Peripheral nerve sensory fibres are recruited with less current and greater selectivity by conventional waveforms compared to high-frequency waveforms

Transcutaneous spinal stimulation aims to target dorsal spinal roots, which deliver sensory information to the spinal cord. However, the stimulation waveform that most effectively recruits sensory fibres at the lowest intensity has not been identified. The purpose of this study was to compare conventional and high-frequency burst-modulated stimulation waveforms in their ability to recruit sensory fibres, as assessed by H-reflex threshold, recruitment characteristics, and motor fibre activation. In participants with intact neurological function (n=12), soleus H-reflex recruitment curves were recorded for ten stimulation waveforms: a conventional waveform (400 μs), serving as the reference condition, high-frequency burst-modulated waveforms (2, 5, and 10 kHz) with the same total phase duration, and a longer conventional waveform (1000 μs), each delivered as biphasic and monophasic pulses. H-reflex threshold was higher for high-frequency waveforms: 2 kHz (biphasic: +63%; monophasic: +35%); 5 kHz (biphasic: +179%, monophasic: +109%); 10 kHz (biphasic: +307%, monophasic: +249%); and lower for the conventional 1000 μs waveform (biphasic: -39%, monophasic: -44%). Similarly, recruitment curve peak slope (mV/mA) was less steep for high-frequency waveforms: 2 kHz (biphasic: -43%, monophasic: -40%); 5 kHz (biphasic: -77%, monophasic: -40%); 10 kHz (biphasic: -82%, monophasic: -48%); and was steeper (+38%) for the monophasic conventional 1000 μs waveform. At stimulus intensity near H-reflex threshold, M-wave amplitude was larger for high-frequency waveforms. Overall, high-frequency waveforms were less effective than conventional waveforms of equal total phase duration at eliciting H-reflexes and showed more motor activation at near-threshold intensities.