Spinal cord injury (SCI) interrupts signal transmission between the brain and muscles, often leading to permanent motor impairments. Improving hand function is the highest priority for people with tetraplegia. Electrically engaging spinal circuits using spinal cord stimulation has been demonstrated to improve hand function in people with paralysis post-SCI. Here, we used a noninvasive intervention, transcutaneous spinal cord stimulation (tSCS), to facilitate voluntary hand function after SCI. We used a multi-cathode tSCS array to study recruitment patterns across various upper-limb muscles, including forearm subcompartments, in five neurologically intact (NI) participants and five participants with SCI. Our primary objectives was to use tSCS over the cervical spinal cord to delineate the stimulation-evoked response patterns and assess the effects of tSCS on hand motor function in both groups. We demonstrated that tonic tSCS targeting hand muscles enhanced muscle activity (by up to 21%), increased grip strength (by up to 55%), and improved activation patterns in the participants with SCI. Furthermore, using high-density electromyography-based extraction of motor unit activity, we provided experimental evidence that tSCS can transsynaptically modulate the activity of individual motor units, enabling integration of supraspinal inputs within these networks. Our results indicate that targeted tSCS can immediately improve hand motor function after SCI and suggest potential mechanisms for its facilitatory effects. Similar facilitation of motor unit activity, enhanced muscle activation (up to 65%), and, in some cases, grip strength increases of up to 66%, were also observed in NI participants, indicating that cervical tSCS engages spinal sensorimotor circuits consistently across populations.NEW & NOTEWORTHY A multicathode transcutaneous spinal cord stimulation (tSCS) array, combined with high-density electromyography (HDEMG) was used to precisely characterize selectivity in recruitment of motor pools in the cervical spinal cord. Through surface HDEMG-based motor unit extraction, the study demonstrated that tonic, subthreshold tSCS transiently facilitates motor unit firing, via recruitment of sensory afferents, resulting in enhanced muscle output, grip strength, and task-specific muscle activation patterns in individuals with tetraplegia post-SCI.
Keywords: EMG; hand motor function; motor unit decomposition; spinal cord injury; transcutaneous spinal cord stimulation.