Operant Conditioning Of Rectus Femoris H-Reflexes: A Proof-Of-Concept With Implications For Post-Stroke Quadriceps Hyperreflexia

Hyperreflexia is common after neurological injury such as stroke, yet clinical interventions have had mixed success. Our previous research has shown that hyperreflexia of the rectus femoris (RF) during pre-swing is closely associated with reduced swing phase knee flexion in those with post-stroke Stiff-Knee gait (SKG). Thus, reduction of RF hyperreflexia may improve walking function in those with SKG after stroke. A non-pharmacological and non-surgical procedure for reducing hyperreflexia has emerged based on operant conditioning of spinal reflexes elicited using electrical stimulation of the peripheral nerve, known as an H-reflex. Operant H-reflex conditioning training of the soleus has improved clinical gait function in those with spinal cord injury. It is currently unknown whether operant conditioning can be applied to the RF or those after stroke. This feasibility study trained 7 participants (5 neurologically intact, 2 post-stroke) over a period of 3 months, up to 3 times per week, to down-condition the RF H-reflex using visual feedback. We found an overall decrease in average RF H-reflex amplitude among 7 participants (44% drop, p<0.001, paired t-test), of which the post-stroke individuals contributed (49% drop). We observed a generalized training effect across quadriceps muscles. Post-stroke individuals exhibited improvements in peak knee-flexion velocity, reflex excitability during walking, and clinical measures of spasticity. These aforementioned neuromuscular outcomes provide promising initial results that operant RF H-reflex conditioning is feasible both in healthy and post-stroke individuals. This procedure could provide a targeted alternative to spasticity management and avoid the drawbacks of drugs and surgery.