Morphological features and activation patterns of motor units.

The force that a muscle exerts depends on which motor units have been recruited and at what rate they are discharging action potentials. Because of differences in motor-unit morphology (innervation ratio, average cross-sectional area, specific force, and geometric distribution of muscle fibers), the maximum motor-unit force within a motor-neuron pool is not constant but rather can vary by approximately 50 times. Consequently, muscle force is affected by which motor units have been activated. Similarly, the rate at which a motor neuron discharges action potentials depends on the pattern and quantity of the synaptic input it receives and its intrinsic frequency-current relation. The force that a single motor unit can exert will vary by approximately 3 to 15 times when discharge rate is increased from a minimum to a maximum. In the performance of voluntary contractions, therefore, humans appear to have an infinite number of combinations of motor-unit recruitment and discharge rate that can be used to vary muscle force. However, control strategies have evolved that reduce these options substantially. From experiments on low-force, isometric contractions, it appears that the recruitment order of motor units is relatively fixed and that muscle force is graded by concurrent variations in recruitment and discharge rate over much of the force range. Whereas the recruitment order of motor units appears to be robust in other tasks also, perhaps with the exception of lengthening contractions, the use of discharge rate to grade muscle force seems more variable. When subjected to acute and chronic challenges, motor-unit properties can adapt within limited ranges, and motor-unit discharge rate appears to be more affected than recruitment order, although the extent of recruitment may be affected by changes in use. Because movement is controlled by motor-unit activity, an understanding of motor-unit physiology can have a significant impact on the evaluation and treatment of movement disorders.