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

Area of Science:

• Biomedical Engineering
• Rehabilitation Engineering
• Prosthetics and Orthotics

Background:

• Lower limb prostheses aim to restore mobility and function for amputees.
• Current prosthetic feet often have fixed stiffness, limiting adaptability to diverse activities.
• A need exists for prosthetic devices that can dynamically adjust to user needs and environmental conditions.

Purpose of the Study:

• To design and validate a novel variable stiffness foot (VSF) prosthesis.
• To enable dynamic adjustment of forefoot stiffness in response to user gait.
• To minimize size, mass, and power consumption in a semi-active prosthetic device.

Main Methods:

• Designed a semi-active prosthetic foot with an overhung composite beam forefoot keel.
• Implemented a mechanism to vary forefoot stiffness by adjusting the overhang length.
• Utilized foot trajectory reconstruction from an embedded inertial sensor to program stiffness modulation.
• Conducted mechanical, actuation, and human subject testing.

Main Results:

• The VSF prototype has a mass of 649 g and a build height of 87 mm.
• Achieved a threefold stiffness variation range (10-32 N/mm) in the forefoot.
• Demonstrated stiffness adjustment within three strides and tracking of speed variations within one swing phase.
• Human testing showed greater energy storage and return at lower stiffness settings.

Conclusions:

• The VSF successfully demonstrates dynamic stiffness modulation for lower limb prostheses.
• The ability to adjust stiffness may improve energy return, mimicking natural ankle function.
• Subjective feedback confirms user perception of stiffness variations and potential benefits.
• Future applications include optimizing prosthetic performance on stairs, ramps, and during varied activities.