Design and Validation of a Bionic Variable Stiffness Semi-Active Ankle–Foot Prosthesis

Abstract

Energy Storage and Return (ESR) prosthetic feet are commonly used by individuals with tibial amputations, but their fixed stiffness cannot accommodate varying walking tasks and user preferences. In contrast, semi-active prostheses can adjust stiffness through control mechanisms, though their design still leaves room for improvement in biomimetic characteristics, mass, and dynamic response. In this paper, we present a bionic, variable-stiffness, semi-active ankle–foot prosthesis. Stiffness is adjusted before each ground contact by controlling the motor to alter the length of the moment arm from the ankle joint center to the point of force application, with a range of 1.78–7.01 N·m/°. Additionally, the device features a biomimetic footplate design based on the transverse arch of the human foot, which reduces weight (total weight: 933 g) while guaranteeing support capacity. The prosthesis can precisely identify the phase for stiffness adjustment and achieve the widest range of adjustment during this phase. A unilateral tibial amputee participated in a preliminary clinical test involving variable-speed walking, stairs, and ramps. Preliminary single-subject data suggest that, compared to the subject's previous fixed-stiffness prosthesis, the newly designed prosthesis increases the range of motion, peak power, and energy storage during level walking, while reducing the adduction moment on the sound limb's knee joint. These findings offer initial support for the prosthesis' potential to improve the biomechanics of walking and other activities, providing insights for the development of biomimetic design.