Changes in wrist postures during a prolonged typing task

Abstract

Twenty experienced typists participated in a laboratory based study to determine whether wrist and forearm postures changed over a 4 hour period of intensive keyboard use. Subjects were randomly assigned to use a conventional keyboard or a fixed split keyboard. Posture data was acquired using electrogoniometers after a 10 warm-up period and at the end of each hour. Wrist and forearm postures did not change significantly over the four hour period among subjects using the split geometry keyboard. On the conventional keyboard, all joint postures were stable except right wrist extension and left forearm pronation. The right wrist extension increased by 5° over the four hour period (p=.002) and left pronation decreased by approximately 9° (p=.001). Wrist postures among typists exposed for the first time to a split keyboard remained constant throughout a four hour period of intensive typing. On the conventional keyboard, some postures drifted over the four hour period.

Wrist and Forearm Postures of Users of Conventional Computer Keyboards

“…The greater mean ulnar deviation of the left wrist (a 5°difference in our participants) generally agrees with the results of Hedge and Powers (1995), who found that participants ulnarly deviated their left wrists 2°more than their right wrists (15°-13°ulnar deviation for left and right wrists, respectively) when they typed on conventional keyboards. Honan et al (1996) also reported a 3°differ-ence between the left (15°) and right (12°) wrists.…”

“…Prior literature provides some information in that regard. The average position of the wrist in ulnar deviation during typing has been reported to be between 11°and 25° (Chen et al, 1994;Honan, Serina, Tal, & Rempel, 1995;Nakaseko, Grandjean, Hunting, & Gierer, 1985;Smith & Cronin, 1993;Sommerich & Marras, 1994;Sommerich, Marras, & Parnianpour, 1996); some authors have reported that the amount of ulnar deviation on the left wrist was a few degrees greater than on the right (Hedge & Powers, 1995;Honan, Jacobson, Tal, & Rempel, 1996). Similarly for wrist extension, the average position of the wrist has been reported as between 13°and 33° (Hedge & Powers, 1995;Honan et al, 1995;Honan et al, 1996;Sommerich & Marras, 1994).…”

“…Similarly for wrist extension, the average position of the wrist has been reported as between 13°and 33° (Hedge & Powers, 1995;Honan et al, 1995;Honan et al, 1996;Sommerich & Marras, 1994). Limited data exist on forearm pronation, which was reported to range from 69°to 79° (Honan et al, 1995;Honan et al, 1996).…”

Design Features of Alternative Computer Keyboards: A Review of Experimental Data

“…5,10,11,19,27 If a split keyboard has a slant angle of approximately 12.5°(25°opening angle), then wrist ulnar deviation is reduced to almost a neutral position (0°) in the radial/ulnar plane. 5,10,11,19,27 In general, it appears that ulnar deviation at the wrist is reduced by nearly 1°for each degree of slant angle incorporated in the keyboard. These findings provide some justification to the use of a fixed 12.5°slant angle, given that the left and right wrists are in 15°a nd 10°of ulnar deviation, respectively, when typing on a conventional keyboard.…”

“…5,10,11,18,19,27 The reduction of ulnar deviation to almost a neutral position is beneficial biomechanically because, as the wrist angle approaches neutral, biomechanical modeling indicates that the net reaction forces from the carpal bones and carpal ligament on the tendons and their sheaths decrease. 2,23 Less net reaction force pressing against the sides of the tendons and their sheaths would theoretically decrease the incidence of tenosynovitis of the wrist and finger flexor/extensor tendons.…”