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

  • ️Mon Jan 01 2001

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

D P Ferris et al. J Physiol. 2001.

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.

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Figures

Figure 1
Figure 1. Reduced gravity simulator

The subjects wore a modified rock climbing harness attached to a series of rubber springs via a metal cable and multiple pulleys. Upward support force fluctuations were small within each stride (± 0.03 G).

Figure 7
Figure 7. Mean rectified EMG amplitudes for soleus during walking and running at each gravity level for all subjects

We broke the stride period into 16 equal bins. Each data point represents the average for a bin. The size of the symbol reflects the gravity level (i.e. largest symbols are 1.0 G, smallest symbols are 0.25 G). Bin number 1 is the first time slice after heel strike. Error bars are standard errors of the mean.

Figure 2
Figure 2. Typical ankle movement patterns for a single subject walking and running at 1.0 G and 0.25 G

The ankle movement patterns were very similar at all gravity levels, but there was slightly less ankle dorsiflexion during the first part of the stance phase under simulated reduced gravity. We defined 0 deg as each subject’s standing posture. The curves represent the average of seven to 10 strides at each condition. Heel strike is at 0% normalised stride time.

Figure 4
Figure 4. Rectified averaged low-pass-filtered EMG (cutoff frequency 10 Hz) for a typical subject walking (A) and running (B) at 1.0 and 0.25 G

Heel strike is at 0% normalised stride time. A, during walking, vastus lateralis activity was very similar for both gravity levels. The 75% decrease in gravity resulted in only a slight reduction in soleus activity but a substantial reduction in medial gastrocnemius activity. Tibialis anterior activity was similar for both gravity levels. B, during running, vastus lateralis activity was much lower at 0.25 G than at 1.0 G. Soleus and medial gastrocnemius activity showed similar reductions for the 75% decrease in gravity level. Although this subject demonstrated a slight increase in tibialis anterior activity under simulated reduced gravity, tibialis anterior activity remained constant for most subjects.

Figure 3
Figure 3. Mean peak ankle displacements for walking and running at four gravity levels

During the first part of the stance phase the ankle joint dorsiflexes and during the second part of the stance phase the ankle joint plantarflexes. Although dorsiflexion slightly decreased under reduced gravity (*P < 0.05), plantarflexion was statistically independent of gravity level (P > 0.05). Error bars are standard errors of the mean and are sometimes small enough to be hidden by the symbol.

Figure 5
Figure 5. Mean EMG for walking and running at four gravity levels for all subjects

We took the mean rectified averaged EMG over the complete stride for each subject and then normalised to the maximum value recorded for that subject regardless of gait. We used a repeated measures ANOVA to determine if gravity level had a significant effect on mean EMG for each gait (*P < 0.05). Error bars are standard errors of the mean and are sometimes small enough to be hidden by the symbol.

Figure 6
Figure 6. Mean maximum M-wave amplitudes (Mmax) during walking and running at each gravity level for all subjects

We broke the stride period into 16 equal bins. Each data point represents the average for a bin. The size of the symbol reflects the gravity level (i.e. largest symbols are 1.0 G, smallest symbols are 0.25 G). The dashed line is 100% of Mmax standing value. Bin number 1 is the first time slice after heel strike. Error bars are standard errors of the mean.

Figure 8
Figure 8. Mean H-reflex amplitudes during walking and running at each gravity for all subjects

We broke the stride period into 16 equal bins. Each data point represents the average for a bin. The size of the symbol reflects the gravity level (i.e. largest symbols are 1.0 G, smallest symbols are 0.25 G). Bin number 1 is the first time slice after heel strike. Error bars are standard errors of the mean.

Figure 9
Figure 9. Example of linear regression data for one subject

Triangles denote walking data and circles denote running data. Each data point reflects data for a single time bin. The size of the symbol reflects the gravity level (i.e. largest symbols are 1.0 G, smallest symbols are 0.25 G). We fitted a linear least-squares regression to data points for each gravity level and gait for each subject. The calculated slope is a measure of H-reflex gain.

Figure 10
Figure 10. Slope and y-intercept of the linear regressions at each gravity levels for all subjects

Gravity level did not significantly change either linear regression slope (i.e. H-reflex gain) or linear regression y-intercept for either gait (P > 0.05). Slope was not significantly different for walking or running (P > 0.6), but y-intercept was significantly higher for walking compared to running (P < 0.001). Triangles denote walking data and circles denote running data. Error bars are standard errors of the mean and lines are linear least-squares regressions.

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