You should use a 9-pound (4 kg) medicine ball for an abs workout. Begin with one circuit and work your way up to three circuit sets. Put the medicine ball on the ground and rest the fingertips of both hands on top of it from a kneeling position. Lean forward with knees bent, keeping your back flat, core engaged and arms straight, and allow your hands and arms to roll over the medicine ball. For the double crunch and reverse crunch, use a slow, controlled movement.