Effect of mental imagery of a motor task on the Hoffmann reflex

Research has found that mental imagery of a motor task may influence the Hoffmann reflex (H-reflex). However, this work has not examined the potential influence of background EMG (BEMG) on the H-reflex. In this study 23 adult participants (M=23.3 years, S.D.=3.2) were instructed to mentally image plantar flexion of the right foot at four intensities: 40, 60, 80 and 100% of maximum voluntary contraction (MVC) after completing practice trials of actual contractions at these intensities. Dependent measures were the BEMG activity and the peak-to-peak amplitude of the H-reflex. On each trial the peak-to-peak amplitude of the soleus H-reflex was measured in the right leg and averaged, BEMG (40 ms bin) was measured in the soleus and tibialis anterior of both legs. Following trials of plantar flexion at the target intensities participants completed 5 imagery trials at each intensity and 15 trials while performing this motor task. Five resting control trials were administered prior to and following the completion of all test trials. Administration of test trials was randomized within conditions. A main effect (P<0.05) for trial blocks was observed for H-reflex amplitude but not BEMG. The H-reflex increased linearly throughout testing, suggesting that the H-reflex was modified by the practice of imagery rather than the intensity of the imagined task.