Soleus a-reflex gain in humans walking and running under simulated reducedgravity

1. The Hoffmann (H-) reflex is an electrical analogue of the monosynaptic s tretch reflex, elicited by bypassing the muscle spindle and directly stimul ating the afferent nerve. Studying H-reflex modulation provides insight int o how the nervous system centrally modulates stretch reflex responses. 2. A common measure of H-reflex gain is the slope of the relationship betwe en H-reflex amplitude and EMG amplitude. To examine soleus H-reflex gain ac ross a range of EMC: levels during human locomotion, we used simulated redu ced gravity to reduce muscle activity. We hypothesised that H-reflex gain w ould 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 gravi ty)). We normalised the stimulus M-wave and resulting H-reflex to the maxim al M-wave amplitude (M-max) elicited throughout the stride to correct for m ovement of stimulus and recording electrodes relative to nerve and muscle f ibres. 4. Peak soleus EMG amplitude decreased by similar to 30% for walking and fo r running over the fourfold change in gravity. As hypothesised, slopes of l inear regressions fitted to H-reflex versus EMG data were independent of gr avity for walking and running (ANOVA, P > 0.8). The slopes were also indepe ndent of gait (P > 0.6), contrary to previous studies. Walking had a greate r y-intercept (19.9% M-max) than running (-2.5% M-max; P < 0.001). At all l evels 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 in to potential neural mechanisms responsible for spinal modulation of the str etch reflex during human locomotion.