On exploring muscle aging of the biceps brachii in the middle-aged population using HD-sEMG signal analysis

Although neuromuscular decline is well documented with aging, emerging evidence indicates that it may begin as early as midlife, around age 50. As this stage represents a critical window for early intervention, the present study investigated age- and sex-related differences in muscle activation using high-density surface electromyography (HD-sEMG) of the biceps brachii (BB). Physically active individuals were categorized into three age groups: young (20-30 years), middle-aged (45-55 years), and older adults (65-75 years). HD-sEMG signals were recorded during isometric contractions at 20%, 40%, and 60% of maximal voluntary contraction (MVC). Muscle activation amplitude, spatial distribution, and signal complexity were analyzed. Although strength remained similar across groups, RMS amplitude was primarily influenced by contraction level, with age-related differences emerging in an intensity-dependent manner. In men, older participants exhibited lower RMS amplitudes compared to younger men at higher contraction levels (60% MVC, p<0.05). In women, middle-aged participants consistently exhibited lower RMS amplitudes across contraction levels, accompanied by altered spatial organization of muscle activation, reflected by higher RMS CoV and lower modified entropy at moderate-to-high contraction intensities (p<0.05). Signal complexity, assessed using sample entropy, did not show robust age-related differences, although descriptive trends toward lower values were observed in older adults at low contraction level. Taken together, these findings suggest that midlife, in women, may be characterized by subtle, task-dependent neuromuscular reorganization rather than a generalized decline. Early identification of such changes using HD-sEMG metrics may support timely interventions aimed at preserving neuromuscular function across the lifespan.