The Effects of Motor Imagery on the Hoffmann Reflex and Presynaptic Inhibition

Abstract

Mental imagery of a motor task may influence the amplitude of the Hoffmann reflex (H-reflex) (Hale et al. 2003, Oishi et al. 1995), but these investigations have not examined the potential influence that presynaptic inhibition might have on the H-reflex during mental imagery of a motor task. PURPOSE: The purpose of this project was to investigate the effects of mental imagery of a motor task on the H-reflex and on presynaptic inhibition of the H-reflex. METHODS: In this study 25 adult participants (M=22.8 years, SD=5.4; 15 Females, 10 Males) were examined on one day of testing. The participants mentally imaged a simple plantar flexion movement of the right foot at two intensities: 20 and 60% of maximum voluntary contraction after completing a series of practice trials of actual contractions at these intensities. Five participants served as control subjects who completed the entire testing protocol but did not perform mental imagery. The dependent measures were peak-to-peak amplitude of the H-reflex and the peak to peak amplitude of the conditioned H-reflex. Data analysis was conducted using a 2X2 ANOVA to determine if any changes in H-reflex amplitude occurred from rest and imagery percentage. Additionally, a One-way ANOVA was conducted to determine if any changes in presynaptic inhibition were noted during the imagery trials. RESULTS: Repeated measures ANOVA revealed a significant (P < 0.05) condition effect for H-reflex amplitude during imagery trials. Specifically, H-reflex amplitude increased between 5 and 14% during imagery trials above rest (Mean at Rest = 1.62, Mean at 20% = 1.84, Mean at 60% = 1.71). Additionally, a significant (P < 0.05) difference was also observed between percent of contraction imagined. A significant (P < 0.05) difference was observed in presynaptic inhibition between percent of imagined contraction Mean at 20% = .36, Mean at 60% = .39). Control subjects demonstrated no significant difference for H-reflex or presynaptic inhibition. CONCLUSION: These findings indicate that the amplitude of the H-reflex is increased while performing mental imagery of plantar flexion. Additionally, the measured differences in the conditioned H-reflex indicate that presynaptic inhibition might influence H-reflex response during mental imagery.