Altering prosthetic foot stiffness influences foot and muscle function during below-knee amputee walking: A modeling and simulation analysis

Most prosthetic feet are designed to improve amputee gait by storing and releasing elastic energy during stance. However, how prosthetic foot stiffness influences muscle and foot function is unclear. Identifying these relationships would provide quantitative rationale for prosthetic foot prescription that may lead to improved amputee gait. The purpose of this study was to identify the influence of altered prosthetic foot stiffness on muscle and foot function using forward dynamics simulations of amputee walking. Three 2D muscle-actuated forward dynamics simulations of unilateral below-knee amputee walking with a range of foot stiffness levels were generated, and muscle and prosthetic foot contributions to body support and propulsion and residual leg swing were quantified. As stiffness decreased, the prosthetic keel provided increased support and braking (negative propulsion) during the first half of stance while the heel contribution to support decreased. During the second half of stance, the keel provided decreased propulsion and increased support. In addition, the keel absorbed less power from the leg, contributing more to swing initiation. Thus, several muscle compensations were necessary. During the first half of stance, the residual leg hamstrings provided decreased support and increased propulsion. During the second half of stance, the intact leg vasti provided increased support and the residual leg rectus femoris transferred increased energy from the leg to the trunk for propulsion. These results highlight the influence prosthetic foot stiffness has on muscle and foot function throughout the gait cycle and may aid in prescribing feet of appropriate stiffness.