The fusimotor system. Its role in fatigue.

Several lines of evidence point to an important role of the fusimotor system in the "muscle-wisdom" phenomenon during peripheral fatigue of some human voluntary contractions: 1) muscle afferents provide a net amplification of skeletomotor output, with the only known afferent species capable of this being the muscle spindle; 2) muscle spindle firing rates decline during constant-force voluntary contractions, so fusimotor support to skeletomotor output decreases; 3) this waning support can be offset by application of high-frequency vibration to the fatiguing muscle, which excites spindle endings; and 4) the progressive decline in motor unit firing rates during maximal voluntary contractions is abolished by blocking muscle afferent inputs, and it is argued that, at least in the initial stages of a contraction, this must be due to a progressive withdrawal of spindle support.