(48) Posterior Root-Muscle Reflex Recruitment in Healthy Individuals of Different Ages

Introduction: The Hoffman’s (H)-reflex is evoked by electrically stimulating a peripheral nerve. Stimulation results in a reflex response that can be recorded in the homonymous muscle using electromyography (EMG) [1]. The H-reflex threshold, which is the current required to elicit the H-reflex, increases with age, indicating reduced spinal cord excitability as people get older [2]. The posterior root-muscle (PRM) reflex is a compound of the H-reflex and cutaneous reflexes [3]. The PRM reflex is elicited by electrically stimulating the dorsal roots of the spinal cord using transcutaneous spinal cord stimulation (tSCS). Unlike the H-reflex, it remains unclear how the PRM reflex threshold changes with age. This study aims to characterize PRM reflex thresholds in healthy participants of different age groups to develop a database to compare spinal cord excitability changes with individuals with neural injury.

Methods: PRM reflexes were evoked using a 1 ms-long monophasic, cathodic square pulse delivered through adhesive electrodes placed lateral to the T12-L1 vertebrae. A return electrode was placed on the ipsilateral anterior superior iliac spine. PRM reflexes were recorded in 21 neurologically-intact participants (11 female; 23-49 years old). Wireless EMG sensors were placed over the soleus muscle of the ipsilateral leg, relative to the stimulating electrode. Stimulation was first delivered at a sub-threshold level, then increased in 5 mA increments until PRM reflex responses first appeared in the EMG recording (threshold), after which the threshold was identified with 1 mA precision. Stimulation was then increased in 5 mA increments until the responses no longer increased in amplitude (maximum).

Results: PRM reflex thresholds were 35±9.9 mA for ages 20-29 (n=14), 40±19 mA for ages 30-39 (n=5), and 42±7.8 mA for ages 40-49 (n=2). Linear regression of the PRM reflex thresholds over age resulted in a correlation coefficient of R=0.20. There was an increase in PRM thresholds in older participants; however, the individuals in this study were relatively young, with fewer participants in older age ranges. Additional reflex thresholds per decade are expected to strengthen this correlation. An example PRM reflex response at threshold and at maximum are shown in Figure 1.

Conclusion: By characterizing how PRM reflex thresholds change with age, we can develop a better understanding of how spinal cord excitability changes over time in healthy individuals, as well as compare thresholds from healthy individuals to those with neural injury.