This paper presents the design of a passive two leg orthosis to gravity balance human legs for use by subjects with both leg impairment. This design combines the use of auxiliary parallelograms to locate the center of mass along with springs to achieve balancing of each leg. The gravity balancing for all motions of the leg and pelvis is achieved by assuming the center of mass of the pelvis to be located on the line joining the two hip joints. A method for deriving the effectiveness of the gravity balancing due to practical limitations is also outlined. We illustrate the gravity balancing feature of the orthosis and examine the assumptions through computer simulations.

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