Sex-related differences in muscle size explained by amplitudes of higher-threshold motor unit action potentials and muscle fibre typing.

Abstract

To investigate the relationships between motor unit action potential amplitudes (MUAPAMP ), muscle cross-sectional area (mCSA) and composition (mEI), per cent myosin heavy chain (%MHC) areas and sex in the vastus lateralis (VL). Ten males and 10 females performed a submaximal isometric trapezoid muscle action that included a linearly increasing, steady torque at 40% maximal voluntary contraction, and linearly decreasing segments. Surface electromyographic decomposition techniques were utilized to determine MUAPAMPS in relation to recruitment thresholds (RT). Ultrasound images were taken to quantify muscle mCSA and mEI. Muscle biopsies were collected to calculate %MHC areas. Y-intercepts and slopes were calculated for the MUAPAMP vs RT relationships for each subject. Independent-samples t tests and ANOVA models examined sex-related differences in mCSA, mEI, slopes and y-intercepts for the MUAPAMP vs RT relationships and %MHC areas. Correlations were performed among type IIA and total type II %MHC area, mCSA and the slopes and y-intercepts for the MUAPAMP vs RT relationships. Males exhibited greater slopes for the MUAPAMP vs RT relationships (P=.003), mCSA (P<.001) and type IIA %MHC (P=.011), whereas females had greater type I %MHC area (P=.010) and mEI (P=.024). The mCSA, type IIA and total II %MHC area variables were correlated (P<.001-.015, r=.596-.836) with the slopes from the MUAPAMP vs RT relationships. Sex-related differences in mCSA and MUAPAMPS of the higher-threshold MUs were likely the result of larger muscle fibres expressing type II characteristics for males.