynovitis.

Based on the literature, it appears that deviated wrist posture in the flexion/extension plane is implicated in the etiology of WMSDs of the wrist (Armstrong, 1986). Studies measuring carpal tunnel pressure have shown that carpal tunnel pressure decreases as the wrist moves toward a neutral posture in the flexion/extension plane (Rempel, Kier, Smutz, & Hargen, 1997; Weiss, Gordon, Bloom, So, & Rempel, 1995). Less pressure in the carpal tunnel is beneficial because the median nerve, which passes through the carpal tunnel, is under less compression and is less likely to show the symptoms of carpal tunnel syndrome. In theory if a typist were to type on a negatively sloped keyboard with a wrist extension angle closer to a neutral posture than the approximately 200 extension with a conventional keyboard, then the typist would be less susceptible to WMSDs.


Research on how a negatively sloped keyboard affects wrist position -- wrist extension in particular -- has been sparse. In a study of 12 office workers, Hedge and Powers (1995) allowed their participants to make a downward adjustment to the slope angle of a platform that supported a conventional keyboard with an integrated palm rest. The mean self-selected slope angle of the keyboard platform was -12[degrees], which resulted in a keyboard slope close to 0[degrees]. The wrist extension angle averaged 13[degrees] with the conventional keyboard supported on a horizontal platform, whereas the wrist angle averaged 1[degrees] flexion with the platform sloped negatively.

In another study, Hedge, McCrobie, Land, Morimoto, and Rodriguez (1995) investigated negatively sloped keyboards in a corporate office in Phoenix, Arizona. …

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