The job of the skeleton is to counteract gravity

It follows that any posture is acceptable in itself as long as it does not conflict with the law of nature, which is that the skeletal structure should counteract the pull of gravity, leaving the muscles free for movement. The nervous system and the frame develop together under the influence of gravity in such a way that the skeleton will hold up the body without expending energy despite the pull of gravity. If, on the other hand, the muscles have to carry out the job of the skeleton, not only do they use energy needlessly, but they are then prevented from carrying out their main function of changing the position of the body, that is, of movement.

In poor posture the muscles are doing a part of the job of the bones.

Moshe Feldenkrais in "Awareness through movement" page 68