Sets: 3 Reps: 10 (each side)

Hold a light dumb-bell in your left hand and lie on your back on a Swiss ball with your feet flat on the floor. Hold the dumb-bell above your head and, keeping your torso in a straight line, rotate to the right until your left arm is parallel to the floor, then rotate back again. Repeat ten times, then switch arms and do another ten reps.