Single motor units (MUs) activity was investigated in human m. biceps brachii during movements against an elastic load. A total of sixty-five MUs were studied by means of subcutaneously placed fine-wire branched electrodes. Subjects were asked to perform active shortening and lengthening of the muscle with approximately constant velocities at two different speeds--slow and fast. Both recruitment (RT) and decruitment (DT) thresholds of MU were found to be lower in movement with higher velocity. The recruitment order of MUs was approximately one and the same during concentric movements with a different but constant velocity. The firing onset of MUs is organized so that the peak of the first twitch contraction occurs at approximately the same force level irrespective of how fast the movement is. In contrast, during the eccentric movements the peak of the last twitch contraction of MU occurs at different torque levels depending on the velocity. The decruitment of the MUs during eccentric movement was in a reverse order to their recruitment during concentric movements. Generally, at one and the same velocity the RT of a given MU was lower than DT. Nevertheless, the peaks of the first and the last twitch contractions during concentric and eccentric movements with one and the same velocity occurred at approximately one and the same torque level.