Normal subjects (n = 7) performed rapid voluntary isometric contractions of the adductor pollicis and the tibialis anterior. Within the first minute of this low-intensity exercise, the speed of tension development slowed, accompanied by a prolongation in EMG burst duration. In contrast, electrically evoked contractions either became more rapid (twitch) or did not change (tetanus), suggesting a fatigue of central origin. 31P NMR measurements of high-energy phosphates showed that the relationship between the fall of maximum force and changes of phosphates and pH was similar to that produced by other high-intensity fatiguing exercise protocols. Thus, rapid movements produce fatigue through two major mechanisms. First, there is slowing of the speed of tension development which appears secondary to central fatigue. Second, the decline of muscle force is primarily attributable to changes in muscle pH or inorganic phosphate, which usually occurs only after high-intensity (but non-rapid) exercise.