Energy Loss and Stiffness Properties of Dynamic Elastic Response Prosthetic Feet

Dynamic elastic response prosthetic feet are designed to store and return energy during the gait cycle to assist the amputee with limb advancement. In so doing, the structural ability of the feet to store and return energy is significant, as is foot stiffness and the amount of input energy dissipated by the foot before it unloads. Manufacturers do not generally make subjective measures of stiffness and hysteresis available; instead, feet are classified according to manufacturer-specific stiffness categories. This investigation attempted to provide an independent measure of stiffness and hysteresis for eleven prosthetic feet. The feet were tested in constant strain rate and in cyclic tests on an Instron material-testing machine. Each foot demonstrated a stiffness value in one of only four categories, with multiple models of feet and even feet from multiple manufacturers showing essentially the same stiffness. Energy loss showed a broad range of feet in the middle of all values, with the greatest amount of loss occurring in the College Park feet and the least amount occurring in the Flex-Foot. Although structural property values do not fully explain a foot’s function in gait, data suggest the need for an independent classification scheme for stiffness and hysteresis among all manufacturers. Such a scheme would aid clinicians’ ability to appropriately prescribe and fit prosthetic feet.