Transspinal Stimulation Downregulates Flexion Reflex Pathways During Walking in Healthy Humans

Academic Background

Human walking is a complex motor process involving the coordinated work of the central and peripheral nervous systems. The spinal cord, as a crucial part of the central nervous system, plays a key role in controlling motor reflexes and maintaining gait. The flexion reflex is a protective reflex typically triggered when the foot is stimulated, helping individuals avoid injury. However, in patients with spinal cord injury (SCI), the flexion reflex may become abnormally enhanced, leading to gait disturbances and walking difficulties. Therefore, studying how neuromodulation techniques can regulate the excitability of the flexion reflex is of great significance for helping SCI patients regain normal walking function.

In recent years, transcutaneous spinal cord stimulation (TSS) has emerged as a non-invasive neuromodulation technique, gradually becoming an important tool for researching and treating SCI. This technology uses electrical stimulation of the spinal cord to modulate the excitability of neural networks, thereby improving motor function. However, the specific effects of transspinal stimulation on the flexion reflex, particularly its dynamic modulation mechanisms during walking, remain unclear. This study aims to explore the regulatory effects of transspinal stimulation at different frequencies and intensities on the tibialis anterior (TA) flexion reflex during walking in healthy humans.

Source of Research

This paper was co-authored by Maria Knikou and Abdullah M. Sayed Ahmad, affiliated with the KLab4Recovery Sci research program and the Department of Physical Therapy at the City University of New York, Staten Island. The study was accepted on December 27, 2024, and was first published on January 8, 2025, in the Journal of Neurophysiology.

Research Process and Results

Research Process

1. Participant Recruitment

The study recruited nine healthy adults (6 males, 3 females) aged 19 to 26 years. All participants had no history of neurological, orthopedic, or systemic diseases. Each participant signed an informed consent form before the experiment, which was approved by the Institutional Review Board of the City University of New York.

2. Experimental Design

The study employed transspinal stimulation at frequencies of 15 Hz, 30 Hz, and 50 Hz, with stimulation intensities both above and below the sensory threshold. All experiments were conducted during treadmill walking. The flexion reflex was elicited by applying a 30 ms pulse train to the medial arch of the foot, with stimuli randomly distributed across 16 equal bins of the step cycle.

3. Data Collection and Analysis

Surface electromyography (EMG) signals were recorded during treadmill walking, and data were extracted from the tibialis anterior (TA) and soleus (SOL) muscles. The amplitude of the flexion reflex was measured by calculating the EMG area from 20 ms to 200 ms after the end of the pulse train and normalized to the maximum gait-related EMG activity.

Key Results

1. Phase-Dependent Modulation of the Flexion Reflex: The study found that the unconditioned flexion reflex exhibited significant phase-dependent modulation during the step cycle, consistent with previous observations in healthy humans.

2. Effects of Transspinal Stimulation on the Flexion Reflex: Regardless of stimulation frequency or intensity, transspinal stimulation significantly suppressed the flexion reflex during walking. This suppression effect was significant across multiple bins of the step cycle, and the phase-dependent modulation pattern of the flexion reflex was maintained.

3. Changes in Background EMG Activity: The study found that transspinal stimulation did not significantly alter the background EMG activity of the TA muscle, suggesting that the suppression effect on the flexion reflex primarily resulted from the modulation of spinal neural networks rather than changes in muscle excitability.

4. Stability of Slope and Intercept: Through linear regression analysis, researchers found that the slope and intercept of the relationship between the flexion reflex and background EMG activity remained stable before and after transspinal stimulation. This indicates that transspinal stimulation did not alter the gain of spinal neural networks or the excitability threshold of the flexion reflex.

Conclusions and Significance

The findings of this study demonstrate for the first time that transspinal stimulation, regardless of frequency or intensity, significantly suppresses the TA flexion reflex during walking in healthy humans. This suppression coincided with the maintenance of the phase-dependent modulation pattern of the flexion reflex without altering the gain or excitability threshold of spinal neural networks. This discovery provides an important theoretical foundation for the rehabilitation of SCI patients. By suppressing the exaggerated flexion reflex, transspinal stimulation has the potential to help patients restore normal gait patterns, particularly during the stance-to-swing transition.

Research Highlights

1. Key Finding: Transspinal stimulation significantly suppresses the TA flexion reflex during walking, and this suppression effect is independent of stimulation frequency and intensity.
2. Problem Solved: The study offers a new neuromodulation strategy for treating gait disorders in SCI patients, with potential applications in suppressing exaggerated flexion reflexes.
3. Methodological Innovation: The study employed novel transspinal stimulation techniques combined with treadmill walking experiments to dynamically assess the modulation patterns of the flexion reflex.

Other Valuable Information

The researchers also noted that while the mechanisms underlying the modulation of the flexion reflex by transspinal stimulation are not fully understood, they may involve presynaptic and postsynaptic inhibition of spinal neural networks. Future studies could further explore the effects of transspinal stimulation in SCI patients to optimize neuromodulation parameters and maximize the recovery of motor function.

Summary

The findings of this study provide new insights into the rehabilitation of SCI patients, particularly through the modulation of flexion reflex excitability via transspinal stimulation, which may improve patients’ gait patterns and quality of life. Additionally, the study demonstrates the potential of transspinal stimulation as a non-invasive neuromodulation technique, laying the groundwork for future clinical research and applications.