Sex-related Differences In H-reflex Responses, Co-activation, And Physical Activity

Males and females display distinct differences in spinal synaptic transmission when assessed using the Hoffmann reflex (H-reflex). However, the origin of these differences in H-reflex excitability and inhibition remains ambiguous. Mechanisms thought to influence the spinal synaptic response pathway include antagonistic co-activation and daily physical activity levels. PURPOSE: To investigate the relationship between H-reflex excitability and inhibition and each of physical activity and co-activation in males and females. METHODS: Thirty healthy young individuals, 15 M (24 ± 2 yrs) and 15F (23 ± 3 yrs) participated in the study. The H-reflex recruitment curves were generated for the soleus by stimulating the tibial nerve, beginning at a low-intensity voltage, and increasing incrementally past the H-max until the maximal amplitude of the M-wave (M-max) was reached, while participants contracted to 10% of maximal force. Excitability was quantified as the H-max/M-max ratio. Reciprocal inhibition (RI) was assessed by stimulating the deep peroneal nerve prior to the tibial nerve and measuring the relative decline in the amplitude of the stimulus-reflex. Co-activation was assessed via surface electromyography from the tibialis anterior and soleus. Physical activity was measured in 4 domains over 7 days with the International Physical Activity Questionnaire (IPAQ). Primary outcome variables were compared between sexes. Relationships between H-reflex excitability and inhibition and each of physical activity and co-activation were assessed with regression analyses. RESULTS: Excitability was not significantly different between sexes, but males exhibited significantly greater RI than females (p = 0.018, d = 0.93; 14 M:15F). Males had higher IPAQ scores than females (p = 0.036, d = 0.94; 12 M:12F). No significant differences were identified in co-activation between sexes (p = 0.82, g = 0.12; 5 M:10F). There were no significant relationships between excitability and inhibition in either physical activity or co-activation for males (R2 = 0.27, p = 0.36) or for females (R2 = 0.04, p = 0.74). CONCLUSION: Males had greater RI than females. However, these results suggest spinal excitability and inhibition are not associated with physical activity level or co-activation in either males or females. Supported by: Natural Sciences and Engineering Research Council (NSERC) (RGPIN-2019-05127)