Ruth Revere offered an exercise to iron out the curves of the spine, strengthen the muscles around the spinal vertebrae, and integrate the pelvis with the spine for firm support in the upright position. It involves lying on your back on the mat or rug. Bend your knees up over your body. Clasp the tips of your fingers around your knees. Round your elbows and ease your shoulders. Now, with the right knee in line with the right shoulder and the left knee in line with the left shoulder, rock your whole body slowly from side to side. Go as far as you can without flopping or resting your elbows on the floor. Keep your arms round. Start to the other side by using your inner, centering muscles (abdomen). This forces you to straighten out your spine with every effort. In time you will get the feel of it and every attempt will help. It relaxes the back muscles.
-- Notebooks Category 5: The Body > Chapter 5: Exercise > # 29