Abstract
This study investigated the neuromodulatory effects of transspinal stimulation on soleus H-reflex excitability and electromyographic (EMG) activity during stepping in humans with and without spinal cord injury (SCI). Thirteen able-bodied adults and 5 individuals with SCI participated in the study. EMG activity from both legs was determined for steps without, during, and after a single-pulse or pulse train transspinal stimulation delivered during stepping randomly at different phases of the step cycle. The soleus H-reflex was recorded in both subject groups under control conditions and following single-pulse transspinal stimulation at an individualized exactly similar positive and negative conditioning-test interval. The EMG activity was decreased in both subject groups at the steps during transspinal stimulation, while intralimb and interlimb coordination were altered only in SCI subjects. At the steps immediately after transspinal stimulation, the physiological phase-dependent EMG modulation pattern remained unaffected in able-bodied subjects. The conditioned soleus H-reflex was depressed throughout the step cycle in both subject groups. Transspinal stimulation modulated depolarization of motoneurons over multiple segments, limb coordination, and soleus H-reflex excitability during assisted stepping. The soleus H-reflex depression may be the result of complex spinal inhibitory interneuronal circuits activated by transspinal stimulation and collision between orthodromic and antidromic volleys in the peripheral mixed nerve. The soleus H-reflex depression by transspinal stimulation suggests a potential application for normalization of spinal reflex excitability after SCI.
Highlights
Spinal cord injury (SCI) impairs the integration of neuromodulatory inputs from supraspinal centers and sensory organs (Rossignol et al, 2006; Côté et al, 2018)
The SOL, medial gastrocnemius (MG), peroneus longus (PL), and tibialis anterior (TA) EMG activity from both legs was decreased in amplitude at the steps during transspinal stimulation when compared to the steps without stimulation (Figure 2A)
Because the EMG patterns at steps during and after a single pulse or pulse train transspinal stimulation were strongly correlated to the EMG patterns at steps without stimulation (Spearman’s rank correlation; rs = 0.684–0.961, p < 0.001; Table 2), we can theorize that transspinal stimulation decreases locomotor EMG amplitude but does not affect the phase-dependent EMG modulation pattern in control subjects (Figure 2A)
Summary
Spinal cord injury (SCI) impairs the integration of neuromodulatory inputs from supraspinal centers and sensory organs (Rossignol et al, 2006; Côté et al, 2018). Proprioceptive reflexes refine muscle activity during locomotion and contribute to recovery after SCI (Lam and Pearson, 2002; Rossignol et al, 2006; Côté et al, 2018). Muscle spindle proprioceptive reflexes are uniquely important to locomotor recovery after SCI. Proprioceptive reflexes direct and maintain reorganization of spinal circuit dynamics promoting a more physiological phasedependent reflex modulation and coordinated muscle activity (Knikou et al, 2009; Knikou, 2013a; Takeoka et al, 2014; Takeoka and Arber, 2019). The modulation of the H-reflex during walking and how it is affected by transspinal stimulation can provide significant information on neural interactions between transspinal stimulation, afferent inputs, and spinal monosynaptic-polysynaptic reflex circuits
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