Menstrual cycle effects on sex differences in motor control during dynamic balance perturbations.

This study investigated sex differences in neuromuscular responses during dynamic balance perturbations and isometric maximal voluntary contractions (IMVCs) by examining if menstrual cycle (MC) phase and the fluctuations in endogenous hormones influence the observation of sex differences. Twenty-seven young adults (17 males: 28.5 ± 6.4 years; 10 females: 31.2 ± 6.5 years) were measured twice: females were measured at the early-follicular (EF) phase, that is 2-4 days after onset of bleeding, and at the mid-luteal phase (LUT), coinciding with peak levels of progesterone. Males were measured twice with 7-14 days between measurements. During dynamic balance perturbations, H-reflex and V-wave (neural drive) responses were recorded from the posterior perturbations at four latencies (40 ms, 70 ms, 100 ms, and 130 ms) coinciding with short- (SLR), medium- (MLR), and long- (LLR) latency responses and voluntary activation. Further, IMVC and its associated neural drive were assessed. Sex differences in either the H-reflex (P = 0.217) or the V-wave (P = 0.475) were not observed during balance perturbation tasks when females at EF were compared to males. However, when the measurements were conducted at LUT, a significant sex difference was revealed in both H-reflex (P = 0.029) and V-wave (P = 0.049) responses. Similarly, in neural drive during IMVC, no sex differences were observed at EF (P = 0.440), but a significant sex difference at LUT was observed (P = 0.024). Our results suggest that hormonal fluctuation in two distinct MC phases lead to females having lower neural responses during dynamic balance and voluntary force generation tasks at LUT compared to males.