View source for R. Van Ham, B. Vanderborght, M. Van Damme, B. Verrelst and D. Lefeber (2006). "MACCEPA: the mechanically adjustable compliance and controllable equilibrium position actuator for 'controlled passive walking'

=Summary=
==Main idea==
Robots can be stiff and rigid, but that is not the only option that one has when creating a robot. In order to make robots move more human-like, a mechanism is introduced where actuators with "adaptive compliance" are created tested: 
*“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.”