title: Soleus H-reflex gain in humans walking and running under simulated reduced gravity.
abstract: 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.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.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. 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. 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.
authors.affiliation: Department of Integrative Biology, University of California, Berkeley, CA 94720-3140, USA. dferris@u.washington.edu
authors.forename: D P
authors.initials: DP
authors.lastname: Ferris
authors.forename: P
authors.initials: P
authors.lastname: Aagaard
authors.forename: E B
authors.initials: EB
authors.lastname: Simonsen
authors.forename: C T
authors.initials: CT
authors.lastname: Farley
authors.forename: P
authors.initials: P
authors.lastname: Dyhre-Poulsen
mesh_terms: D000328:Adult; D004576:Electromyography; D005528:Foot / Q000502:physiology; D006112:Gravitation; D006181:H-Reflex / Q000502:physiology*; D006801:Humans; D008297:Male; D018482:Muscle, Skeletal / Q000294:innervation / Q000502:physiology*; D009475:Neurons, Afferent / Q000502:physiology; D012044:Regression Analysis; D012420:Running / Q000502:physiology*; D016138:Walking / Q000502:physiology*