Abstract
The difference in RT for right and left biceps, acting on the forearm in two different movement patterns, flexion and supination, was examined for 14 normal Ss, seven right-handed and seven left-handed. The task was to flex or supinate both forearms simultaneously in response to a sound stimulus. Median RTs of each S were computed for each movement task. The analysis indicated that RT of supination is faster than that of flexion. Concerning left-right difference of RT, the flexion of the non-preferred hand is faster than that of the preferred hand and the supination of the preferred hand is faster than that of the non-preferred hand. Even in a simple movement there are differences in RTs for the right and left hands which do not depend on the muscles but on the movement patterns. Hemispheric dominance is not established by comparing the rapid initiation of movement.

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