Pairing FES cycling with the right spinal stimulation waveform

More FES cyclists are asking about adding transcutaneous spinal cord stimulation (tSCS) to their sessions. The reason is straightforward. FES cycling moves the legs, builds muscle, and conditions the cardiovascular system. tSCS, applied through electrodes on the surface of the back, aims to prime the spinal cord so that the work done on the bike has a better chance of producing lasting neural change.

The combination is attractive in theory. A new paper in Nature Biomedical Engineering, published on 12 May 2026, helps explain why it matters which kind of tSCS you pair with the cycling.

What we hope tSCS does for the FES cyclist

The hope is not that tSCS makes the legs move on its own. FES cycling already does that by stimulating the leg muscles in sync with pedal movement. The hope is that the stimulation lowers the threshold for voluntary input, calms spasticity that interferes with smooth pedalling, and increases the chance that the nervous system rewires around the task.

That rewiring depends on what neurophysiologists call afferent recruitment. The large sensory fibres running through the dorsal roots, when activated synchronously, depolarise networks of spinal interneurons. Those interneurons sit at the heart of the locomotor circuits we are trying to re-engage. If we can get them firing in time with the cycling pattern and any residual voluntary drive from above the lesion, we have the conditions for use-dependent plasticity. The old Hebbian principle ("cells that fire together wire together") describes it well enough.

So the question becomes: does the tSCS we are using actually recruit those afferent fibres? Or is it doing something else?

The new finding

The Keesey et al. paper, with authors from Washington University in St. Louis and the Medical University of Vienna, looked at exactly this question. They tested how different tSCS waveforms recruit nerve fibres in 28 human participants and used a computational model to project the results into the geometry of dorsal root anatomy.

The headline: kilohertz-frequency carrier waveforms, which a new generation of commercial tSCS devices use, raise the threshold for sensory afferent recruitment and bias recruitment towards motor efferent fibres instead. The bias is most pronounced at cervical levels, but the same principle applies more broadly.

Conventional longer-duration biphasic waveforms (pulse widths around 1 ms, frequencies between 15 and 50 Hz) recruit afferents preferentially and produce the synchronised volleys that spinal interneurons respond to.

Translated for the FES cyclist:

A kilohertz-carrier device may make muscles fire during the session, but it is less likely to be driving the spinal circuits that turn cycling into recovery. A more conventional waveform, as produced by Stim2Go, is designed to do exactly that and is supported by research.

Why this happens

Three principles do the work.

First, chronaxie. Each nerve fibre has a pulse duration at which it is most efficiently excited. For the sensory afferents we want, that sits in the half-millisecond (500 microseconds) to one-millisecond range. Kilohertz pulses are shorter than this window, and threshold rises sharply, particularly for afferents.

Second, synchrony. When pulses arrive faster than a fibre can recover, firing becomes irregular. Some fibres fail to fire at all. The volley reaching the spinal cord is desynchronised, which weakens the postsynaptic response of the interneurons we are trying to drive.

Third, spatial summation. Spinal interneurons fire when synchronised input arrives from many afferent fibres at once. Lose the synchrony and you lose the summation that lights up the locomotor network.

The conventional waveform we use sits where the physiology wants it. The Vienna group, alongside colleagues at UCLA and elsewhere, established this experimentally in humans long before the technology became commercially attractive enough for new entrants to design around different waveforms.

Why this matters when you are stacking tSCS with FES cycling

Anyone combining tSCS with FES cycling is trying to do two things at once: drive the legs through the cycling motion, and PRIME the spinal cord to integrate that input into recovery. The two are complementary, but only if spinal priming is doing what we think it does.

If the tSCS is mostly recruiting motor efferents, you are essentially adding a second source of motor stimulation on top of the FES cycling. Useful for muscle activation, perhaps, but redundant with what the cycling is already providing.

