Age and sex modulate scapular muscle excitation strategies during loaded shoulder flexion

Age and sex are primary risk factors for shoulder pain, yet the underlying neuromuscular adaptations in scapular control are poorly understood, partly due to methodological inconsistencies in prior research, particularly in load standardization. This study aimed to define age- and sex-related differences in scapular muscle excitation during a shoulder flexion task with a standardized relative load. Fifty-two healthy younger (18-39y) and older (60-74y) males and females performed shoulder flexion against a load equivalent to 30 % of their maximal strength. High-density and bipolar surface electromyography (EMG) quantified the excitation magnitude (%MVIC) and time-to-peak (%TTP) of the serratus anterior (SA), and upper, middle, and lower trapezius (UT, MT, LT). Compared to younger adults, older adults demonstrated significantly greater SA excitation magnitude (p = 0.044) and a delayed time to peak SA excitation (p = 0.002). Separately, females exhibited significantly greater LT excitation than males (p = 0.033). No significant differences were observed for the UT or MT muscles between any groups. Our findings reveal distinct neuromuscular strategies that are dependent on age and sex. The greater and delayed SA excitation in older adults may be a critical compensatory mechanism to preserve shoulder function in the face of age-related neuromuscular decline. Similarly, the heightened LT excitation in females points to a sex-specific motor control strategy. By using a standardized relative load, this study clarifies conflicting reports in the literature and provides novel insights into the adaptive neuromuscular patterns that may influence long-term shoulder health.