The relationship between the speed of the fastest possible voluntary contractions and their amplitude was examined for several hand- and forearm muscles under isometric and isotonic conditions. The consistent finding was the amplitude dependence of the speed of the fastest voluntary efforts: the larger the amplitude, the faster the contraction. The increase of the rate of rise of isometric tension or of the velocity of isotonic movements with rising amplitude was linear. The slope of this relationship was the same for three different hand- and forearm muscles examined. The duration of the contractions measured from onset to peak was approximately constant for all amplitudes. The duration of the EMG-burst recorded from the contracting muscle was similar as the time from onset to peak of the contraction. These results show that the skeleto-motor speed control system operates by adjusting the velocity of a contraction to its amplitude in such a way that the contraction time remains approximately constant. It is suggested that this type of speed control is a necessary requirement for the synchrony of synergistic muscle contractions.