Start: Place a stability ball next to a sturdy object (a wall or bench) you can touch for balance, then lie with your back on the ball, feet flat on the floor. Lower back should be pressed into the ball with navel centered over ball. Hold wall with one hand; place other hand behind head.
Move: Lift leg closest to wall off the ground, keeping knee bent. As you lift, twist upper body toward lifted leg, opposite arm and leg meeting above stomach. Use shoulder muscles to move your arm rather than pulling elbow forward. Lift knee toward chest, not chest to knee.
Repeat: Perform 10 to 12 reps, then reposition and repeat on opposite side. Aim for two sets per side with one minute of rest between sets.