Muscle action in lower extremities during double support phase of gait was ana-lyzed quantita tively using optimization techniques.Two optimization problems ,one linear and one non-linear,were formulated to solve the indeterminate problem of muscle load sharing.The re-sults show that the predicted muscle action is consistent with the qualitative analysis by other investigators.Two-joint muscles have better mechanical advantage in counter-current move ment between syn ergistic muscles.In a muscle group,greater muscle force is allocated to the muscle with larger physiological cross sectional area or higher percentage of type I muscle fiber.The moment arm of the muscle is not a dominant factor in muscle load sharing.For clinical use,the eccentric control of the two-joint muscles in the knee joint is very important for knee sta-bility during stance phase,and is suggested to be essential in programs of physical therapy for training in double limb support.