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A Novel Framework for Quantitatively Connecting the Mechanical Design of Passive Prosthetic Feet to Lower Leg Trajectory

This paper presents a novel framework that quantitatively connects the mechanical design of a prosthetic foot to its anticipated biomechanical performance. The framework uses kinetic inputs (ground reaction forces and center of pressure) to predict kinematic outputs of the lower leg segment by knowing the geometry and stiffness of the foot. The error between the predicted and target kinematics is evaluated using a root-mean-square error function called the Lower Leg Trajectory Error (LLTE). Using physiological kinetic inputs and kinematic targets, three model foot architectures were optimized to minimize the LLTE. The resulting predicted lower leg kinematics were compared to those of the same foot architectures optimized for physiological roll-over geometry. The feet with minimized LLTE had lower leg kinematics closer to physiological than those optimized for roll-over geometry. A prosthetic foot that exactly mimics physiological roll-over geometry may result in gait kinematics that differ greatly from physiological, as roll-over geometry omits information about the foot-ground contact constraint, lower leg orientation, and temporal progression of a step. The LLTE-based framework is agnostic to specific foot designs provided their constitutive behavior can be characterized, and it can accept alternate inputs and targets depending on what performance and clinical objectives are desired.