The effects of electrode configuration and location on motor thresholds, tolerance, and muscle selectivity of transcutaneous spinal cord stimulation in stroke.

Identifying optimal conditions for transcutaneous spinal cord stimulation (tSCS), such as electrode configuration, is essential for clinical translation, particularly as this promising neuromodulation therapy expands to include diverse neurological populations. However, before clinical application, it is important to understand how different configurations affect dosage and muscle recruitment. We define dosage as the percentage of the current needed to elicit a motor response (the resting motor threshold, or RMT) of the current value tolerated during walking when stimulation is delivered continuously. Another consideration is whether electrode configurations can selectively recruit specific muscles, potentially guiding tSCS protocols to prioritize the muscles most critical for functional improvement. This study tested five electrode configurations in individuals with chronic stroke: one round electrode at T11-T12 vertebrae, one round electrode at L1-L2, two separate round electrodes at T11-T12 and L1-L2 from separate current channels, two separate round electrodes at T11-T12 and L1-L2 from the same current channel, and a rectangular electrode spanning T11-T12 and L1-L2. No single electrode configuration clearly outperformed others in terms of tolerated dosage, with most supporting stimulation near 50% of RMT except the two-channel setup with separate current channels, which showed lower dosage (∼35% RMT). Additionally, muscle selectivity did not significantly differ across configurations. These findings inform stimulation dosage feasibility for future tSCS protocols using different electrode configurations and the need for future studies to evaluate whether current dosage levels are adequate to induce meaningful neuromodulation and clinical benefits in the stroke population.