Fusimotor system in muscle fatigue.

Evidence is reviewed that a reflex increase in fusimotor activity develops during fatiguing muscle contraction in decerebrate and/or spinal cats. This phenomenon is apparently due to an increased inflow from small-diameter afferents that is elicited by metabolic products liberated and/or accumulated in the muscle tissue. It occurs in fusimotor neurones to the contracting muscle as well as in those to its close synergists, inactive remote muscle groups, and in a smaller number of those to antagonist muscles. It is finely modulated by afferent inflow of other origin and results in changes of muscle spindle outflow and responsiveness. According to the magnitude and time course of the responses, the fusimotor-spindle system might contribute to: (1) optimization of skeletomotor activity directed to the contracting muscle so as to relieve the fatigue-induced changes in its contractile properties; (2) spreading of activity to initially inactive muscle groups when the agonist gets fatigued; (3) providing more information to skeletomotor neurones and higher motor centres both during and after the fatiguing contraction until the muscle recovers. However, it remains to be assessed whether such a reflex increase plays a role in muscle fatigue in intact animals and/or humans.