Transcutaneous Spinal Stimulation Modulates Spinal Reflex Circuit Excitability in Persons with Spinal Cord Injury

Area of Science:

• Neuroscience
• Rehabilitation Medicine
• Biomedical Engineering

Background:

• Transcutaneous spinal stimulation (TSS) is a noninvasive method to modulate spinal reflex excitability.
• Posterior root muscle (PRM) reflexes are used to assess spinal circuit excitability changes post-TSS.
• Stimulus parameters significantly influence PRM reflex responses.

Purpose of the Study:

• To compare the effects of three TSS conditions (single-site continuous, single-site burst, dual-site continuous) on soleus PRM reflex responses.
• To investigate the impact of TSS on electrophysiologic measures of spasticity.
• To assess the correlation between biomechanical and electrophysiologic measures of spasticity.

Main Methods:

• Compared three TSS conditions: single-site continuous (SS-CONT), single-site burst (SS-BURST), and dual-site continuous (DS-CONT).
• Stimulation parameters: 50 Hz, 80% soleus reflex threshold (RT), 30 min duration.
• Measured soleus PRM reflex indices (RT, RA1.2xRT, slope, AUC) and paired-pulse indices (PPD, AUCdep).
• Assessed ankle clonus first drop excursion (FDE) for biomechanical spasticity.

Main Results:

• Whole-group analysis revealed significant decreases in PPD and AUCdep.
• These significant decreases in PPD and AUCdep were specifically observed after the SS-CONT condition.
• No significant changes were found in other PRM reflex indices or FDE across conditions.
• No baseline correlation was identified between FDE and PRM reflex parameters.

Conclusions:

• Single-site continuous TSS (80% soleus RT) modulated targeted spinal reflex circuits, evidenced by changes in conditioning and test stimuli responses.
• The study suggests that stretch-induced soleus spasticity may not consistently correlate with electrophysiologic testing results.
• TSS parameters, particularly the continuous single-site application, are crucial for effective spinal reflex modulation.