Fatigue may be defined as a reduction in the maximal force-generating capacity of a muscle. It may result from peripheral processes distal to the neuromuscular junction and from central processes controlling the discharge rate of motoneurons. When assessed with a sensitive test using twitch interpolation, most 'maximal' voluntary contractions approach but do not attain optimal muscle output. During fatigue, reflex inputs from intramuscular receptors may contribute to a decline in motor unit discharge rate--a decline which optimises force production during maximal efforts. Further studies should investigate how the central nervous system controls the discharge rate of motor units during fatigue produced by different forms of exercise.