The crunch is a dual motion situp where your legs move in toward your head and you situp. Do not confuse it with a "half situp" style crunch where the head and upper body only raise slightly.In the standard crunch situp the arms are to your sides and the head and back are just slightly above the ground. The feet are up and just slightly above the ground so all of your weight is on your rear. Maintain balance in this position. Now crunch your body together, bringing your knees to your head and your head toward your knees. Maintain core tension as you release toward the floor. Do not let your feet or upper back touch the floor between repetitions.
Other variations include twisting as you sit up, holding the arms over your head, bringing one leg up at a time (and holding the other out straight). A challenging variations is called the V-Up where the knees and arms are locked out. The motion is the same, except the knees don't bend. You will situp into a V shape with only your rear on the ground.
