The effects of stimulation waveform and carrier frequency on tolerance and motor thresholds elicited by...

With growing interest in translating transcutaneous spinal cord stimulation (tSCS) into rehabilitation for different neurologic injuries, understanding the effects of various combinations of stimulation parameters becomes essential.

Twenty-one participants post-stroke completed an assessment to determine their resting motor threshold (RMT) (minimum current required to elicit a muscle response) and tolerance levels (uncomfortable current intensity) to 12 stimulation configurations: two square waveforms, biphasic and monophasic, paired with six carrier frequencies (unmodulated: 0, and modulated: 1, 3, 5, 7, and 10 kHz).

The results demonstrated that increasing carrier frequency increased participants' tolerance level and RMTs. Carrier frequency nor waveform type significantly altered discomfort when tolerance was normalized to the motor threshold, with 57 ± 23 % tolerated across all configurations. However, higher carrier frequencies, particularly biphasic waveforms at frequencies > 5 kHz, required more charge to reach a muscle activation and activated fewer muscles compared to unmodulated waveforms. No significant differences in discomfort relative to RMT were found between monophasic and biphasic waveforms.

Higher carrier frequency allows stimulation to be more comfortable at a given intensity, but it also requires more current to reach RMTs.

This study provides an essential feasibility assessment of tSCS configurations in a neurological population.