Basis of segmental motor control: motoneuron size or motor unit type?

The principles of organization of motor control at the segmental level are developed and discussed in this review. Consideration is given to the concepts of the motor unit, and the motoneuron pool. Recent studies from our laboratories that have significance for hypotheses regarding segmental motor control are presented. These studies indicate that the critical factor controlling motor unit recruitment in heterogenous muscles is motor unit type. This results in motor unit recruitment in the order of increasing contraction strength and fatigability: slow twitch, fatigue-resistant first; fast twitch, fatigue-resistant second; fast twitch with intermediate fatigue resistance third; and fast twitch, fatigable units last. A recruitment model that incorporates this hypothesis is presented in which there is an orderly recruitment of motor units by type. This recruitment model, based on data from cat medial gastrocnemius motor units, closely approximates a theoretical optimal recruitment strategy and is consistent with actual medial gastrocnemius muscle forces generated during free movements in intact animals.