The one I think is the toughest is the medicine ball walk-out, but the one I think is the best ab exercise (and I realize abs do NOT make up the entire core section), is what I call the “medicine ball reach”. I learned it from a trainer certified in Pilates. A soft 6-8″ ball is placed tightly against the lumbar region as you sit knees bent and feet flat. The arms hold a light medicine ball and are kept entirely straight as you lean back slightly to hold the softer ball in place. The concentric motion is to use the abs and do very small pulses forward, gradually raising the arms higher until they are above or even slightly behind the head, thus moving from a tiny crunch motion, to a rib cage “extension”. In this one, higher arm position = harder. The best part is the leg position eliminates any hip flexor involvement, thus isolating the rectus abdominis for the entire workload.