Reduced Post-Activation Depression in Humans with Spinal Cord Injury is not related to Stretch Reflex Hyperexcitability

Post-activation depression of spinal reflexes is reduced following central motor lesions, including spinal cord injury (SCI), and has long been proposed to contribute to signs and symptoms associated with spasticity. To evaluate this hypothesis, we assessed post-activation depression in individuals with chronic SCI, both males and females with and without stretch reflex hyperexcitability as well as in age-matched controls. Post-activation depression was assessed by recording the soleus H-reflex every 10 seconds using paired pulses at interstimulus intervals of 1, 2, and 4 seconds, and test reflexes amplitudes set at 20% and 40% of maximal motor response (M-max). Stretch reflexes were elicited by rapid (>300°/s) passive dorsiflexion of the ankle, and spasticity was clinically assessed in all SCI participants using the Modified Ashworth Scale (MAS) in plantarflexor muscles. We found that post-activation depression was more pronounced at shorter interstimulus intervals and at lower test reflex amplitudes (20% M-max) across all groups. However, post-activation depression was significantly reduced in individuals with SCI compared to controls, with no differences observed between SCI participants with and without stretch reflex hyperexcitability. Moreover, the magnitude of post-activation depression did not correlate with stretch reflex amplitude or MAS scores. These findings indicate that while post-activation depression is reduced after SCI, this decrease does not correlate with either stretch reflex hyperexcitability or the severity of clinical spasticity as assessed by the MAS. Significance Statement Decades of research have suggested that, following spinal cord injury (SCI), reduced suppression of the soleus H-reflex by repeated stimulation of sensory fibers—a phenomenon known as post-activation depression—is associated with the development of clinical signs and symptoms of spasticity. Our findings show that post-activation depression is similarly reduced in individuals with SCI, regardless of the presence or absence of stretch reflex hyperexcitability, compared to control subjects. The reduction in post-activation depression did not correlate with stretch reflex size or clinical spasticity severity, as measured by the Modified Ashworth Scale.