The circular motion of this "figure-of-eight crunch" will really work your abdominal muscles. See the details in the video from Men's Health, which shows you how to do a really powerful crunches exercise.
Lie on your back with your knees bent at a 90º angle, squeezing a light medicine ball tightly between them. Place your feet flat on the floor. Place your fingers behind your head. Slowly raise your head, shoulders and feet off the ground. Keeping your torso up, move your knees in a figure-of-eight motion. Each figure-of-eight you make counts as one rep.
Keep your back as flat as you can on the floor and avoid pulling on your head with your hands as you execute each rep. This exercise requires a bit of practise so we do recommend you start off with the lightest ball available.
Too easy? Try going for eight sets of eight reps on the figure-of-eight crunch and take just eight seconds rest between sets. When you can do that, aim for a heavier ball and apply the same techniques.