Sex differences in the cardiovascular and ventilatory responses to arm or leg mechanoreflex activation

The mechanoreflex mediates the initial cardiorespiratory response during dynamic exercise, via input from the mechanical deformation of the exercising muscle. In response to passive movement (PM), engaging a greater number of limbs elicits a higher heart rate response. Additionally, males have larger pressor responses to arm PM; however, no sex differences exist in the pressor response to leg PM. Considering that the legs are a larger muscle mass and the pressor response to leg PM was not different between sexes, this may suggest inherent limb-dependant differences. This study aimed to determine sex differences and the role of limb volume and muscle strength on the cardiorespiratory responses to arm and leg PM. Females were smaller (i.e., stature, limb volume), less fit and had weaker handgrip strength compared to males (all p < 0.05); although, both sexes had similar plantarflexion strength (p = 0.3). During arm PM, only males experienced an increased MAP response to arm PM (p < 0.001), compared to females (p < 0.05); however, this sex difference was eliminated when accounting for disparities in forearm volume. Males and females had similar cardiorespiratory responses to leg PM (all p > 0.05). Further, greater plantarflexion strength enhanced the ventilatory response to leg PM in both sexes (p = 0.024). Based on covariate analysis, differences in limb volume and muscle strength contributed almost half of the variability in the cardiorespiratory responses to arm or leg PM. Contrary to our hypotheses, the results of the current study suggest that the previously observed sex and limb-dependant differences were influenced by disparities in limb characteristics (i.e., volume or strength).