The electrical activity of four pairs of synergistic muscles-the long head of triceps brachii and anconeus, lateral gastrocnemius and soleus, the long and short heads of biceps brachii, and rectus femoris and vastus medialis-was studied during isometric contractions of varying speed. Each muscle pair consists of a two-joint muscle and a one-joint muscle. Two of the one-joint muscles, anconeus and soleus, are composed predominantly of red muscle fibers and are called red muscles; all the remaining muscles are composed predominantly of pale fibers and are therefore called pale muscles. Synergistic pairs of muscles in which the muscles are both pale become active simultaneously in movement of all speeds. No difference in usage of such muscles was found as a function of movement speed. In synergistic muscle pairs composed of a red and a pale muscle, a slow movement was always initiated by the red muscle, whose electrical activity predominated throughout the movement. In a rapid movement, the pale muscle could be initially most active. However, movements of equal speed could be initiated by the red muscle. Thus the speed of a contraction is a necessary but not a sufficient condition for reversals of muscle activation.