The Abdominal Crunch exercises your middle stomach muscles. Grab the handles and crunch by pulling your elbows towards your knees. Muscles in the side of your stomach can be worked if the seat is twisted. Crunch machines usually use extra resistance in the form of selectorized weight stacks or plate loading, and are often performed for moderate to high reps, like 8-12 reps per set or higher, as part of the ab-focused portion of a workout.