Why does perfect structure make any movement easy?
Foundational muscles work (30% contraction) gently holding you up, so outer muscles (the ones that move you) are at their correct length, not wasting energy holding you up. Energy is saved.
Joints are free, greased with no restrictions, inner organs no longer compressed and free to pump all the energy-producing ingredients around a no longer restricted body.
The forces of gravity fall equally through the centre of the body and every joint, so the body requires the least possible amount of energy to move, in any direction. Movement is easy.