Adaptive control of a variable-impedance ankle-foot orthosis to assist drop-foot gait

TLDR

The study presents an active ankle‑foot orthosis that modulates joint impedance during walking to treat drop‑foot gait. The orthosis employs a biomimetic torsional spring that actively adjusts joint stiffness throughout the gait cycle—minimizing impedance in late stance, applying spring‑damper control during swing for toe clearance, and reducing forefoot collisions during plantar flexion—while kinetic and kinematic data from two drop‑foot patients were compared to matched normals under zero, constant, and variable impedance conditions. Actively adjusting joint impedance reduced slap foot, allowed greater powered plantar flexion, and produced less kinematic difference during swing compared to normals, indicating clinical benefits over conventional orthoses with zero or constant stiffness.

Abstract

An active ankle-foot orthoses (AAFO) is presented where the impedance of the orthotic joint is modulated throughout the walking cycle to treat drop-foot gait. During controlled plantar flexion, a biomimetic torsional spring control is applied where orthotic joint stiffness is actively adjusted to minimize forefoot collisions with the ground. Throughout late stance, joint impedance is minimized so as not to impede powered plantar flexion movements, and during the swing phase, a torsional spring-damper control lifts the foot to provide toe clearance. To assess the clinical effects of variable-impedance control, kinetic and kinematic gait data were collected on two drop-foot participants wearing the AAFO. For each participant, zero, constant, and variable impedance control strategies were evaluated and the results were compared to the mechanics of three age, weight, and height matched normals. We find that actively adjusting joint impedance reduces the occurrence of slap foot allows greater powered plantar flexion and provides for less kinematic difference during swing when compared to normals. These results indicate that a variable-impedance orthosis may have certain clinical benefits for the treatment of drop-foot gait compared to conventional ankle-foot orthoses having zero or constant stiffness joint behaviors.