The effects of keyboard height, wrist support and keying time on wrist posture and trapezius EMG during keyboarding

The purpose of this study was to investigate the effects of keyboard height, the presence or absence of a wrist support, and keying time on different risk factors associated with musculoskeletal disorders experienced by VDT operators. Measurements of wrist posture, activity of the trapezius muscles, shoulder abduction and typing performance were recorded at fixed intervals during a controlled 40 minute keying task. Ratings of musculoskeletal comfort were recorded before and after each keying task. Six subjects performed the keying tasks on six different days. A different keyboard height and wrist support condition was tested on each day.

For wrist extension, ANOVA showed that the effects of Keyboard Height, Wrist Support, and Height x Keying Time were significant. For ulnar deviation, the main effects of Keyboard Height and Keying Time were significant. For forearm pronation, only the effect of Wrist Support was significant. Mean shoulder abduction ranged between 0 and 2 degrees. Mean EMG activity for all keying tasks was 5.3 %MVC. Subjects made an average of 26.5 errors during each keying task (.6 errors/min.), and an average of 1.64 errors for each time interval (.8 errors/min.). For the reported change in musculoskeletal comfort of the back, neck, left and right shoulders, left and right upper arms, left and right forearms, left wrist, and left hand, the main effect of Wrist Support was significant. In all cases the decrease in musculoskeletal comfort was greater when the wrist support was absent. Subjects preferred wrist supports, and heights at or greater than seated elbow height. The keying condition that was preferred most by subjects was when the keyboard was positioned at elbow height and when the wrist support present. The condition preferred least was when the keyboard was positioned 5 cm below elbow height and when the wrist support was absent.

These results indicate that the use of wrist supports can decrease wrist extension and musculoskeletal discomfort during keying tasks. Keyboard heights at or above seated elbow height may help decrease wrist extension but keyboard heights above elbow height may increase ulnar deviation.