In addition to the ability of individual muscle cells to modify their force development, the force of the entire muscle can be modulated. Every muscle cell is not contracted at the same time within a muscle. Since the functional unit of the muscle is the motor unit, the extent of force development will be determined by the number of muscle fibers associated with that motor unit and the way in which those motor units are recruited. As more motor units are simultaneously recruited, the force developed by the muscle will be increased. The extent of force developed depends upon the number of muscle cells present within a given motor unit, which can vary from several hundreds to thousands. Muscles that are involved in fine movement will have fewer muscle cells per motor unit than muscles involved in more gross movement. The specific motor units activated are determined by the size principle. This principle states that there is a correspondence between the size of motor units activated and their order of recruitment. The physiological determinant of which motor units are activated is the sensitivity of the motor neurons to action potential frequency; smaller motor neurons are stimulated to generate an action potential at a lower frequency of action potentials than larger diameter motor neurons. The smaller motor units are innervated by smaller diameter motor neurons. Thus, the size of the motor units corresponds with the diameter of the motor neuron. The larger motor units are resistant to
depolarization. Thus, a higher frequency of action potentials is required to activate the larger motor units. At a lower frequency, the smaller motor units will be activated. As the force required to move an object is increased, the frequency of the action potentials from the motor center is increased, thereby recruiting more of the larger fibers that are capable of generating greater force.
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