Equilibrium and buojancy

At a joint, two muscles are attached, one for each direction. If one of them is active, the joint moves, if the both are active at the same time, the joint is stiff.

Buoyancy

The upright standing is maintained by permanent small muscle activity against the tendency to collapse. The more equilibrity the body has, the less energy is needed.

Such floating occures e.g. in the knees, which are bend and which keeps the upper body springy at a certain level.

In architecture, there is the word tensegrity. This is also true for a sceleton, which is held upright by muscles. Tensegrity is the combination of equilibrium and buojancy in a system of stiff and elastic elements. (Or something like that)