PREVIOUS work from this laboratory indicates that there is a relation between the speed of contraction of a motor unit and the size of the motor neuron innervating it.1,2Motor units which contract rapidly and develop large amounts of tension are innervated by large motor neurons, whereas motor units which contract slowly and develop small amounts of tension are innervated by small motor neurons. Moreover, the size of a motor neuron determines its susceptibility to discharge: the smaller an anterior horn cell, the lower its threshold for excitation.3It follows that the contraction speed of muscle fibers may be a function of the amount of contractile activity they perform. The available evidence1-3suggests that muscle fibers which are used intensively have slow speeds of contraction. If this inference is correct, experimental interventions which alter the amount of activity or usage of a muscle should produce changes in