If you complete 300 crunches a day, you might build a little muscle — but, more likely, you'll be left with a sore stomach and not much else. The crunch and its big brother, the sit-up, can play a role in an abdominal workout routine to boost strength, but it's far from the best exercise you can do — and it won't get you in shape.
Instead, to get in shape — that is, to lose fat and build muscle so you have excellent cardiovascular endurance and muscular strength — you must undertake a comprehensive healthy-eating plan and workout routine that includes both cardio and strength-training. Although crunches can be part of that strength-training, it shouldn't be the only exercise you do.
If you want to do crunches as part of your workout routine, stick to a moderate amount like you would with any other exercise — three sets of 10 to 12 reps is generally sufficient.
Vary your crunches by doing bicycle crunches, reverse crunches and oblique crunches to hit multiple muscles. However, keep in mind that even if you build muscle by doing crunches, you won't see the results if it's hidden under a layer of stomach fat.
How to Get in Shape
Getting in shape takes more than doing crunches, no matter how many you might do. Instead, work on your eating plan and increase your physical activity.