What would the world be like without this exercise? It’s a modified sit-up that’s every gym teacher’s favorite torture tool and the first line of defense against a paunchy midriff. It’s also far friendlier to back muscles, which can be strained by improper sit-ups.
Technically, the abdominal crunch isn’t a weight-bearing exercise. It’s often called a “core-strength” exercise because it exercises the core set of abdominal and back muscles that promote good posture and reduce your risk of injury. But because you don’t choose the heftiness of the weight you’re lifting when performing an abdominal crunch, you need to deviate from the standard 8/14 rule.
That doesn’t mean you should stomach-crunch the day away. As you become a skilled cruncher, you should strive to perform two or three sets of 25 or fewer crunches. The goal is most decidedly not to perform as many crunches as you can. If you can breeze through 50 without a care in the world, you need to get fanatical about good form, find ways to intensify your effort, or switch to a more difficult variation (such as doing abdominal crunches on an exercise ball).
Here’s how to perform a basic crunch:
1. Lie down flat on your back. Bend your knees, keep your feet flat on the floor and hip-width apart, and place your hands across your chest.
2. As you breathe out, contract your abdominal muscles as tightly as you can, and slowly raise your head and shoulders just a few inches off the floor.
3. Hold your raised position for a few seconds, breathing in and out continually, and contract your stomach as hard as you can.
4. Slowly lower back down, but don’t relax all the way. This is a key point to getting the most out of a crunch—if you simply drop your body back down under the effect of gravity, you’ll lose half the benefit of the exercise.
To get the best results out of your abdominal crunch, concentrate on making sure that your abdominal muscles are doing all the work. Resist the urge to push with your feet, pull up with your arms, or lead with your chin. You can watch a basic instructional video at www.mayoclinic.com/health/abdominal-crunch/MM00725.
Source of Information : Oreilly - Your Body Missing Manual