View source for R. Van Ham et al. (2006). "MACCEPA: the mechanically adjustable compliance and controllable equilibrium position actuator for 'controlled passive walking'.

“Nowadays, more and more research groups working on bipeds have started to believe that natural biped walking is a combination of both approaches, requiring actuators with adaptable compliance (inverse of stiffness), resulting in energy efficient walking at different speeds.”

“Human joints are actuated by at least two muscle groups, giving them the possibility to change the stiffness of a joint and to control the equilibrium position. By controlling both the compliance and the equilibrium positions, a variety of natural motions is possible, requiring a minimal energy input to the system.”

The paper then mentions some of the leading research regarding designs of adaptive compliance combined with actuators: University of Pisa, Italy, [9] the Variable Stiffness Actuator (VIA), Georgia Institute of Technology, USA, [10] a Biologically Inspired Joint Stiffness Control, North-western University, MARIONET  [11]. These all describe mechanisms that use some elastic component to achieve features like human/animal joints. 

The paper then proposes its own design that is complex to explain and involves a lot of physics.

An experimental design is then built, named Veronica, for the purpose of showing the applications of this design. The robot is in the shape of humanoid legs, with 6 degrees of freedom:

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Although not yet fully functional, the robot managed to walk a small distance before falling.