Conventional transcutaneous spinal stimulation waveforms recruit sensory pathways more effectively than burst-modulated high-frequency waveforms

Transcutaneous spinal stimulation (TSS) can improve motor function in people with paresis, although outcomes remain inconsistent across studies. Different stimulation parameters may influence the effectiveness and sensory fiber selectivity of TSS. Here, we systematically compared the ability of ten TSS waveforms applied over the T11-L1 vertebral levels to evoke a soleus muscle response in people with intact neurological function (n = 12). Burst-modulated high-frequency waveforms (2 kHz, 5 kHz, 10 kHz) were compared with a conventional square waveform of the same total cathodal phase duration (400 μ s); a longer conventional waveform (1,000 μ s) was also tested. All waveforms were delivered in biphasic and monophasic pulse types. Relative to the conventional 400 μ s waveform, response threshold intensities were lower with the 1,000 μ s waveform (-28 to -27%) and higher with high-frequency stimulation: 2 kHz (+26 to +43%), 5 kHz (+111 to +129%), and 10 kHz (+254 to +273%). Recruitment curve slopes were also steeper with the 1,000 μ s waveform (+33 to +46%) and generally less steep with high-frequency stimulation: 2 kHz (biphasic: -25%), 5 kHz (-50 to -46%), and 10 kHz (-75 to -64%). Paired-pulse depression was weak at intensities >100%-120% of threshold but increased markedly at higher intensities across all waveforms. The 10 kHz waveforms showed the weakest paired-pulse depression, consistent with a smaller sensory contribution than the conventional 400 μ s waveform. Onset latencies were comparable to peripheral conduction times at near-threshold intensities but became shorter at higher stimulation intensities, suggesting changes in the relative contributions of sensory and motor fiber activation. Overall, burst-modulated high-frequency waveforms required higher stimulation intensities than conventional waveforms to evoke spinally-evoked motor responses and provided no physiological evidence of an advantage over conventional waveforms.