On carrier frequency in transcutaneous spinal cord electrical stimulation: a narrative review.

Objective.Transcutaneous spinal cord stimulation (tSCS) using kilohertz frequency carrier modulation has emerged as a non-invasive neuromodulation approach to improve motor recovery and reduce pain. Early application of 5-10 kHz modulated pulses for tSCS has shown promising results in spinal cord (SC) injury and post-stroke rehabilitation, but the mechanisms underlying these effects remain poorly understood.Approach.This narrative review synthesizes electrophysiological, computational and clinical evidence to assess how kilohertz modulation influences spinal and corticospinal excitability and analgesia. A total of 20 preclinical and clinical studies comparing the effects of kHz-modulated and conventional stimulation pulses were reviewed.Main results.The results indicate that kilohertz modulated tSCS increases tolerance to stimulation, but requires a higher charge to evoke motor responses in healthy participants and individuals with post-stroke motor disorder. Compared to conventional stimulation, modulated stimulation recruits afferents less efficiently at motor threshold intensity but appears to engage broader corticospinal circuits, especially near or below threshold. Frequency-specific effects include prolonged spinal inhibition, frequency-dependent modulation of supraspinal input, and selective activation of inhibitory interneurons in the dorsal horn. Computational study supports these observations, showing that kilohertz pulses produce delayed action potential initiation due to alternating depolarization cycles. A comparative functional study has shown that modulated tSCS improves motor function in individuals with SC injury more significantly than conventional stimulation.Significance.This narrative review highlights gaps in our understanding of the mechanisms of modulated tSCS, suggests directions for further research and will be useful in planning studies on the mechanisms behind tSCS with and without carrier frequency. It also holds engineering relevance for the optimal design of stimulation devices.