Muscles are functionally composed of motor units, the smallest functional unit within muscles. They represent the smallest controllable unit in muscles. Patterns of how units are recruited and discharge provide insight into the complex motor control scheme of the nervous system and generate more accurate neural control and biomechanical models. Understanding motor control schemes can will inform development of new rehabilitative strategies for neuromuscular injuries or motor control deficits. This dissertation examines motor unit activation and control in healthy adults and in children with cerebral palsy. Analysis of recruitment and discharge parameters of individual units and motor unit pool in primary elbow flexors allowed development of a nuanced understanding of how motor unit control is varied over a variety of conditions.

In the isometric condition, in which muscle length is fixed, elbow flexor firing rates were found to correlate with muscle forces, but not changes in muscle length. To measure the versatility of motor unit function, the temperature of the muscle was lowered, altering muscle contractile mechanics. This reduced firing rates of the active biceps brachii and brachioradialis and altered the recruitment behavior of flexor motor units compared to the unaltered normal muscle temperature condition. When observing motor control in individuals with cerebral palsy to their typically developing peers we found that there was a significant decrease in motor unit firing rates compared across the same percent of maximal joint torque. This was accompanied with a unique pattern of motor unit recruitment. These results indicate a compensatory recruitment strategy for physiological deficits. The final aim of this work found that populations of motor units within activations had firing rate distributions that were good predictors of muscle forces and joint torques. These distribution shape characteristics included the higher-order mathematical moments of the peak mean motor unit firing rates. This suggests that while observing individual motor units can provide some information about neural control strategies, analysis of motor unit populations and global parameters may capture important aspects of overall motor control schemes.