Optimizing the Mechanics of a Variable-Stiffness Orthosis With Energy Recycling to Mitigate Foot Drop

In ankle-foot orthosis development, it is challenging to both specify the appropriate ankle mechanics and design systems that can physically render them. Recently, a new ankle-foot orthosis - the Variable Stiffness Orthosis (VSO) - was introduced to allow customization of the shape of the joint's torque-angle relationship via a cam-based transmission. A module in the VSO permits switching between two coupled torque-angle relationships at desired kinematic transitions. This module decouples energy storage and return (DESR), enabling new functionality, including varying the ankle's equilibrium position and exchanging energy between gait phases. However, the torque-angle relationships are defined by many parameters and subject to substantial constraints. We developed an optimization framework to design two versions of the DESR module to address foot drop. The <i>angle module</i> was designed to maximize swing ankle angle, and the <i>energy module</i> was designed to maximize energy recycled from early stance phase to augment push off. We validated the results of the optimization with brute-force searching and empirically tested the DESR mechanics in a rotary dynamometer. The angle module facilitated swing angles of up to 0.63° dorsiflexion, while simultaneously permitting a plantarflexed standing angle, and the energy module recycled up to 1.84 J.