Specific recruitment properties of spinal reflex of thigh muscle in sprinter

Thigh muscles activation pattern plays a key role in high-performance sprint running. We investigated spinal neural mechanisms underlying sprinters' specific thigh muscle control by examining spinal reflex recruitment properties and reciprocal inhibition of the thigh muscles in sprinters. Ten high-level national sprinters and ten controls participated in this study. Transcutaneous spinal cord stimulation (tSCS) was used to evoke spinal reflexes in biceps femoris (BF) and soleus (SOL) muscles. To assess the recruitment properties of thigh muscles, in Experiment 1, tSCS was applied at varying intensities to analyze recruitment curves. In Experiments 2-4, reciprocal inhibition of the thigh muscles was assessed under electrical stimulation, voluntary contraction, and muscle vibration. Sprinters exhibited a significantly steeper recruitment curve slope for BF spinal reflex compared to controls, whereas the groups did not differ for the SOL. Reciprocal inhibition from the rectus femoris to BF showed no significant differences between groups. These findings suggest that spinal neural circuits in sprinters undergo plastic changes, allowing higher motor output induced by reflexes with small changes in sensory input. This sprinter-specific spinal reflex recruitment, particularly in BF, likely aligns with their specific thigh muscle activation patterns.