Closed-Loop Control of Electrical Stimulation through Spared Motor Unit Ensembles Restores Foot Movements after Spinal Cord Injury

Restoring movement of a paralyzed foot is a key challenge in helping individuals with neurological conditions such as spinal cord injury (SCI) to improve their quality of life. Neuroprostheses based on functional electrical stimulation (FES) can restore the physiological range of motion by stimulating the affected muscles using surface electrodes. We have previously shown that, despite chronic motor-complete SCI, it is possible to capture paralyzed hand movements in individuals with tetraplegia using spared and modulated motor unit (MU) activity decoded with non-invasive electromyography (EMG) sensors. This study investigated whether a wearable high-density surface EMG system could capture and control paralyzed foot kinematics in closed-loop control with an FES system. We found that all our participants with SCI (2 with chronic SCI and 3 with acute SCI) retained distinct spared EMG activity for at least three ankle movements, which allowed them to reliably control a digital cursor using their spared tibialis anterior and triceps surae MU activity. Movement separability was further reconfirmed by extracting task-modulated MU activity during foot flexion/extension (3-7 modulated MUs/participant). Three participants were further able to modulate and maintain their foot flexion/extension EMG levels with an accuracy of >70%. Lastly, we show that real-time control of a FES system using EMG from the affected limb can restore foot movements in a highly intuitive way, significantly improving the lost or pathological foot range of motion. Our system provides an intuitive approach for closed-loop control of FES that has the potential to assist individuals with SCI in regaining lost motor functions.