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. 2023 Jun 29;4:1198679. doi: 10.3389/fresc.2023.1198679

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© 2023 Wolpaw and Thompson.

This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

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Soleus H-reflex down-conditioning and its effects on walking in people with chronic incomplete spinal cord injury (SCI). (A) Reflex conditioning setup and visual feedback. Soleus H-reflexes are elicited while the participant maintains a stable standing posture and stable soleus and tibialis anterior (TA) electromyography (EMG) activity. During reflex trials, the visual feedback screen shows the number of trials completed in the current 75-trial block and ongoing (background) soleus EMG activity (left panel). When soleus EMG stays in the correct (shaded) range for at least 2 s, the tibial nerve is stimulated and an H-reflex is elicited. During control trials (not shown), H-reflex size is not displayed. During conditioning trials, the shading in the Target Response (right) panel indicates the rewarded H-reflex size range for down-conditioning. The dark horizontal line indicates the average H-reflex size for the 6 baseline sessions. The vertical bar (i.e., reflex feedback bar) shows the size of the most recent H-reflex. If H-reflex size falls in the shaded area, the bar is green, and the trial is a success. If it falls outside the shaded area, the bar is red and the trial is a failure. The running success rate for the current 75-trial block is also shown. (B) Soleus H-reflex during standing (top) and soleus EMG activity during walking (bottom), measured before and after 30 down-conditioning sessions and then 6 months after conditioning ended in a person with chronic incomplete SCI. Successful conditioning resulted in smaller H-reflexes during standing, and phasic (swing-phase) suppression of soleus EMG activity during walking. The smaller H-reflex and swing-phase suppression of soleus EMG remained present 6 months after conditioning ended. (C) Bilateral soleus and TA EMG activity during walking before (dashed line) and after (solid line) 30 down-conditioning sessions in another person with SCI. The step cycle is divided into 12 equal bins, starting from foot contact. Thus, bins 1–7 are the stance phase and bins 8–12 are the swing phase. After successful down-conditioning reduced H-reflex size on the more impaired side, the soleus stance burst was increased and the concurrent inappropriate TA activity was decreased in both legs. Down-conditioning the soleus H-reflex in the more impaired leg improved locomotion in both legs. (D) Ten-meter walking speed [mean(±SE) in % of baseline speed] and step-cycle symmetry (mean ± SE) before and after 30 down-conditioning sessions reduced H-reflex size in people with chronic SCI. Step cycle symmetry was measured as the ratio of the time between the nonconditioned leg's foot contact (nFC) and the conditioned leg's foot contact (cFC) to the time between cFC and nFC. Thus, a perfectly symmetrical gait has a value of 1. Initially, the ratio was much greater than 1. After 30 conditioning sessions, the ratio had decreased to just above 1; gait was nearly symmetrical. [(A) is modified from Thompson et al. (20)]; (B–D) are modified from Thompson et al. (17).