Sex-related Differences In Mechanomyographic Amplitude Of The Biceps Brachii During Motor Unit Activation And Deactivation

PURPOSE: To investigate mechanomyographic amplitude (MMGRMS)-force relationships of the biceps brachii (BB) between females and males during a high-intensity muscle action. METHODS: Ten healthy females (age = 20 ± 3 yrs) and twelve healthy males (age = 24 ± 4 yrs) participated in this study. Participants performed maximal voluntary contractions (MVCs) of the elbow flexors on a dynamometer, followed by an isometric trapezoidal muscle action at 70% MVC. Surface MMG was recorded from the BB. Subcutaneous fat (sFAT) of the BB was measured via ultrasonography. Individual a (gain) and b (slope) terms were calculated from the log-transformed MMGRMS-force relationships for the linearly increasing and decreasing segments. MMGRMS during the steady force segment was normalized (N-MMGRMS) to MVC MMGRMS. A 2-way mixed factorial repeated measures ANOVA (sex [males vs. females] x segment [increase vs. decrease]) examined the a and b terms. Independent samples t-tests compared N-MMGRMS and sFAT between sexes. Alpha was set at ≤0.05. RESULTS: For b terms, there were neither a significant two-way interaction (p = 0.856) nor a main effect for segment (p = 0.248). However, there was a main effect for sex (p < 0.001). The b terms were greater for the males (0.800 ± 0.231) than the females (0.548 ± 0.170) when collapsed across segment. For a terms, there was neither a significant two-way interaction nor main effects for sex and segment (p = 0.564 - 0.910). Additionally, there were no significant differences between groups for N-MMGRMS (p = 0.241) and sFAT (p = 0.219). CONCLUSION: Males exhibited greater b terms (slopes) than the females during the 70% MVC; however, there was no significant difference in the a terms (gain) of the log-transformed MMGRMS-force relationships between sexes. Therefore, motor unit activation and deactivation strategies were influenced as a function of sex. The similar sFAT between sexes provides confidence the differences in the b terms between the males and females was not due to sFAT filtering of MMG signal. Furthermore, the lack of differences between the males and females for N-MMGRMS during the steady force segment may suggest that linearly increasing and decreasing muscle actions are more appropriate for investigating sex-related differences in the mechanical behavior (MMGRMS) of the BB.