John and Marcia Price College of Engineering

16 Using the Posterior Root Muscle (PRM) Reflex to Infer Changes in Spinal Cord Excitability

Hana Habib; Abigail T Harrison; and Ashley N Dalrymple

Faculty Mentor: Ashley N Dalrymple (Departments of Biomedical Engineering and Physical Medicine & Rehabilitation)

Spinal cord excitability is a dynamic property that changes with age1 and injury, such as limb amputation2. Understanding these changes is essential for optimizing neuromodulation strategies2–4. This study investigated the potential of the posterior root muscle (PRM) reflex, evoked using transcutaneous spinal cord stimulation (tSCS)5–7, as a novel biomarker of spinal cord excitability.

tSCS was applied at the T12-L1 vertebral level to stimulate dorsal roots and evoke PRM reflexes, which were recorded from four lower limb muscles using electromyography (EMG). The study included two groups: neurologically intact participants (n = 30, age = 28 ± 6.5 years) and individuals with limb amputation (n = 4, age = 53 ± 13.4 years). Outcome measures included the threshold, which is the minimum amount of current required to evoke the PRM reflex, peak- to-peak amplitude of the PRM reflex, and the latency of onset of the reflex response.

We successfully evoked PRM reflex responses in both neurologically intact participants and individuals with lower-limb amputation using tSCS, adding to our prior work2,5. Reflexes were characterized across a range of stimulation amplitudes, from threshold to maximum intensity (Figure 1). Within this range, we constructed a recruitment curve (Figure 2), which describes the change in peak-to-peak amplitude of the reflex response as the stimulation intensity was increased.

Overlapping traces of the PRM reflex from a neurologically intact participant, aged 26 years old, with the reflex growing in size with increasing stimulation amplitude. The stimulation amplitudes ranged from 22 to 65 uC. A scale bar indicates the amplitude and timescales of the reflexes, and stimulation onset is marked with an arrow. Lower-intensity traces have smaller peaks, with peak-to-peak amplitude increasing smoothly as current increases.
Figure 1. PRM reflexes with increasing stimulation amplitude from A) a neurologically intact individual and B) an individual with a lower-limb amputation. Legend ranges from below threshold to maximum.

Among intact participants, we observed a small trend indicating that PRM reflex thresholds may increase with age (Figure 3). Although this suggests a potential age-related modulation of spinal cord excitability, the relationship was weak (R2 = 0.017), and conclusions are limited by the sample size.

Scatterplot showing the relationship between PRM reflex threshold (in µC) and participant age (in years).
Figure 2. Recruitment curve showing the mean peak-to-peak amplitude of the PRM reflex versus stimulation amplitude from a neurologically intact individual.

Preliminary findings from the amputee group exhibited higher thresholds and greater variability in waveform morphology compared to intact controls. Additionally, the peak-to-peak amplitudes of the PRM reflexes were orders of magnitudes smaller than in intact individuals. The reflex latency was slightly higher than those in neurologically intact participants (Table 1).

 

Scatterplot of mean peak-to-peak EMG amplitude (mV) versus stimulation amplitude (µC) for a single neurologically intact participant. The curve rises sigmoidally from threshold to maximum intensity, illustrating the recruitment of reflex responses as current increases.
Figure 3. PRM reflex threshold versus age. A weak increasing trend is shown with R2 = 0.017.
A three-column table listing mean PRM reflex latency (milliseconds) by age decade for neurologically intact participants and amputee participants. For each decade (18-29, 30-39, 40-49, 50-59, 60-69), the intact group averages are shown with standard deviation, and the amputee group averages are shown side by side where available. Intact participants had a PRM reflex latency of 21.4 ± 2 ms in the 18-29 age decade, 21.7 ± 1.4 ms in the 30-39 age decade and 21.1 ± 1.9 ms in the 40-49 age decade. Amputee participants showed a PRM reflex latency of 23.2 ms in the 30-39 age decade, 23.6 ms in the 40-49 age decade, 19 ms in the 50-59 age decade and 20.8 ms in the 60-69 age decade.
Table 1. PRM reflex latencies grouped by age decade for neurologically intact participants and individuals with lower limb amputation.

The PRM reflex shows promise as a biomarker for spinal cord excitability, with some differences observed between intact and amputee participants. These differences may reflect spinal plasticity or sensorimotor dysfunction following injury. The age-related trends observed in intact participants also support the potential of the PRM reflex to track changes across the lifespan.

Differences observed between intact and amputee participants may point to injury-induced changes in spinal cord function, but the small sample size prevents us from drawing definitive conclusions. Future work will focus on expanding the amputee cohort and performing statistical comparisons. Broader participant inclusion is needed to further validate the PRM reflex as a tool for tracking spinal cord excitability and informing spinal cord plasticity after limb amputation.

References

[1] Henry, M., Duchateau, J. & Baudry, S. Age‐related changes in sensory and motor components of the Hoffmann‐reflex pathway of the flexor carpi radialis. doi:10.1111/ejn.16000.

[2] Dalrymple, A. N., Fisher, L. E. & Weber, D. J. A preliminary study exploring the effects of transcutaneous spinal cord stimulation on spinal excitability and phantom limb pain in people with a transtibial amputation. J. Neural Eng. 21, 046058 (2024).

[3] Thatcher, K. L., Nielsen, K. E., Sandler, E. B., Daliet, O. J., Iddings, J. A., Field-Fote, E. C. Optimizing Transcutaneous Spinal Stimulation: Excitability of Evoked Spinal Reflexes is Dependent on Electrode Montage. Preprint at https://doi.org/10.21203/rs.3.rs-4719031/v1 (2024).

[4] Prat-Ortega, G., Ensel, S., Donadio, S., Borda, L., Boos, A., Yadav, P., Verma, N., Ho, J., Carranza, E., Frazier-Kim, S., Fields, D. P., Fisher, L. E., Weber, D. J., Balzer, J., Duong, T., Weinstein, S. D., Eliasson, M. J. L., Montes, J., Chen, K. S., Clemens, P. R., Gerszten, P., Mentis, G. Z., Pirondini, E., Friedlander, R. M., Capogrosso, M. First-in-human study of epidural spinal cord stimulation in individuals with spinal muscular atrophy. Nat Med 31, 1246–1256 (2025).

[5] Dalrymple, A. N., Hooper, C. A., Kuriakose, M. G., Capogrosso, M. & Weber, D. J. Using a high-frequency carrier does not improve comfort of transcutaneous spinal cord stimulation. J. Neural Eng. 20, 016016 (2023).

[6] Hofstoetter, U. S., Freundl, B., Binder, H. & Minassian, K. Recovery cycles of posterior root- muscle reflexes evoked by transcutaneous spinal cord stimulation and of the H reflex in individuals with intact and injured spinal cord. PLoS ONE 14, e0227057 (2019).

[7] Minassian, K., Freundl, B. & Hofstoetter, U. S. Chapter 18 – The posterior root-muscle reflex. in Neurophysiology in Neurosurgery (Second Edition) (eds. Deletis, V., Shils, J. L., Sala, F. & Seidel, K.) 239–253 (Academic Press, 2020). doi:10.1016/B978-0-12-815000-9.00018-6.

About the authors

name: Hana Habib

name: Abigail Harrison

name: Ashley Dalrymple

institution: University of Utah

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RANGE: Undergraduate Research Journal (2025) Copyright © 2025 by University of Utah is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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