Motor Unit Activity During Maximal-Intent Dynamic Muscle Actions Varies by Intensity and Sex In Healthy Adults

Motor unit firing behavior is regulated by intensity-dependent, hierarchical recruitment patterns. There is also recent evidence of divergent motor unit behavior between sexes during intensity-matched tasks. This study compares the firing behavior of motor unit populations of the biceps brachii between dynamic muscle actions corresponding to ~90% versus ~98% of maximal dynamic strength. Using surface electromyography decomposition, we analyzed the relationships between motor unit action potential amplitude size (MUAP AMP ) and firing rate (FR), identifying the Y-intercepts and slopes. We also identified the average MUAP AMP and FR during each contraction. A total of 152 discrete muscle actions were analyzed from 35 participants. We compared these responses between young adult males ( n = 12) and females ( n = 23). A total of 1,361 motor units were identified ( n = 563 male; n = 798 female) across loading conditions. Our results indicate that the Y-intercept of the MUAP AMP - FR relationship increases with load, while no differences in slope were observed between loading conditions. Notably, females exhibited significantly lower Y-intercepts, FR, and MUAP AMP , and greater slopes (steeper) than males across loading conditions. These findings suggest that, at near-maximal intensities of dynamic strength, motor unit populations adapt their firing properties in a load-dependent manner. The observed sex-dependent differences in motor unit firing behavior are likely related to variations in muscle fiber composition indicative of smaller, more oxidative fiber phenotypes for females. These findings provide novel evidence of load-dependent motor unit firing behavior and divergent firing characteristics between sexes during maximal-intent dynamic muscle actions.