Stiffness and energy storage characteristics of energy storage and return prosthetic feet
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Prosthetists currently lack quantifiable measures to guide prosthesis prescriptions and must rely on experience and manufacturer recommendations. Studies have shown that stiffness and energy storage characteristics of prosthetic feet significantly influence amputee gait. Consequently, several studies have attempted measure these mechanical characteristics, but typically measure only a few orientations in a single plane. This study examined the stiffness and energy storage characteristics of several prostheses over normal gait orientations with the goal of improving prosthesis prescriptions. Feet from five different manufacturers were tested with twenty-five different combinations of foot style, stiffness category and heel wedge inclusion. Force-displacement data were collected at fifteen sagittal orientations and five coronal orientations, and were used to calculate stiffness and energy storage. Loading conditions at each sagittal orientation were determined using a representative amputee’s scaled walking data. Stiffness and energy storage were found to be highly non-linear in both the sagittal and coronal planes. Across all feet, stiffness was greatest near foot flat in the sagittal plane. Generally, stiffness decreased with greater heel, forefoot, medial and lateral loading orientations. Energy storage was greatest for forefoot loading and increased with medial or lateral loading orientations. As stiffness category increased within a foot style, stiffness increased and energy stored decreased. However, the recommended weight for a given foot was not linearly related to stiffness or energy storage. In addition, feet with similar manufacturer recommended weight ranges had varied energy storage over all orientations and varied stiffness over heel and foot flat loading orientations. Inclusion of heel wedges increased stiffness and decreased energy storage over heel and foot flat loading for the Vari-Flex with EVO foot, but not the Sierra foot. These results may help improve clinical prescriptions by providing prosthetists with quantitative measures to compare feet.