EFFECT OF SPEED AND PRECISION DEMANDS ON HUMAN SHOULDER MUSCLE ELECTROMYOGRAPHY DURING A REPETITIVE TASK

Effects of speed and precision on electromyography (EMG) in human shou lder muscles were studied during a hand movement task where five point s were marked repeatedly with a pencil. Six female subjects performed with three precision demands and at four speeds. Three of the speeds w ere predefined, while the last speed was performed as fast as possible . The EMG were recorded from 13 shoulder muscles or parts of muscles. Elbow velocity, acceleration and rectified EMG were calculated for eac h task. The mean elbow velocity and acceleration increased with speed and precision demands. There was an increase in EMG as the speed deman d increased for all three precision demands (P < 0.001), and as the pr ecision demand increased for the two highest predefined speed demands (P < 0.05). The combination of a high speed and a high precision deman d resulted in the highest EMG. Different EMG levels were attained for the 13 muscles and the supraspinatus muscle always showed the highest normalized EMG. However, analysis of variance showed the same relative increase for all muscles with speed and precision demands. The EMG ch anges in response to precision demand can only be explained in part by the differences in movement velocity and acceleration, and other fact ors such as increased co-contraction must also be taken into account.