This is the relatively static and involuntary posture that our body or parts of our body take due to the configuration of muscle, ligament and bone. This is particularly true of the chest and the face.
The structural part of our posture is made up of a configuration of muscle, tendon, ligament and bone. Bones are rigid and generally do not flex at all, nor do they stretch. They supply unmoving, rigid structure. The tendons and ligaments are elastic. They are the springs in the body, and will stretch slightly when we move but always returning to their original shape. Ligaments join bones together at the joints and tendons join muscle to bone. Muscles themselves are flexible and their main purpose is to supply movement, but they contribute to structural posture by filling in many of the gaps between bones and ligaments. In short, our structural posture is maintained by this system of muscles, ligaments, tendons and bones and without each of those components, will fall apart.
Our structural posture is essentially involuntary, largely due to the stiff elasticity of tendons and ligaments, which prevent us from voluntarily changing this structure with any ease.