From Paige Waehner,
Your Guide to Exercise.
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The following exercises are part of the American Council on Exercise's study to determine the most effective ab exercises. Keep in mind that ab exercises won't reduce fat over the abs or create the coveted 'six-pack' abs. That comes with good genes and low body fat, although you should strengthen the abs along with the rest of the body to build muscle and raise your metabolism. For more about flat abs, visit my Abs Page for articles, workouts, FAQs and more.

The Bicycle exercise is the best move to target the rectus abdominis (i.e., the 'six pack') and the obliques (the waist), according to a study done by the American Council on Exercise. To do this exercise correctly:

1. Lie face up on the floor and lace your fingers behind your head.
2. Bring the knees in towards the chest and lift the shoulder blades off the ground without pulling on the neck.
3. Straight the left leg out to about a 45-degree angle while simultaneously turning the upper body to the right, bringing the left elbow towards the right knee.
4. Switch sides, bringing the right elbow towards the left knee.
5. Continue alternating sides in a 'pedaling' motion for 12-16 reps.