A legged robotic device is disclosed. The legged robotic device can include a mechanism formed at least in part by a plurality of support members coupled together for relative movement defining a plurality of degrees of freedom, at least some of the plurality of degrees of freedom corresponding to degrees of freedom of a human leg. The legged robotic device can also include a primary drive actuator operable to apply a force or a torque to the support members in a first of the plurality of degrees of freedom. In addition, the legged robotic device can include a second actuator operable to apply a force or a torque to the support members in a second of the plurality of degrees of freedom. The mechanism can be configured to move in a gait-like motion that emulates human gait. The primary drive actuator can be sufficient to actuate the mechanism to move the mechanism in the gait-like motion. The mechanism can be dynamically modified by actuating the second actuator.