Biplanar Ankle Assistance for Dropfoot Gait Post-Stroke with Multi-Objective Human-In-the-Loop Optimization: A Case Study

Exoskeletons are frequently explored for their efficacy in rehabilitation and in assisting in daily activities in people with motor disorders, yet relatively few have convincing evidence for use, nor do many aim to personalize assistance for maximum benefit of each user. Here, we describe a cabledriven ankle exoskeleton that assists persons with dropfoot and excessive inversion after stroke. We propose a multi-objective human-in-the-loop optimization that simultaneously adjusts exoskeleton assistive profiles to minimize two independent gait quality indices: foot kinematics deviation and step length asymmetry. This optimization proposes, rather than a single solution, a group of solutions that minimize both indices. We present experimental findings from one participant with chronic stroke, specifically the feasibility of the device in assisting ankle motion in two anatomical planes, the resulting gait pattern and individualized optimal assistive profiles. Following the design goals, the exoskeleton improved the dropfoot and excessive inversion gait pattern; with the exoskeleton, the participant's foot inclination angle at initial contact increased from $-2^{\circ}$ to 10°, ankle inversion angle decreased from 2° to 0°, foot clearance during swing increased from 31 mm to 44 mm, and step length asymmetry decreased from 12 % to 6 %. The optimal solutions are an inventory of profiles from which the user can choose based on priorities. We believe the device and approach have a large potential in achieving individualized assistance.