Lie face down on a Resista-Ball with your knees slightly flexed and your legs spread out for balance. Place your arms next to the body, off to the side, or overhead, depending on the desired resistance. Begin with your trunk flexed yet with tension in the erector spinae (the muscles that run on either side of the spine).
Pull your shoulder blades together and down toward your buttocks. Begin to contract the erector spinae slowly, extending the trunk one vertebra at a time. Hold, continue to contract the erector spinae, then slowly lower your trunk back to its original start position. Repeat.