Feed-forward transcutaneous spinal stimulation modulates reciprocal gait dynamics after spinal cord injury

Abstract Background Transcutaneous spinal stimulation (TSS) can enhance motor output after spinal cord injury (SCI), but continuous TSS might have limited benefit during dynamic tasks like stepping. We hypothesized that a feed-forward controlled (FFC) TSS strategy, dynamically timed to specific gait phases, would significantly improve phase-dependent muscle activation and mechanical output compared to continuous TSS. Methods Five adults with chronic thoracic SCI (AIS A/B) received multi-electrode TSS targeting the lumbosacral enlargement. Interventions were evaluated during (1) supine stepping with isometric force measurements and (2) overground stepping in a robotic exoskeleton. Electromyography (EMG) was recorded bilaterally from four lower-limb muscles. Linear mixed-effects models were used to compare motor responses between continuous TSS and FFC TSS conditions. Results FFC TSS substantially outperformed continuous TSS across both tasks. During supine stepping, FFC TSS increased isometric swing-phase knee extension force by up to 309% and ankle dorsiflexion force by 151% relative to continuous TSS. During overground stepping, FFC TSS amplified stance-phase rectus femoris activation by 352% compared to continuous TSS. Crucially, when compared to continuous TSS, FFC TSS significantly reduced antagonistic co-contraction ratios at the knee (from 104% to 69%) and ankle (from 102% to 69%) during swing phases, restoring reciprocal dynamics. Furthermore, FFC TSS significantly increased the Phase Selectivity Index across the majority of recorded muscles in both tasks ( p < 0.05), demonstrating superior spatial and temporal targeting compared to the tonic interference induced by continuous TSS. Conclusions Gait-synchronized FFC TSS more effectively engages functional muscle synergies than continuous stimulation. By suppressing antagonistic co-contraction and delivering temporally precise, phase-specific activation, FFC TSS addresses key limitations of conventional continuous paradigms, representing a critical step forward in optimizing non-invasive neuromodulation for dynamic motor recovery.