Soleus H‐reflex gain in humans walking and running under simulated reduced gravity

1 The Hoffmann (H‐) reflex is an electrical analogue of the monosynaptic stretch reflex, elicited by bypassing the muscle spindle and directly stimulating the afferent nerve. Studying H‐reflex modulation provides insight into how the nervous system centrally modulates stretch reflex responses. 2 A common measure of H‐reflex gain is the slope of the relationship between H‐reflex amplitude and EMG amplitude. To examine soleus H‐reflex gain across a range of EMG levels during human locomotion, we used simulated reduced gravity to reduce muscle activity. We hypothesised that H‐reflex gain would be independent of gravity level. 3 We recorded EMG from eight subjects walking (1.25 m s−1) and running (3.0 m s−1) at four gravity levels (1.0, 0.75, 0.5 and 0.25 G (Earth gravity)). We normalised the stimulus M‐wave and resulting H‐reflex to the maximal M‐wave amplitude (Mmax) elicited throughout the stride to correct for movement of stimulus and recording electrodes relative to nerve and muscle fibres. 4 Peak soleus EMG amplitude decreased by ≈30% for walking and for running over the fourfold change in gravity. As hypothesised, slopes of linear regressions fitted to H‐reflex versus EMG data were independent of gravity for walking and running (ANOVA, P > 0.8). The slopes were also independent of gait (P > 0.6), contrary to previous studies. Walking had a greater y‐intercept (19.9%Mmax) than running (‐2.5%Mmax; P < 0.001). At all levels of EMG, walking H‐reflex amplitudes were higher than running H‐reflex amplitudes by a constant amount. 5 We conclude that the nervous system adjusts H‐reflex threshold but not H‐reflex gain between walking and running. These findings provide insight into potential neural mechanisms responsible for spinal modulation of the stretch reflex during human locomotion.