Sitting back against a chair support is the mother of all weight mis-commitments. We need look no further than this most common of activities to explain why modern societies are rampant with back-injuries and bad posture.
In sitting back against a chair support, we are mis-directing the force of our falling, sending it in the exact opposite direction that it needs to go. There is no anatomical support behind the sit bones to receive and transform the force of our backwards falling. All of our structual support - with the extension power it provides - lies directly underneath us and in front.
By becoming accustomed to the muscular straining and skeletal distortions that are part of our sustaining uprightness while falling backwards - in other words, becoming numb to them - we lose appreciation of the connection between our falling and our uprighting. In losing this connection, we lose the ability to direct the powerful force of falling to our advantage.