Soleus H-reflex modulation during body weight support treadmill walking in spinal cord intact and injured subjects

Abstract

The soleus H-reflex modulation pattern was investigated in ten spinal cord intact subjects during treadmill walking at varying levels of body weight support (BWS), and nine spinal cord injured (SCI) subjects at a BWS level that promoted the best stepping pattern. The soleus H-reflex was elicited by tibial nerve stimulation with a single 1-ms pulse at an intensity that the M-waves ranged from 4 to 8% of the maximal M-wave (M(max)). During treadmill walking, the H-reflex was elicited every four steps, and stimuli were randomly dispersed across the gait cycle which was divided into 16 equal bins. EMGs were recorded with surface electrodes from major left and right hip, knee, and ankle muscles. M-waves and H-reflexes at each bin were normalized to the M(max) elicited at 60-100 ms after the test reflex stimulus. For every subject, the integrated EMG area of each muscle was established and plotted as a function of the step cycle phase. The H-reflex gain was determined as the slope of the relationship between H-reflex and soleus EMG amplitudes at 60 ms before H-reflex elicitation for each bin. In spinal cord intact subjects, the phase-dependent H-reflex modulation, reflex gain, and EMG modulation pattern were constant across all BWS (0, 25, and 50) levels, while tibialis anterior muscle activity increased with less body loading. In three out of nine SCI subjects, a phase-dependent H-reflex modulation pattern was evident during treadmill walking at BWS that ranged from 35 to 60%. In the remaining SCI subjects, the most striking difference was an absent H-reflex depression during the swing phase. The reflex gain was similar for both subject groups, but the y-intercept was increased in SCI subjects. We conclude that the mechanisms underlying cyclic H-reflex modulation during walking are preserved in some individuals after SCI.

Locomotor training improves reciprocal and nonreciprocal inhibitory control of soleus motoneurons in human spinal cord injury

“…With the subject standing at a BWS similar to that utilized during stepping, the stimulation sites were reevaluated, and the MG and/or TA M-wave MT was reestablished. Then, 80 to 130 stimuli were delivered at 0.2 Hz to assemble the soleus M-wave and H-reflex recruitment curves (Knikou et al 2009Smith et al 2015).…”

“…The knee joint during the swing phase moves the stimulating electrode away from the tibial nerve, while knee extension during the stance phase has the opposite effect. To counteract these confounding factors, a supramaximal stimulus to the tibial nerve was delivered at 60 ms after the unconditioned or the conditioned H reflex at each bin of the step cycle (Knikou et al 2009Mummidisetty 2011, 2014). The customized LabVIEW software measured the peak-to-peak amplitude of the M wave and M max recorded during stepping and used a self-teaching algorithm to adjust the stimulus intensity at each bin of the step cycle.…”

“…The customized LabVIEW software measured the peak-to-peak amplitude of the M wave and M max recorded during stepping and used a self-teaching algorithm to adjust the stimulus intensity at each bin of the step cycle. Adjustment of stimulation intensity was based on two criteria: the amplitude of the M wave as a percentage of the M max that was set to range from 2 to 12% of the M max , and the stimulation intensities that evoked H reflexes only on the ascending limb of the recruitment curve (Knikou 2008(Knikou , 2013Knikou et al 2009. These criteria applied to both conditioned and unconditioned H reflexes, which were randomly recorded during stepping.…”

H reflex modulation by transcranial magnetic stimulation in spinal cord injury subjects after gait training with electromechanical systems

“…In healthy subjects, the H reflex amplitude modulation by TMS shows an early phase, peaking at 10-30 ms, and a late phase, peaking at 60-130 ms. [12][13][14] According to Valls-Solé and co-workers, 13 the first phase may result from the generation of an excitatory postsynaptic potential (EPSP) in the a-motoneuron, whereas the mechanisms responsible for the second phase are not clear. In a study of patients with SCI, Wolfe et al 17 reported the absence of modulation in patients with complete motor lesions, and a partial facilitation present on incomplete motor lesions (ASIA C and D).…”

“…These intervals were chosen in view of their relevance for definition of the pattern of TMS modulation of the H reflex. 12,14 We collected five responses at each interval. The intertrial interval was at least 10 s to prevent carryover effects.…”