Design and Mechanical Characterization of a Variable Stiffness ESR Foot Prosthesis

In this article, MyFlex-
$\epsilon $
, an ESR foot prosthesis equipped with a light and manually adjustable mechanism that allows for varying its stiffness in the sagittal plane, and a systematic approach to calculate its rotation-stiffness curves are presented. Through a design of experiment conducted numerically using a two-dimensional (2D) finite element (FE) model, calibrated experimentally, a geometric parameter whose variation alters the sagittal plane stiffness of a prosthesis originally designed with invariable stiffness, MyFlex-
$\delta $
, was determined. After building the mechanism and integrating it into MyFlex-
$\delta $
to obtain MyFlex-
$\epsilon $
, the displacement-force curves of the latter through tests equivalent to the static tests specified in ISO 10328 were determined. Based on the experimental results, the 2D FE model of MyFlex-
$\epsilon $
was built and calibrated to determine its rotation-stiffness curves in the sagittal plane. Comparing the rotation-stiffness curves obtained with the most compliant setting to the stiffest setting, stiffness variations of 119%, 122%, 138%, and 162% at plantarflexion angles of −5° and −2.5°, and dorsiflexion angles of 7.5° and 15°, respectively, were found.