Differential effects of stimulation waveform and intensity on the neural structures activated by lumbar transcutaneous spinal cord stimulation

Lumbar transcutaneous spinal cord stimulation (TSS) evokes synchronized muscle responses, termed spinally evoked motor response (sEMR). Whether the structures TSS activates to evoke sEMRs differ when TSS intensity and waveform are varied is unknown. In 15 participants (9 F, 6 M), sEMRs were evoked by TSS over L1-L3 (at sEMR threshold and suprathreshold intensities) with conventional (one 400-µs biphasic pulse) or high-frequency burst (ten 40-µs biphasic pulses at 10 kHz) stimulus waveforms in vastus medialis (VM), tibialis anterior (TA), and medial gastrocnemius (MG) muscles. TSS was paired with transcranial magnetic stimulation (TMS) over the contralateral motor cortex at relative interstimulus intervals (ISIs) (-10 ms to 11 ms), centered on the ISI when TSS and TMS inputs simultaneously activated VM motoneurons. Doublet TSS was delivered at 80-ms ISI. For VM, the area of the combined response evoked by paired TMS and TSS was not facilitated at any ISI. For TA and MG, combined responses were facilitated by ∼40-100% when TMS activated the motoneurons before or at a similar time as TSS, particularly with suprathreshold TSS. Additionally, for TA, there was greater suppression of the second sEMR evoked by TSS doublets using suprathreshold conventional TSS compared to high-frequency burst TSS (P < 0.001). The results suggest that for VM TSS activated predominantly motor axons, but for TA and MG facilitation of the sEMR by TMS suggests that TSS activated sensory axons. Stimulation waveforms had similar outcomes in most conditions.NEW & NOTEWORTHY Transcutaneous spinal cord stimulation (TSS) can evoke muscle responses by activation of sensory and/or motor axons. The relative contribution of these varies across the muscles tested. We found evidence for activation of sensory axons with TSS for tibialis anterior and medial gastrocnemius but not for vastus medialis. In cases where sensory axons were activated, conventional TSS activated relatively more sensory axons than high-frequency burst TSS.