Factors that determine the magnitude and time course of human H- reflexes in locomotion

Abstract

The soleus H-reflex amplitude is deeply modulated during locomotion in humans (Capaday and Stein, 1986). Moreover, at a constant stimulus intensity, the slope of the relationship between the amplitude of the soleus H-reflex and the background electromyogram (EMG) changes with different locomotor tasks (Capaday and Stein, 1987a). Two further aspects are studied here. First, we recorded the reflex during overlapping speeds of walking (2.0-7.5 km/hr) and running (5-9 km/hr) to determine whether the speed, the motor output, or the form of locomotion was most important in setting the slope of this relationship between H-reflex and background EMG. Second, we determined the time course of change in the H-reflex amplitude and the possible site of action for the reflex depression during the transition from standing to walking. The primary determinant of the slope was found to be the form of locomotion. The differences between running and walking could not be explained entirely by either movement speed or motor output. For walking, the slope varied inversely with the speed and the motor output of locomotion. This compensation in slope as a function of motor output may prevent saturation of the motoneuron pool. The appropriate reflex amplitudes for a particular locomotor pattern are activated rapidly and completely within a reaction time, and simultaneously with the activation of muscle activity for the initiation of walking. Mechanisms for the rapid change seen during the initiation of locomotion most likely act presynaptically on the muscle spindle afferents. The time course and magnitude of this change are correlated with the activity of the tibialis anterior muscle.

Modulation of Soleus H-Reflexes During Gait in Children With Cerebral Palsy

“…The size of the H-reflex is essentially dependent on the background EMG of the same muscle (Edamura et al 1991). If reflexes were simply a function of background EMG, one would expect the larger reflexes in CP children to be reflected by a larger background EMG.…”

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.…”

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

“…4 in Capaday & Stein, 1987a and Fig. 2 in Edamura et al 1991). This could have potentially forced the linear regressions through the origin and altered the calculations of H-reflex gain (i.e.…”

“…Previous studies on reflex modulation during human locomotion have concluded that soleus H-reflex gain is greater for walking than for running (Capaday & Stein, 1987a;Edamura et al 1991). Measurements of H-reflex gain are not calculated directly but are inferred based on an operational definition of reflex gain.…”

“…Furthermore, measuring H-reflex amplitudes at different muscle activation levels is necessary to test the hypothesis that H-reflex gain is independent of muscle recruitment during locomotion. Edamura et al (1991) attempted to increase the range of muscle activation levels for their H-reflex measurements by examining humans walking and running at multiple speeds. However, in addition to increasing muscle activation, increasing locomotion speed also increased H-reflex inhibition (Edamura et al 1991;Brooke et al 1997).…”