Day nr. 3 – Plank Leg-Lifts. Lie on your belly on an exercise ball, and walk your hands out so the ball is underneath your shins. Your hands should be underneath your shoulders. Firm your abs to keep your spine straight and your body stabilized. Lift your right leg into the air, then slowly lower it back toward the ball, but don’t let it touch.Complete three sets of 20 reps with the right leg, and then repeat with the left.