A legged robotic device is disclosed. The legged robotic device can include a plurality of support members coupled together for relative movement defining a plurality of degrees of freedom, which can correspond to degrees of freedom of a human leg. The legged robotic device can also include actuators to apply forces or torques to the support members in the degrees of freedom. In addition, the legged robotic device can include potential energy storage mechanisms associated with the degrees of freedom operable to store potential energy as a result of relative movement of the support members in the degrees of freedom and to provide at least a portion of the stored potential energy to the support members as compensating forces or torques to assist the actuators. In one aspect, elastic potential energy can be stored. A spring rate and/or a zero position of the potential energy storage mechanisms can be dynamically variable.