Previous research demonstrates modulation of the Hoffmann reflex ampli
tude and gain during changes in environmental conditions. H-reflex gai
n (defined in this study as the ratio of H-reflex amplitude to average
soleus background EMG) is considered a functional measure of reflex m
odulation. In this study the effects of manipulating visual input and
surface stability were investigated in 17 subjects under four experime
ntal conditions: (1) vision-stable surface, (2) no vision-stable surfa
ce, (3) vision-unstable surface, and (4) no vision-unstable surface. I
n each condition, subjects performed fifteen trials of a single leg st
ance for 7 s. The H-reflex was electrically elicited at the end of eac
h trial by delivering a 1 ms square wave stimulation to the tibial ner
ve in the popliteal fossa of the dominant leg. Average background EMG
(40 ms window) and peak to peak amplitude of the H-reflex were measure
d online for each trial (sampling rate = 2 kHz). An analysis of varian
ce 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), b
etween these variables. These results suggest that supraspinal mechani
sms, possibly presynaptic inhibition, modulate H-reflex gain across en
vironmental conditions. We conclude that visual and possibly cutaneous
inputs were responsible for driving presynaptic inhibition and thus d
ecreasing H-reflex gain.