How to: Lie on an exercise ball, so that the ball is supporting your lower back. Keep your knees at a 90-degree angle, your feet flat on the floor, and keep your head in line with the rest of your spine, as above. Raise your arms, take a ball from your training partner, and lie back, taking the ball over your head. Draw your abs in, exhale and, with straight arms, crunch forward, raising your shoulders off the supporting ball to give the smaller ball you are holding back to your partner.