Does Gait Assistance Modulate Reflex Excitability in Those with Spasticity? A Preliminary Study

Stiff-Knee gait (SKG) is a common post-stroke impairment, marked by insufficient knee flexion during the swing phase, which reduces toe clearance, increases fall risk, and requires compensatory movements that can have adverse biomechanical effects. To address muscle weakness and paresis in SKG, assistive technologies, such as functional electrical stimulation (FES) and robotic exoskeletons, have been introduced. However, robotic assistance may inadvertently increase spinal reflex excitability by activating muscle stretch receptors, potentially worsening SKG. Previous studies suggest that SKG may stem from quadriceps hyperreflexia, particularly in the rectus femoris (RF) muscle. As such, assistive interventions could exacerbate, rather than alleviate, this condition via artificial activation of stretch receptors. In this study, we examined the impact of two FES modalities, plantarflexor and knee flexor assistance, on RF reflex excitability at rest and during gait in six post-stroke individuals with SKG. Results showed that, at rest, plantarflexor FES facilitated the RF reflex excitability, while knee flexor FES led to a slight inhibition. During gait, we observed that some individuals exhibited changes in reflex excitability during FES, but this effect was not as consistent as during rest. These preliminary results suggest that assistive technology such as FES modulates reflex excitability dependent on context.