The ankle joint is a vital component of human biomechanics and it is highly susceptible to injury due to its constant functional demand, often requiring long-term and repetitive rehabilitation exercises. While beneficial, conventional exercises frequently lack the ability to provide objective information on patient progress. This two-part study proposes three portable rehabilitation devices based on a spatial four-bar mechanism. The improved designs allow patients to perform exercises in the comfort of their homes while ensuring precise alignment between the anatomical center of the ankle joint and the intersection point of the rotation axes of the sole-supporting plate. The first part of the study focuses on key aspects of the proposed devices, while the second part presents simulation results. The simulation results indicate that the mechanisms achieve the full range of movements necessary for comprehensive ankle rehabilitation.