How can I get rid of my potbelly? It's a concern of men and women who exercise vigorously and regularly as well as those who don't. For example, some exercisers do hundreds of sit-ups every day and still can't get rid of a rounded belly, which is more accurately known as a protruding lower abdominal wall.

You can help trim this area by doing reverse sit-ups (sometimes called knee-ups or pelvic roll-ups). This is a key exercise for working the lower abdominal muscles. By strengthening and tightening these muscles, your protruding abdominal wall should diminish.

However, keep in mind that Madison Avenue advertising and the human body are two different things. The main abdominal muscle, the rectus abdominis, is not straight -- and never will be. It has a slight curvature as it goes under the inner muscles to attach to the pubic bone. Don't expect to see a flat abdomen. Your lower abdomen is slightly curved.

Why is the reverse sit-up more effective than regular sit-ups? Different exercises cause different parts of a muscle to contract, which is what tightens and strengthens them. The regular sit-up works only the upper abdominal muscles. Conversely, reverse sit-ups work the lower abdominals and don't particularly affect the upper abdominals. Of course, the lower abdominals are not a separate muscle. But the rectus abdominis is a very long muscle, and when the pelvis is moved upward during the exercise, only the lower portion does the work. The Muscles Worked

The muscles involved in reverse sit-ups are the rectus abdominis and the internal and external obliques. The rectus abdominis is a relatively long, slender muscle running vertically across the abdominal wall from the crest of the pubic bone to the cartilages of the fifth, sixth and seventh ribs. It has a right and left half and is separated by a tendinous strip about one inch wide called the linea alba.

The internal and external obliques cover the sides and front of the abdomen. The upper end of the external oblique is attached to the lower eight ribs, and the lower end connects to the ilium, pubis and linea alba. The fibers run diagonally upward and sideward and form the letter V.

The internal oblique is located directly underneath the external oblique. At the upper part of the abdominal wall, its fibers run at nearly right angles to the external oblique fibers and form an inverted letter V. In the lower abdomen, however, the internal oblique fibers are almost horizontal. The internal oblique muscle connects with lumbar tissue, the ilium and the inguinal ligament in the lower abdomen and attaches to the cartilages of the eighth, ninth and 10th ribs and linea alba. How to Do It

To do it right, lie on your back with arms at your side and palms dowm. Keep your knees bent. Then raise your legs until your knees are directly in front of your face and your pelvis is rotated away from the floor. This will also raise your buttocks off the floor. In this position, the lower abdominal fibers are fully contracted.

When you first do this exercise it may seem difficult because of very weak lower abdominal muscles. (After all, it' s the upper abdominals you've probably been working so diligently.) In time they will become stronger, and you'll be capable of doing many more repetitions.

In the beginning, though, it may help you to press against the floor with your hands to help push your buttocks up and over. Don't do this all the time, though, because you want the abdominal muscles to do the work.

Also, it is not always necessary to return to the starting position with your feet on the floor. The initial raising of the legs from this position is executed by the hip joint flexors, not the abdominals. (While the hip joint flexors do most of the work, the abdominals contract to stabilize the pelvic area and allow the legs to be pulled upward. The abdominal muscles involved are thus pretensed and capable of a stronger contraction to lift the buttocks upward when the hip flexion ceases.)

Warning: If this -- or any other abdominal exercise -- is not done properly, it could be stressful to the back. People who have back problems should not begin with this exercise, but should check with a doctor and consider starting with an easier exercise such as the abdominal curl -- which is done in the same starting position, but curls the head, shoulders and torso up and forward off the floor in one fluid motion, exhaling at the top of the movement. Be sure the lower back never arches off the floor throughout the exercise. Workout Schedule

Beginners should do between eight and 16 repetitions, working up to 30 repetitions. If you can't do eight, do as many as you can using proper alignment. Keep in mind that you must go through the needed range of motion on each repetition. Don't cheat!

At first, perform the reverse sit-ups three days a week on alternate days. After several months, when you can do 30 repetitions with comparative ease, you can do the exercise every day. Points to Remember

*Don't expect your abdomen to flatten -- it's naturally curved.

*Keep knees slightly bent. If that's too difficult, then bend your knees more.

*Use your hands for support.

*Make sure buttocks are entirely off the floor.

*Concentrate on tensing the lower abdominals.

1985, Shape Magazine; Los Angeles Times Syndicate