Motor unit physiology: some unresolved issues

The purpose of this review was to examine three issues that limit our understanding of motor unit physiology: (1) the range and distribution of the innervation ratios in a muscle; (2) the association between discharge rate and force; and (3) the variation in motor unit activity across contractions that differ in speed and type. We suggest that if more data were available on these issues, the understanding of neuromuscular function would be enhanced substantially, especially with regard to plasticity in the motor neuron pool, adequacy of the neural drive to muscle, and flexibility of activation patterns across various types of contractions. Current data are limited and these limitations influence our ability to interpret adaptations in muscle function in health and disease.