THE EFFECTS OF VISION AND TASK COMPLEXITY ON HOFFMANN REFLEX GAIN

Previous research demonstrates modulation of the Hoffmann reflex amplitude and gain during changes in environmental conditions. H-reflex gain (defined in this study as the ratio of H-reflex amplitude to average soleus background EMG) is considered a functional measure of reflex modulation. In this study the effects of manipulating visual input and surface stability were investigated in 17 subjects under four experimental conditions: (1) vision-stable surface, (2) no vision-stable surface, (3) vision-unstable surface, and (4) no vision-unstable surface. In each condition, subjects performed fifteen trials of a single leg stance for 7 s. The H-reflex was electrically elicited at the end of each trial by delivering a 1 ms square wave stimulation to the tibial nerve in the popliteal fossa of the dominant leg. Average background EMG (40 ms window) and peak to peak amplitude of the H-reflex were measured online for each trial (sampling rate = 2 kHz). An analysis of variance revealed significant decreases in H-reflex gain for the visual (F-1,F-16 = 4.71, P < 0.05) and, surface conditions (F-1,F-16 = 7.67, P < 0.05), however there was no interaction (F-1,F-16 = 0.48, P < 0.05), between these variables. These results suggest that supraspinal mechanisms, possibly presynaptic inhibition, modulate H-reflex gain across environmental conditions. We conclude that visual and possibly cutaneous inputs were responsible for driving presynaptic inhibition and thus decreasing H-reflex gain.