Wrist and Forearm Posture from Typing on Split and Vertically Inclined Computer Keyboards

Article excerpt

A study was conducted on 90 experienced office workers to determine how commercially available alternative computer keyboards affected wrist and forearm posture. The alternative keyboards tested had the QWERTY layout of keys and were of three designs: split fixed angle, split adjustable angle, and vertically inclined (tilted or tented). When set up correctly, commercially available split keyboards reduced mean ulnar deviation of the right and left wrists from 12[degrees] to within 5[degrees] of a neutral position compared with a conventional keyboard. The finding that split keyboards place the wrist closer to a neutral posture in the radial/ulnar plane substantially reduces one occupational risk factor of work-related musculoskeletal disorders (WMSDs): ulnar deviation of the wrist. Applications of this research include commercially available computer keyboard designs that typists can use and ergonomists can recommend to their clients in order to minimize wrist ulnar deviation from typing.

INTRODUCTION

Many workers in the clerical sector, which has an employment base of more than 18 million in the U.S. (Statistical Abstract of the United States, 1992), use a computer keyboard during a majority of their working hours, resulting in 50 000 to 100 000 key strokes a day (40 words per minute [wpm] for 8 h). Within the last three decades of published literature, upper-extremity, work-related musculoskeletal disorders (WMSDs) have often been attributed to mechanical and electronic keyboard use (Bergqvist, 1995; Kroemer, 1972; Sauter, Schleifer, & Knutson, 1991). Thus it appears that the design of the keyboard is implicated in the etiology of upper extremity WMSDs among keyboard users for the following reasons: (a) The often-cited occupational risk factors of repetitive movements and deviated posture of the wrist in the flexion/extension and radial/ulnar planes are an inherent part of typing on a computer keyboard. (b) Cross-sectional studies have demonstrated a strong positive relationship between musculoskeletal discomfort and keyboard use (Bergqvist, 1995; Duncan & Ferguson, 1974; Sauter et al., 1991).

Conventional and Alternative Computer Keyboards

The conventional flat keyboard requires operators to hold their hands and forearms in a relatively awkward position. With this keyboard they must substantially pronate the forearms substantially in order to hold their palms almost horizontally. In addition, they must deviate both wrists in the ulnar direction in order to rest their fingers on the home keys, as shown in Figure 1. Most computer keyboard users also hold their wrists with some extension (Simoneau, Marklin, & Monroe, 1999).

Usually the design of alternative computer keyboards differs from that of conventional keyboards in the slant angle, slope, or tilt angle. A conventional keyboard has a slant angle of 0[degrees], a slope ranging from 0[degrees] to 15[degrees], and a tilt angle of 0[degrees] (see Figure 2). Several studies have investigated whether commercially available alternative keyboards place the wrist in a more neutral posture than conventional keyboards (Nakaseko, Grandjean, H[ddot{u}]nting, & Gierer, 1985; Chen, et al. 1994; Honan, Serina, Tal, & Rempel, 1995; Honan, Jacobson, Tal, & Rempel, 1996). If a commercially available split keyboard has an opening angle of approximately 25[degrees] (12.5[degrees] slant angle), then wrist ulnar deviation is reduced to almost a neutral position in the radial/ulnar plane. The ulnar deviation for participants typing on a conventional keyboard is typically at least 10[degrees]. However, the studies that investigated alternative keyboards were limited in two ways. First, in those wrist posture studies in which practice time was stated, the participants were given only 3-30 mm practice time. Second, the pronation/supination angle of the forearm was measured in only two of the studies that were part of our literature review (Honan, et al. …