Initial wrist posture during typing as a function of keyboard height and slope

Abstract

Carpal tunnel syndrome has been linked to occupations which use a computer workstation. Two factors thought to be responsible for this problem are repetition and awkward wrist postures. This experiment examined wrist flexion-extension, radial-ulnar deviation, and pronation-supination for 24 righthanded subjects at 25 combinations of keyboard height and slope. Keyboard heights tested were: -10, -5, 0, 5, and 10 cm from elbow height, and keyboard slopes tested were: -45, -22, 0, 22, and 45 degrees from horizontal. Keyboard slopes were considered negative if they sloped away from the subject and positive if they sloped towards the subject. Subjects wore a wrist monitor, comprised of metal strips with potentiometers, on each hand and typed a text passage for two minutes in each experimental condition. The number of correct words per minute was also measured in each experimental condition.

Results indicated that flexion was minimum when the keyboard was 45 degrees from horizontal, and that overall the left wrist exhibited extension while the right wrist exhibited flexion. Ulnar deviation was minimized when the keyboard height was -10 cm below elbow height, and both ulnar and radial deviation were minimized at slope conditions 22 and 45 degrees from horizontal. Higher keyboard heights coupled with positive slopes reduced radial and ulnar deviation as did lower keyboard heights coupled with negative slopes.

For low keyboard heights, the right hand exhibited more extreme ulnar deviation than the left hand. Pronation was minimum when the keyboard was 10 cm above elbow height and -45 degrees from horizontal, and was maximum when the keyboard was -5 cm below operator elbow height and 45 degrees from horizontal. Correct words typed per minute was maximun1 at 0 degrees from horizontal, and decreased quadratically as slope was both increased and decreased.