There's a useful arrangement where you have two opposed springs powered by separate actuators. If you tighten up both springs, the system stiffens up; if you loosen both, you get better energy recovery but worse position control. This is a reasonable muscle model, and can recovery energy, but is bulky mechanically. Movement without load is cheap; changing the compliance is expensive from an energy standpoint. There's also a pneumatic version of this, where you pressurize an air cylinder from both ends. That works well if the valves are close to the cylinder.

There are mechanical approaches to variable compliance, but they're complex mechanically and usually seem to involve string and pulleys.[1] The trend seems to be towards all-electric systems, rather than clever mechanics.

[1] http://mime.oregonstate.edu/research/drl/publications/_docum...