Why people always fidget so much when they sit in a chair and why...

we have to consider getting people with motor impairment to supine positions?

The chair as a health hazard

The downward gravity force pushes the bottom of the pelvis (sacrum) down and forward.  Eventually the sitter finds himself sitting mostly on a crocked pelvis which creates a c-curve slouch.  (from The Chair by Galen Cranz).  These distortions are not easy to self-restore by a person with movement dysfunction.

Therapeutic Positioning can help counteract these distortions.