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, whereas 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.NEW & NOTEWORTHY We show that during dynamic, maximal-intent muscle actions, a subtle increase in load (∼1.3 kg) elevates the operating point of the motor unit pool, despite no change in recruitment gain or individual motor unit firing rates. We further demonstrate that sex-based differences in motor unit activity were evident, with males exhibiting greater motor unit firing rates, action potential amplitudes, and firing rate relationships favorable for generating high contraction forces.