The crunch is an exercise that strengthens the core, specifically the upper abdominal muscles. It is different from the sit-up because you only flex your abs in a limited range of motion. To perform this exercise you start lying on the floor with your knees bent. Simply raise your shoulders off of the floor, breathe out, and squeeze your stomach muscles simultaneously. This exercise should be done slowly in order to get a deep contraction, unless they are being used in a workout that is timed.