Sex-related Differences In H-reflex Excitability And Inhibition

The Hoffmann reflex (H-reflex), when elicited at the soleus (SOL) in healthy males and females, produces a response that can be interpreted as the efficacy of synaptic transmission from sensory to motor fibres. However, the current literature on sex-related differences on spinal excitability, measured via the H-reflex are equivocal. Some studies demonstrate greater or lesser excitability, while others show no difference. PURPOSE: To determine whether H-reflex excitability and pre-synaptic inhibition at the SOL is different between males and females. METHODS: Ten healthy young individuals, 5 males (24.2 ± 1.1 yr) and 5 females (24.0 ± 2.9 yr), participated in the study. Surface electromyography (EMG) electrodes were used to record the H-reflex at the soleus. The tibial nerve was stimulated at increasing intensity from sub-threshold to supramaximal M-wave, to produce a stimulus response curve, based on peak-to-peak amplitude, for both the H-reflex and M-wave. Pre-synaptic inhibition was assessed as the percent decline in the H-reflex when conditioned by stimulating the common peroneal nerve 100 ms before the stimulation of the tibial nerve. The following measures were obtained and compared between males and females: the maximal amplitude of the H-reflex (Hmax) and M-wave (Mmax), Hmax/Mmax ratio, slope of the H-reflex (Hslope) and M-wave (Mslope) stimulus-response curves, Hslope/Mslope ratio, and percent inhibition. RESULTS: The Hslope was greater in males (0.063 ± 0.001) than in females (0.026 ± 0.0001; p = 0.04; ES = 1.6). There were no statistically significant differences between sexes in any other variable: Mmax (p = 0.15; ES = 1.0), Hmax (p = 0.29; ES = 0.72), Hmax/Mmax ratio (p = 0.39; ES = 0.57), Mslope (p = 0.12; ES = 1.1), Hslope/Mslope ratio (p = 0.82; ES = 0.15). There was also no significant difference in pre-synaptic inhibition between males and females (p = 0.40; ES = 0.57). CONCLUSION: Although there were no significant differences found between males and females for Mmax, Hmax, Hmax/Mmax ratio, Mslope, Hslope/Mslope ratio, and percent inhibition, their effect sizes were large indicating differences may become apparent with a larger sample size. Natural Sciences and Engineering Research Council (NSERC) (RGPIN-2019-05127)