Posture

Posture is the position assumed by the body either by means of the integrated action of muscles working to counteract the force of gravity, or when supported during muscular inactivity. Many postures are assumed by an individual during the course of 24 hour, and at any given moment posture comprises the positions of all the joints of the body.
Dynamically and statically, an efficient posture:

1. is stable
2. minimises stress and strain on the tissues
3. minimises energy cost

Subtle departure from an optimal posture is common and subjects the body to routine, eccentric and increased mechanical stress.

Alison Middleditch & Jean Oliver "Functional Anatomy of the Spine" page 327