Low Body Fat is ideal for those 6-packs to show, But doing proper excersises where you are focusing on your stomach and not your back or hips, can develope the abdominal muscles stronger and increase size, leading to more viewable abs with your body fat at a maintainable %
Here are a few excersises to Try:
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Exercise #1 : Lay flat on the floor, legs drawn up at an angle, feet flat on the floor. This is similar to the "oldstyle" situp position. Place your hands on opposite shoulders, elbows pointing at the ceiling. Now try to touch your elbows to your hips by "curling" your upper body into a ball. You probably wont be able to get all the way there (At least I cant....), but you should feel a tension in your midsection just below your breastbone down to about your navel. Hold this for a second or two, then slowly relax back down. These muscles respond just as well to a slower movement, so there is no need to thrash back and forth like a whale trying to unbeach itself. ;-)
Exercise #2: This is similar to the first exercise. Lay on your back with your hands palm down on the floor along your body (Or extended out in a cross type fashion.) Bring your knees and legs up to your body (Think of the old cannonball dive position, except don't wrap your hands around them.) Now "curl" from the bottom up, trying to drive your knees through your shoulders. Hold and relax.