Post-activation Depression Is Modulated By Electrical Stimulation Frequency In Individuals With Spinal Cord Injury

Abstract

People with chronic spinal cord injury (SCI) lose the ability to suppress the H-reflex when it is elicited repetitively, a process known as post-activation depression. The loss of H-reflex depression is believed to be one adaptation that contributes to spasticity. Electrical stimulation of paralyzed muscle has many known benefits including reducing spasticity; however, the mechanism that causes changes in muscle spasticity has not been clearly delineated. In this study we examine the mechanism, post-activation depression, after electrical stimulation of paralyzed muscles. Importantly, we assess whether frequency of neuromuscular electrical stimulation (NMES) affects post-activation depression. PURPOSE: To determine the effect of 1-Hz and 5-Hz NMES on post-activation depression in individuals with complete quadriplegia and paraplegia. METHODS: Nine individuals with SCI (5 quadriplegics & 4 paraplegics) participated in this study. Each participant received 1-Hz and 5-Hz NMES on two different days. NMES was applied to the bilateral quadriceps and hamstring muscles for 20 minutes at an intensity of 50% of maximum. Soleus H reflex was elicited by doublets before, 0 min, 10 min, and 30 min after NMES. Repeated measures ANOVAs were used to compare differences across 2 protocols, 4 time points, and 2 groups. RESULTS: The amplitude of the first H reflex during the H-reflex doublet was not affected by either 1-Hz or 5-Hz NMES (all P > 0.51). In addition, 5-Hz NMES decreased post-activation depression for at least 30 minutes (all P < 0.007), whereas 1-Hz NMES did not change post-activation depression at any time points after NMES (all p > 0.769). Furthermore, following 5-Hz NMES, there was a trend (P = 0.051) that people with quadriplegia showed a significant disinhibition at 0 (P = 0.004) and 10 min (P = 0.016) post; while people with paraplegia showed a significant disinhibition at 10 (P = 0.016) and 30 min (P = 0.011) post. CONCLUSIONS: 5-Hz, but not 1-Hz NMES applied to the bilateral quadriceps and hamstring muscles diminished post-activation depression of soleus H reflex in people with SCI. These findings suggest that 1-Hz stimulation would induce less spasticity in people with SCI. Supported By NIH Grant RO1-HD-062507 and VA Grant 1-01RX000149-01