how?

The spine is a chain of bones that transmits the weight of the head and ribcage to the pelvis. It changes shape according to their counterbalancing movement in different positions.

Posture is determined by the alignment of these body parts in relation to each other. The more aligned, the better the posture and the lower the load to the joints and muscles.

In standing, the pelvis tilts forwards and the upper body shifts backwards. The spine forms an S-shape around the line of gravity with the head balanced on top.

In unsupported sitting, the pelvis tips backwards and the upper body shifts forwards. The spine drops into a C-shape around the line of gravity with the head falling forward above.