If the tSCS is recruiting synchronised afferents, you are putting the spinal interneurons into a primed state at the same time as the cycling drives the locomotor pattern. That combination is what the published functional outcome studies have been built on.

KEY POINT

When pairing tSCS with FES cycling, choose a stimulator that delivers conventional biphasic waveforms (around 1 ms pulse width, 15 to 50 Hz frequency, sub-motor threshold intensity). This is the waveform family that the new mechanism evidences and twenty years of functional outcome research support.

Where Stim2Go fits

Stim2Go, the wearable stimulator developed by SensorStim Neurotechnology in Berlin and manufactured by Pajunk, delivers conventional biphasic tSCS waveforms in the parameter range supported by the published literature. The tSCS priming programmes available in the Stim2Go use 1 ms pulses at frequencies between 30 and 50 Hz, with intensity calibrated to the individual's posterior root muscle reflex threshold.

It also runs the FES cycling in the same device. One platform, five channels, one app, with the fifth channel typically reserved for the spinal stimulation electrode. The protocols sit alongside each other. That practical integration matters when the goal is consistent, repeated use at home over months, not isolated clinic sessions.

Outcomes worth knowing

The clinical evidence for conventional-waveform tSCS is not abstract.

Gerasimenko and colleagues showed in 2015 that motor-complete (AIS B) participants regained voluntary stepping-like movements with 30 Hz, 1 ms biphasic lumbar tSCS over 18 weeks. Inanici and colleagues demonstrated in 2018 that cervical conventional tSCS plus physical therapy in chronic tetraplegia produced large GRASSP gains and several-fold pinch strength increases, with benefit sustained at three-month follow-up after the stimulation had stopped. That is the signature of plasticity rather than acute facilitation.

Hofstoetter and colleagues have shown that single sessions at sub-reflex intensity produce clinically meaningful reductions in spasticity that persist for at least two hours, with cumulative carryover over weeks of home use.

These are the outcomes for which the conventional waveform family has been validated. The waveform Stim2Go delivers sits in this family.

Caveats worth stating

The new mechanism evidence does not invalidate every published outcome from kilohertz-carrier devices. Those devices have produced functional gains in trial settings. The question the new paper sharpens is what kind of gain, and how durable.

The Keesey et al. study used 28 unimpaired participants and a computational model. Lumbar tSCS retains some afferent selectivity even with kilohertz waveforms, so the argument is sharpest for cervical work but applies more broadly than that.

And as ever in this field, individual response varies. Two cyclists with similar injuries can respond differently to the same protocol. That is part of why we assess people individually rather than prescribing a one-size protocol.

What to take from this

If you are considering adding tSCS to your FES cycling regimen, the device's waveform is not a minor specification. It is the thing that determines whether the spinal stimulation is doing what you hoped.

For most FES cycling applications, the waveform you want is conventional biphasic, around 1 ms pulse width, 15 to 50 Hz, delivered at sub-motor threshold intensity once calibrated against your own posterior root muscle reflex. Stim2Go delivers this waveform family and combines it with the cycling in a way that is practical for sustained home use.

If this is something you are weighing up, we are happy to walk through it with you in detail.

References

Keesey R, Hofstoetter US, et al. Fundamental limitations of kilohertz-frequency carriers in afferent fibre recruitment with transcutaneous spinal cord stimulation. Nature Biomedical Engineering. 2026 May 12. DOI: 10.1038/s41551-026-01684-w.

Gerasimenko YP, et al. Noninvasive reactivation of motor descending control after paralysis. Journal of Neurotrauma. 2015;32(24):1968-80.

Inanici F, et al. Transcutaneous electrical spinal stimulation promotes long-term recovery of upper extremity function in chronic tetraplegia. IEEE TNSRE. 2018;26(6):1272-1278.

Hofstoetter US, et al. Transcutaneous spinal cord stimulation induces temporary attenuation of spasticity in individuals with spinal cord injury. Journal of Neurotrauma. 2020;37(3):481-493.