Preferential activation of spinal sensorimotor networks via lateralized transcutaneous spinal stimulation in neurologically intact humans

Transcutaneous spinal stimulation (TSS), a noninvasive technique to modulate sensorimotor circuitry within the spinal cord, has been shown to enable a wide range of functions that were thought to be permanently impaired in humans with spinal cord injury. However, the extent to which TSS can be used to target specific mediolateral spinal cord circuitry remains undefined. We tested the hypothesis that TSS applied unilaterally to the skin ~2 cm lateral to the midline of the lumbosacral spine selectively activates ipsilateral spinal sensorimotor circuitry, resulting in ipsilateral activation of downstream lower extremity neuromusculature. TSS cathodes and anodes were positioned lateral from the midline of the spine in 15 healthy subjects while supine, and the timing of TSS pulses was synchronized to recordings of lower extremity muscle activity and force. At motor threshold, left and right TSS-evoked muscle activity was significantly higher in the ipsilateral leg compared with contralateral recordings from the same muscles. Similarly, we observed a significant increase in force production in the ipsilateral leg compared with the contralateral leg. Delivery of paired TSS pulses, during which an initial stimulus was applied to one side of the spinal cord and 50 ms later a second stimulus was applied to the contralateral side, revealed that ipsilateral leg muscle responses decreased following the initial stimulus, whereas contralateral muscle responses did not decrease, indicating side-specific activation of lateral spinal sensorimotor circuitry. Our results indicate TSS can selectively engage ipsilateral neuromusculature via lumbosacral sensorimotor networks responsible for lower extremity function in healthy humans. NEW & NOTEWORTHY We demonstrate the selectivity of transcutaneous spinal stimulation (TSS), which has been shown to enable function in humans with chronic paralysis. Specifically, we demonstrate that TSS applied to locations lateral to the spinal cord can selectively activate ipsilateral spinal sensorimotor networks. We quantified lumbosacral spinal network activity by recording lower extremity muscle electromyography and force. Our results suggest lumbosacral TSS engages side-specific spinal sensorimotor networks associated with ipsilateral lower extremity function in humans.