Control of velocity in rapid flexion movements of the interphalangeal joint of the thumb was investigated by examining movement trajectories and patterns of activity in the extensor pollicis longus (EPL) and flexor pollicis longus (FPL) muscles. Although velocity was controlled with considerable accuracy, it was not sensed with the same precision. Consistent errors were made when subjects attempted to match the peak velocities under conditions in which the relationship between muscle activity and joint acceleration had been altered, i.e. changing the angle from which movement was initiated or varying the load. Rather than relying on afferent feedback from peripheral sensory receptors for information about velocity during rapid movements, it is suggested that subjects were more likely to base their judgment of velocity on sensations evoked by the voluntary motor command.