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

Muscle strength (MVC) remained similar across age groups despite age-related differences in HD-sEMG metrics.

• Participants were categorized into young (20-30 years), middle-aged (45-55 years), and older adults (65-75 years).
• All participants were physically active individuals.
• Isometric contractions were recorded at 20%, 40%, and 60% of maximal voluntary contraction (MVC).
• Strength similarity across groups indicates that neuromuscular changes may precede or occur independently of observable strength loss.

In men, older participants exhibited lower RMS amplitudes compared to younger men at higher contraction levels.

• The age-related difference in RMS amplitude in men was observed specifically at 60% MVC (p<0.05).
• RMS amplitude was primarily influenced by contraction level across groups.
• Age-related differences in RMS amplitude emerged in an intensity-dependent manner.
• No significant RMS differences were reported between young and middle-aged men.

Middle-aged women exhibited consistently lower RMS amplitudes across contraction levels compared to younger women.

• Lower RMS amplitudes in middle-aged women were observed across multiple contraction intensities (20%, 40%, and 60% MVC).
• This pattern was distinct from the age-related pattern observed in men, where differences were only intensity-dependent at the highest contraction level.
• These findings suggest sex-specific differences in the trajectory of neuromuscular aging.
• The pattern was described as 'consistent' across contraction levels rather than emerging only at high intensities.

Middle-aged women showed altered spatial organization of muscle activation, reflected by higher RMS coefficient of variation (CoV) and lower modified entropy at moderate-to-high contraction intensities.

• Higher RMS CoV and lower modified entropy were observed at moderate-to-high contraction intensities (p<0.05).
• These spatial distribution metrics reflect reorganization of muscle activation patterns across the biceps brachii.
• HD-sEMG allowed spatial analysis of muscle activation that would not be detectable with conventional sEMG.
• These changes were present in the middle-aged group, suggesting neuromuscular reorganization begins as early as midlife in women.

Signal complexity assessed by sample entropy did not show robust age-related differences, though descriptive trends toward lower values were observed in older adults at low contraction levels.

• Sample entropy was used to assess signal complexity from HD-sEMG recordings.
• No statistically significant age-related differences in sample entropy were found.
• A descriptive trend toward lower sample entropy values was observed in older adults specifically at low contraction levels (20% MVC).
• The lack of robust sample entropy differences suggests complexity measures may be less sensitive than amplitude or spatial metrics for detecting early neuromuscular aging in this population.

The study investigated age- and sex-related neuromuscular differences using HD-sEMG in physically active individuals across three age groups.

• High-density surface electromyography (HD-sEMG) was applied to the biceps brachii (BB).
• Three metrics were analyzed: muscle activation amplitude (RMS), spatial distribution (RMS CoV, modified entropy), and signal complexity (sample entropy).
• The study design included both men and women to enable sex-stratified analysis.
• The middle-aged group (45-55 years) was included specifically because emerging evidence indicates neuromuscular decline may begin as early as midlife around age 50.

Emerging evidence indicates that neuromuscular decline may begin as early as midlife, around age 50, representing a critical window for early intervention.

• This framing motivated the inclusion of a middle-aged cohort (45-55 years) in the study design.
• Early identification of neuromuscular changes using HD-sEMG metrics may support timely interventions aimed at preserving neuromuscular function across the lifespan.
• The study positioned midlife as an underexplored but important stage in the continuum of neuromuscular aging.