Control of a Powered Ankle–Foot Prosthesis Based on a Neuromuscular Model

TLDR

Control schemes for powered ankle‑foot prostheses rely on fixed torque‑ankle state relationships derived from intact human walking, which work at target speeds and terrains but cannot adapt to disturbances such as speed transients or terrain changes. The study proposes an adaptive muscle‑reflex controller that uses a Hill‑type plantar‑flexor muscle with positive force‑feedback reflex to enable such adaptation. Model parameters were tuned to match the torque‑angle profile of a weight‑matched intact subject walking at 1 m/s, and the same parameter set was applied in clinical trials with a transtibial amputee across level ground, ramp ascent, and ramp descent. Trials showed prosthetic ankle work adapted to ground slope in a manner comparable to intact subjects, with energy output directly correlated to slope angle, underscoring the value of neuromuscular controllers for terrain‑adaptive prostheses.

Abstract

Control schemes for powered ankle-foot prostheses rely upon fixed torque-ankle state relationships obtained from measurements of intact humans walking at target speeds and across known terrains. Although effective at their intended gait speed and terrain, these controllers do not allow for adaptation to environmental disturbances such as speed transients and terrain variation. Here we present an adaptive muscle-reflex controller, based on simulation studies, that utilizes an ankle plantar flexor comprising a Hill-type muscle with a positive force feedback reflex. The model's parameters were fitted to match the human ankle's torque-angle profile as obtained from level-ground walking measurements of a weight and height-matched intact subject walking at 1 m/s. Using this single parameter set, clinical trials were conducted with a transtibial amputee walking on level ground, ramp ascent, and ramp descent conditions. During these trials, an adaptation of prosthetic ankle work was observed in response to ground slope variation, in a manner comparable to intact subjects, without the difficulties of explicit terrain sensing. Specifically, the energy provided by the prosthesis was directly correlated to the ground slope angle. This study highlights the importance of neuromuscular controllers for enhancing the adaptiveness of powered prosthetic devices across varied terrain surfaces.