By placing your feet on an exercise ball while doing floor crunches, you make your abs work a little harder.
To do this exercise, lay on the floor with either your feet or ankles resting on the ball. You want your legs at a 90 degree angle.
Put your hands on your head, being careful not to pull on your neck.
Tighten your abs and lift your shoulders towards your knees.
Hold for a few seconds. Then release your stomach muscles and return to the starting position.