Motor unit behavior adaptations across lifespan: sex differences in young, middle-aged and old adults.

Aging is associated with neuromuscular decline, but how sex modulates motor unit adaptations across adulthood remains unclear. This study examined age- and sex-related differences in motor unit firing behavior in young (YG), middle-aged (MA), and older (OLD) adults by integrating high-density surface EMG decomposition with assessments of muscle morphology and daily physical activity. Linear mixed-effects models revealed significant effects of age and sex on mean firing rate: females showed higher rates than males at 30% maximal voluntary contraction (MVC) in YG and MA groups, but not in OLD, and no sex differences were observed at 50% MVC. Firing-rate variability was consistently higher in females. During force-increasing contractions, OLD adults showed reduced motor unit discharge modulation; in early-recruited units, reductions were significant in OLD females relative to both YG and MA females, while males showed reductions across both early- and late-recruited units. Males exhibited greater muscle thickness, cross-sectional area, and maximal torque, and daily physical activity was lower in OLD participants. These findings indicate that neuromuscular aging is associated with reduced discharge-rate modulation and a convergence of motor unit behavior between sexes in older age. Physical activity may contribute, underscoring the importance of sex-sensitive strategies to preserve neuromuscular function.