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Gravity balancing is often used in industrial machines to decrease the required actuator efforts during motion. In the literature, a number of methods have been proposed for gravity balancing that include counterweights, springs, and auxiliary parallelograms that determine the center of mass. However, these concepts have not yet been seriously applied to rehabilitation machines. This paper presents the underlying theory and design of an orthosis for the human leg that can fully or partially balance the human leg over the range of its motion. This design combines the use of auxiliary parallelograms to determine the center of mass along with springs to achieve a full or partial gravity balanced orthosis design. A first prototype has been constructed to demonstrate the effectiveness of the idea. Future prototypes will have parameters that will be tuned to the geometry and inertia of a human subject and be tailored to an individual's needs.