Partitioning of the nervous system following exoskeleton and epidural stimulation in spinal cord injury

The effects of percutaneous spinal cord epidural stimulation (SCES) on motor control have yet to be elucidated in persons with spinal cord injury (SCI). We examined the effects of percutaneous SCES on the central and peripheral nervous systems after SCI. We determined the effect of SCES combined with exoskeletal assisted walking (EAW) and resistance training (RT) on motor performance as assessed by the 10 m-walking test, electromyography outcomes and spasticity measurement. Four chronic (6-24 years post-injury) motor complete (C8-T11 AIS A and B) persons with SCI underwent permanent implantation with percutaneous SCES. Participants were either assigned to 12 months of EAW + SCES or EAW without SCES, 3 × weekly for the first 6 months. This was followed by an additional 6 months of EAW + SCES + RT (n = 3) or EAW + delayed SCES + no RT (n = 1) (both groups: 3-5 × weekly). Measurements were conducted at baseline (BL), post-intervention 1 (P1, 6 months following BL) and post-intervention 2 (P2, ~ 12 months following BL). Central nervous system performance was evaluated by measuring volitional knee isometric torques with SCES ON compared to SCES OFF. H-max/M-max ratio was measured for peripheral nervous system performance. For central adaptations, two participants (0882 and 0884) intentionally (SCES ON + active movement) generated knee extensor isometric torques at either 75% sub-motor threshold (0882; 1.87-6.6 × greater than SCES ON) or 100% motor threshold [0882 (1.6 × greater than SCES ON) and 0884 (3.7-16 × greater than SCES ON)]. A third participant initiated torques in the flexion direction and the fourth participant failed to initiate isometric torque. For peripheral adaptations, individual data indicated improvement in H-reflex and M-waves following P1 (0881 and 0882) and P2 (0881 and 0884). Surface neuromuscular electrical stimulation induced greater (24-42%) isometric knee extensor torques at P2 compared to BL in both groups. Overall, 10-m walking test resulted in 30% longer duration and 29% slower speed with reduced EAW assistance (50-70%) during EAW + SCES ON (duration: 79 s and speed: 0.13 m/s) compared to 100% EAW + SCES OFF (duration: 60.5 s and speed: 0.18 m/s) during P1 measurements. In P2, similar pattern was observed and characterized by longer duration (22%) and slower walking speed (24%). Spasticity decreased (0.5-13%) at different angular velocities following P1 but not after P2. EAW + SCES ON improved intention to generate isometric torques accompanied with improvement in peripheral neuromuscular kinetics. This was mirrored by increasing EMG harmony with training without enhancing EAW 10-m performance. The effects of EAW + SCES on spasticity may warrant additional investigation.