Holding a dumbbell at your left shoulder, lie on a stability ball, keeping your hips in line with your knees (A). Push the dumbbell straight up to full extension (B). Grab your left wrist and rotate your body to the left, keeping your hips in position (C). Return your shoulders to the ball and lower the weight to your shoulder. That's 1 rep. Finish the reps on the left, then repeat on the right.