Transcutaneous spinal stimulation with upper extremity robotic training in chronic stroke and spinal cord injury: individual neurophysiological and clinical responses

BACKGROUND: Damage to the corticospinal tract after stroke and spinal cord injury (SCI) often results in persistent upper extremity (UE) impairment. Transcutaneous spinal stimulation (TSS) and robotic technologies have been explored as approaches to facilitate motor training; however, their combined effects on UE sensorimotor recovery remain poorly understood. The purpose of this study was to examine the effects of TSS combined with UE robotic training in individuals with chronic stroke or SCI. METHODS: Five participants with stroke and six with SCI completed a 14-week, sham controlled, single blind crossover study consisting of four total weeks of assessments (one week each pre and post for both training phases), four weeks of UE training with sham TSS, a two-week washout period, and four weeks of UE training with active TSS. Each one-hour session (three days/week) included robotic exoskeleton-assisted UE movements and hand grip training, performed concurrently with sham or active TSS. Assessments included electrophysiological measurements and standardized rehabilitation outcomes. RESULTS: Descriptive analysis revealed meaningful individual improvements masked by group-level heterogeneity. In the stroke group, three participants showed grip strength improvement (assessed without stimulation) after the active phase (+ 9.4 Newtons [N] to + 23.9 N), with two-to-four-fold increases in forearm muscle activation. Mean Fugl-Meyer overall UE scores improved from 89 to 94.2. In the SCI group, two participants showed grip strength gains. One participant exhibited a six-fold immediate force increase (1.0 N to 6.2 N) during stimulation. Another participant achieved improved grip strength without stimulation (23.9 N to 36.8 N) and a three-fold increase in electromyography (EMG) activity from the flexor carpi radialis and first dorsal interosseous muscles, alongside partial pin-prick sensory recovery and self-reported restoration of previously affected perspiration during the active TSS phase. CONCLUSIONS: Varied outcomes in participants confirm that therapeutic effects of combined TSS and robotic UE training are highly individualized. Three critical elements must be blended for the best outcomes of this combinatorial approach: residual UE function, a curated stimulation paradigm, and tailored UE training that provides appropriate challenge, intensity, and salience. The results suggest TSS with UE robotic training hold key potential when considered in the context of the physiological and functional profile of each participant.