Role of muscle afferents in the inhibition of motoneurons during fatigue

In conscious humans, fatiguing muscular contractions are accompanied by a decrease in the discharge rate of alpha motoneurons. The association between alpha motoneuron discharge rate and the generation of force by skeletal muscle has been called 'muscle wisdom' (Marsden et al., 1983). Its purpose is believed to ensure that central neural drive to skeletal muscle, which is fatigued, matches that needed to generate the required force. In addition, muscle wisdom may he one mechanism that functions either to decrease or to postpone central neural fatigue (Enoka and Stuart, 1992). Bigland-Ritchie and colleagues (1986) have suggested that a reflex arising front fatigued skeletal muscle is responsible, at least in part, for muscle wisdom. This chapter has two purposes. The first is to evaluate the evidence that a reflex arising froth fatigued skeletal muscle causes muscle wisdom, and the second is to examine the discharge properties of muscle afferents to determine which ones are most likely to initiate reflexly this phenomenon.