Presynaptic Inhibition of the H-reflex Relative to Test Reflex Size

Abstract

The amplitude of the H-reflex can be suppressed by both (EPI) and intrinsic (IPI) presynaptic factors associated with the discharged terminals of Ia afferent collaterals. Unfortunately, in human experimental paradigms, the relative influences of EPI and IPI on suppression of the H-reflex are hard to differentiate. It is known that the sensitivity of the conditioned H-reflex to inhibition depends on the size of the test reflex, and that the onset of a voluntary contraction has been shown to decrease the amount of presynaptic inhibition observed in the H-reflex pathway. PURPOSE: The purpose of this research was to investigate the effect stimulus intensity (SI) had on both EPI and IPI of the soleus H-reflex, and to examine how a subsequent voluntary contraction influenced each presynaptic mechanism. METHODS: Ten young healthy subjects were administered two conditioning protocols on the H-reflex and motor response recruitment curves (RC); 1) common peroneal nerve stimulation prior to tibial nerve stimulation (i.e. EPI), and 2) paired tibial nerve stimulation (i.e. IPI). The interstimulus interval was 100 ms for both conditioning protocols. Further, a 10% voluntary isometric plantarflexion was held for two seconds prior to stimulation and the conditioning paradigms were repeated. Three measurements from the RC's were extracted and standardized to the maximal motor response; H-reflex threshold (Hth), 50% Hmax, and Hmax. RESULTS: A 3x3x2 (Protocol x SI x BEMG) repeated measures ANOVA indicated a significant interaction between protocol and SI [F(4,36) = 7.05, p < 0.001]. Simple effects analyses showed that 50% Hmax [F(2,54) = 31.11, p < 0.001] and Hmax [F(2,54) = 27.37, p < 0.001] were significantly different across the protocols. Tukey's post hoc analyses revealed that at 50% Hmax and at Hmax both EPI and IPI were significantly inhibited but not different from each other (23 and 28%; 28 and 35%, respectively). A 10% contraction significantly disinhibited both 50% Hmax and Hmax for EPI and IPI (30 and 14%; 13 and 12%), but also significantly disinhibited Hth for EPI but not for IPI (14% vs 6%). CONCLUSION: It was shown that the sensitivity of the discrete H-reflex to EPI and IPI cannot be distinguished across SI's. However, the addition of minimal BEMG revealed that, at very low SI's, EPI was more sensitive to supraspinal input, which may indicate differences in the pathways/mechanisms between EPI and IPI.