Unilateral walking surface stiffness perturbations evoke brain responses: Toward bilaterally informed robot-assisted gait rehabilitation

Gait impairment due to neurological disorders has become an important problem of the 21st century. Stroke is a leading cause of long-term disability with approximately 90% of stroke survivors having some functional disability, with mobility being a major impairment. Despite the growing interest in using robotic devices for rehabilitation of sensorimotor function, their widespread use remains somewhat limited, as results so far in gait rehabilitation do not generally show improved outcomes over traditional treadmill-based therapy. This work focuses on understanding the mechanisms of inter-leg coordination, and based on that, proposing novel methods for gait rehabilitation. Using a novel robotic device, the Variable Stiffness Treadmill (VST), we apply walking surface stiffness perturbations to one leg, and analyze the response of the human nervous system in both low- (muscle) and high- (brain) levels, focusing on the mechanisms involved in the response of the other (unperturbed) leg. We show that the unperturbed leg uniquely responds to unilateral stiffness perturbations, while we provide solid evidence that the brain is involved in this observed inter-leg coordination. From a clinical prospective, the results of this study can be disruptive since they suggest that supraspinal neural activity can be evoked by altering the stiffness of the walking surface. Moreover, our methods provide a safe and targeted way to provide gait rehabilitation in hemiparesis since direct manipulation of the paretic side is not required. The present work provides for the first time evidence that specific robotic intervention in gait rehabilitation can have direct and predictable effects on the brain, opening a new avenue of research on targeted robot-assisted gait rehabilitation.