Many times as children we heard our parents implore us to "sit up straight".
Often, we were sitting at the dinner table slumped over our food. This slumped state was the product of our habit of falling backwards, adjusted for the present task of getting food off of our plates, located down on the table in front of us.
Given the set of conditions present when we heard these words, there was but one set of responses that would enable us to achieve our goal of sitting up straight. We knew what to do. We had to mightily tense the ilio-psoas muscles in front to lift the pelvis out of its backwards fall, and to mightily tense the erector spinae muscles in back to lift the spine out of its forward slump. Because of our overall lack of awareness, we tense too much, producing the classic 'sway-back' posture. This is much too much work to sustain for very long. We will soon return to habit and find ourselves slimping or falling back against the chair-support.