Effects of exercise training on components of the motor unit.

Relatively little is known as to how the motor unit (the motoneurone and its innervated muscle fibres) adapts. Using the model of compensatory overload of the rat plantaris, we have described the complex patterns of adaptation which differ across motor units within a single muscle. These adaptations, measured using histochemical and physiological techniques in combination, emphasize the difficulty of predicting whole muscle responses from those of its constituent muscle fibres and motor units. In addition, they suggest that subtle changes may occur in the way motor units are used during training that are not evident when looking at whole muscles. Evidence of the responses of the motoneurone component of the motor unit to overload is fragmentary; nonetheless, other models of neuromuscular adaptation show the extent to which motor nerves and muscles adapt in a coordinated manner, with functional consequences that may be relevant to the exercise training model. More information on overload-induced changes in motoneurone and muscle properties, how these changes are coordinated, and their functional consequences, is necessary in order to better understand the training model.