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, elicitedby 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 ) 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 (M max ) 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% M max ) than running (_2.5% M max ; 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.

Muscle activation during unilateral stepping occurs in the nonstepping limb of humans with clinically complete spinal cord injury

“…This suggests that the increases in EMG amplitude during limb loading were not caused by stretch reflex activation. Previous studies have found that the EMG amplitude increases in proportion to lower limb loading in nondisabled and spinal cord-injured humans, 4,[16][17][18] but those studies have examined normal bilateral stepping where the limb undergoes considerable flexion and extension with each step.…”

Soleus H-Reflex Modulation After Motor Incomplete Spinal Cord Injury: Effects of Body Position and Walking Speed

“…Previous reports suggest that the soleus H-reflex amplitude decreases with increasing walking speeds (17)(18)(19)(20). However, it is not known if the soleus H-reflexes decrease with an increase in walking speed in patients after SCI.…”

Synchronous stimulation and monitoring of soleus H reflex during robotic body weight-supported ambulation in subjects with spinal cord injury

“…Afferent input from step to step may also vary within a given bin. Given that H reflex amplitudes have been shown to be modulated by both phasedependent muscle activation levels during ambulation [2,[5][6][7][8] and by ankle [9][10] and hip joint positions [11,29], precisely stimulating sequential H reflex cycles during the gait cycle may reduce variability in modulation factors that affect H reflex output. Robotic-controlled BWSTT provides highly repeatable gait characteristics that would be difficult to control during therapist-assisted BWSTT.…”

“…Methods include selecting a constant percentage of the M max to be maintained across conditions. Investigators have used intensities ranging from 10 to 30 percent [7,[30][31][32][33], multiple stimulation intensities with post hoc analysis to match M wave cycles [2,6], or have not reported. Acceptable M wave variance for these studies was between ±3 to 5 percent of selected intensity.…”

“…The amplitude of the soleus H reflex is modulated in a task-dependent manner, such as standing versus walking [2][3][4][5]. A profound phase-dependent modulation, including modulation associated with ambulation speed, also occurs during walking and running [2,[5][6][7][8]. In addition to reflex modulation coupled to muscle activation, isolated joint angledependent modulation of H reflex activity has been demonstrated at the ankle [9][10] and the hip [11] independent of motor neuronal excitation.…”