Humans, like most walking animals, have been walking efficiently by using the kinetic energy and potential energy of the lower limbs. Human joints are actuated by 2 muscles, giving the possibility to change the stiffness of a joint and to control the position.While the shape of bipeds has been inspired by the human body, stiff electrical motors have been used commonly to actuate these joints. These bipeds do not use natural dynamics resulting in high energy consumption. In contrast with these fully actuated bipeds, there are also passive walkers, which exploit the natural dynamics to make a movement.The mechanism is designed in such a way that the natural motions generate a human like walking motion. The big advantage is that they are highly energy efficient and they only need to overcome friction and impact losses. The major drawback is that they have only one walking speed, since the natural frequency is determined by design. Besides the development and implementation of the Pleated Pneumatic Artificial Muscles,there has also been a second actuator with adaptable compliance. Each of these parameters is set by a position controlled servo motor. Howover, the torque is a linear function of the compliance and of the angle between equilibrium position and actual position. This actuator is a adaptable spring with torsion, where one motor sets the stiffness of the torsion spring, and the other sets the equilibrium position.