Modulation of Soleus H-Reflex across different levels of voluntary isometric contractions

Abstract

This study investigates ”How does the modulation of the Soleus H-reflex vary across different levels of isometric voluntary contractions?” The hypothesis that the Soleus H-reflex amplitude follows a biphasic response, with an initial increase at lower contraction levels, followed by a plateau or decrease at higher levels of contractions. To test this, ten healthy subjects were recruited to perform isometric voluntary contractions at different levels of maximal voluntary contraction (MVC) (0%, 20%, 40%, 60%, and 80% MVC). The H-reflex peak-to-peak amplitude was measured and normalized as a percentage of the maximum M-wave (H/Mmax). The data were analyzed using a repeated measures ANOVA to assess the effect of MVC level on the Soleus H-reflex amplitude, followed by pairwise comparisons to identify significant differences between contraction levels. A repeated measures ANOVA revealed a significant effect of MVC level on Soleus H-reflex amplitude (H/Mmax) (F(4,32) = 4.82, p < 0.05). Pairwise comparisons showed a significant increase in H-reflex amplitude between 0% and 20% MVC (p = 0.013), while no significant differences were observed between higher contraction levels (20%, 40%, 60%, and 80% MVC). This pattern is indicative of the biphasic hypothesis, suggesting that the reflex amplitude significantly increases initially but plateaus as contraction intensity increases. These findings suggest that the initial increase and subsequent plateau in reflex amplitude may be influenced by changes in spinal excitability and inhibitory mechanisms as contraction intensity increases.