1. Lie flat on your back, with your arms stretched out to each side and your palms flat on the ground. Squeeze a Swiss ball in between your feet and, by engaging your core, raise your legs to the ceiling.
2. Squeezing the ball and keeping your legs straight, lower your legs to one side as far as possible without letting the upper back or arms come off the ground. Then return the ball back to the center and repeat on the opposite side. That's one rep.