Force sensitivity of robots enhances adaptability and dexterity of manipulation. We can hypothesize that this is also true for legged mechanisms since locomotion fundamentally is a manipulation of the center of mass. Legged mechanisms require high resistance to impacts. Recent studies revealed the efficacy of hydraulic actuator in legged robots, but backdrivability was lost due to high resistance in servo valves. Electro-Hydrostatic Actuators (EHAs) use no servo valves and acquired backdrivability and high efficiency. However, it required high rated power in all actuators when they are used in serial manner. In this paper, we propose to design 3DOF ankle with parallel mechanism, 1DOF knee, and one biarticular actuator connecting ankle and knee to provide redundant actuation in order to enhance total power capacity of the leg. Design concept of the biarticular actuator and the design optimization method of the redundantly actuated legged mechanism is discussed.