Augmentation of Voluntary Locomotor Activity by Transcutaneous Spinal Cord Stimulation in Motor‐Incomplete Spinal Cord‐Injured Individuals

Abstract

The level of sustainable excitability within lumbar spinal cord circuitries is one of the factors determining the functional outcome of locomotor therapy after motor-incomplete spinal cord injury. Here, we present initial data using noninvasive transcutaneous lumbar spinal cord stimulation (tSCS) to modulate this central state of excitability during voluntary treadmill stepping in three motor-incomplete spinal cord-injured individuals. Stimulation was applied at 30 Hz with an intensity that generated tingling sensations in the lower limb dermatomes, yet without producing muscle reflex activity. This stimulation changed muscle activation, gait kinematics, and the amount of manual assistance required from the therapists to maintain stepping with some interindividual differences. The effect on motor outputs during treadmill-stepping was essentially augmentative and step-phase dependent despite the invariant tonic stimulation. The most consistent modification was found in the gait kinematics, with the hip flexion during swing increased by 11.3° ± 5.6° across all subjects. This preliminary work suggests that tSCS provides for a background increase in activation of the lumbar spinal locomotor circuitry that has partially lost its descending drive. Voluntary inputs and step-related feedback build upon the stimulation-induced increased state of excitability in the generation of locomotor activity. Thus, tSCS essentially works as an electrical neuroprosthesis augmenting remaining motor control.

Weight Bearing Over-ground Stepping in an Exoskeleton with Non-invasive Spinal Cord Neuromodulation after Motor Complete Paraplegia

“…The interventions tested included a combination of non-invasive spinal cord stimulation, pharmacological activation via a monoaminergic agonist (Buspirone), and over-ground weight bearing stepping in an exoskeleton. It has been shown that activation of the lumbar enlargement via transcutaneous electrical stimulation facilitates passive locomotion and robust patterns of EMG activity in lower extremity muscles in SCI patients (Minassian et al, 2007; Hofstoetter et al, 2013, 2015). Recently we have shown that transcutaneous spinal cord neuromodulation (pcEmc) can be used to both initiate oscillatory movement and enable voluntary oscillatory movements in motor complete subjects.…”

The relationship between maximum tolerance and motor activation during transcutaneous spinal stimulation is unaffected by the carrier frequency or vibration

“…Two additional 50 × 100 mm self‐adhesive (PALS, Axelgaard, USA) electrodes served as the anodes and these positioned on the abdomen about 2 cm laterally left and right from the participant's midline (corresponding to about T11 to L2). The electrode positions were based on previous work where similar electrode configurations were used to activate spinal sensorimotor networks (Calvert et al, 2019; Danner et al, 2016; Hofstoetter et al, 2015). Participants laid in a prone position for the duration of the experiment.…”

“…As such, mechanisms of spinal neuromodulation may also include activation of spinal interneural networks and antidromic activation of ascending fibers in the dorsal columns. TSS has been used to increase excitability at multiple levels of the spinal neuraxis to enable motor and autonomic functions in individuals with chronic spinal cord injury (SCI) (Gad et al, 2018; Hofstoetter et al, 2013, 2015; Minassian et al, 2013, 2016; Phillips et al, 2018; Rath et al, 2018; Sayenko et al, 2019). Although TSS has been examined as a possible clinical intervention for individuals with SCI, the promising findings with regard to motor recovery and the noninvasive nature of the technique could make TSS suitable for use with other neurologically‐impaired populations.…”

Supraspinal Control Predicts Locomotor Function and Forecasts Responsiveness to Training after Spinal Cord Injury

“…101 While epidural spinal cord stimulation requires an invasive surgical procedure for the implantation of electrodes into the epidural space, more recent evidence indicates that transcutaneous spinal cord stimulation provides a non-invasive alternative for the stimulation of the spinal cord, using electrodes placed on the skin over the spine. 102,103 Modeling of both epidural [104][105][106] and transcutaneous spinal cord stimulation 107 shows that large diameter afferent fibers in the dorsal roots are the likely sites of neural activation, suggesting that both stimulation approaches act through a common mechanism to modulate excitability within the spinal circuits.…”