The effects of the horizontal position of the keyboard on electromyography (EMG) activities of the shoulder and forearm muscles are determined among 100 healthy subjects. The subjects performed a typing task at four difference keyboard placements. [NEAR = the keyboard was placed at 0cm from the edge of the table, MID = the keyboard was placed at 8cm from the edge of the table, FAR = the keyboard was placed at 15cm from the edge of the table, Far with Pad (FWP) = the keyboard was placed at 15cm from the edge of the table with addition of a foam pad]. Surface EMG recordings were done on the upper trapezius (UT) and extensor carpi ulnaris (ECU), both on left and right side. Discomfort level was rated by the subjects by using 100mm visual analogue discomfort scale (VADS). Information of the individual preference for the four keyboard placements was obtained from a set of questionnaire. The result showed significant difference between four horizontal placements of keyboard in the percentage of Maximum Voluntary Contraction (%MVC) of both UT and ECU. However, the %MVC of UT was the lowest values at the NEAR position, whereas the %MVC of ECU was the lowest at FWP position. There was no significant correlation between the forearm length and the muscle contractions. However, the result showed there was significant lower value of the VADS at FWP position compared to NEAR position. There was highest number of subjects who preferred FWP positions due to lowest discomfort level while typing at that position. In conclusion, horizontal position of computer keyboard has an important role in affecting the upper extremity muscle activities.
