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In this article, MyFlex-<inline-formula> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula>, 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-<inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula>, was determined. After building the mechanism and integrating it into MyFlex-<inline-formula> <tex-math notation="LaTeX">$\delta $ </tex-math></inline-formula> to obtain MyFlex-<inline-formula> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula>, 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-<inline-formula> <tex-math notation="LaTeX">$\epsilon $ </tex-math></inline-formula> 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 &#x2212;5&#x00B0; and &#x2212;2.5&#x00B0;, and dorsiflexion angles of 7.5&#x00B0; and 15&#x00B0;, respectively, were found.