Crunches: To isolate your abdominal muscles, lie on your back with your knees slightly bent. Make sure your feet are planted firmly on the floor and about hip-width-distance apart. Keep your knees comfortably apart. Fold your arms on your chest and tighten your abdominal muscles. Exhale as you lift your shoulder-blades just off the floor. Inhale as you return to the floor.
The important thing to remember while you perform this exercise is that you must keep you abdominals pulled in. Imagine that you are pulling your belly button towards your spine. Even as you inhale, keep your muscles tight and pulled in.
Don’t allow your abdominal muscles to “bulge” out – or you’ll end up training your belly to look like a keg, rather than a six pack.
Below is a perfect demonstration photo from fitwirr.com.