Sex differences in soleus muscle H-reflex and V-wave excitability.

We compared H-reflex and V-wave excitability between men and women engaging in similar levels of physical activity. We also explored whether differences in antagonist muscle co-activation between sexes might partially explain sexual dimorphism in the excitability of the H-reflex and V-wave. Fifty-seven young participants were included (29 men: 21.7 ± 2.3 years; 28 women: 22.4 ± 3.3 years). Soleus M- and H-recruitment curves were constructed on a tonic background muscle activation. V-waves were elicited during maximal voluntary contraction (MVC). Besides being stronger than women, men achieved greater Hmax /Mmax values and presented a steeper slope of the ascending limb of the H-reflex recruitment curve (P < 0.05). The current intensity required to elicit Hmax was lower for men (P < 0.05). The co-activation of the tibialis anterior muscle during the sustained plantar flexions was greater in women (ratio between tibialis and soleus normalized EMG: 20.5 vs. 8.3%, P < 0.05). Covariance analysis showed that sexual dimorphism in H-reflex excitability was dissipated when controlling for antagonist co-activation. V-wave normalized amplitude was similar between sexes even after controlling for the effects of Hmax /Mmax and antagonist co-activation as covariates. Thus, women exhibit lower H-reflex excitability than men and this is dependent on their higher level of antagonist muscle co-activation. While sex differences in antagonist co-activation persist during MVCs, this is not the case for V-wave normalized amplitude. Thus, although the efficacy of the transmission between Ia afferent fibres to α-motoneurons is lower in women because of a greater level of antagonist co-activation, our findings are consistent with similar supraspinal drive between sexes.