For Flat Abdominals, Go for the Burn : Fitness: The only way to strengthen stomach muscles, say the experts, is with exercise--sit-ups, leg lifts and stomach curls.

They grow slack with age and are the muscles that challenge even the most highly paid professional athletes to keep in shape.

“Abdominals are the weak spot for most people,” said Ronnie Barnes, head athletic trainer for the New York Giants football team. “They are a highly neglected area.”

Those who make the mistake of letting abdominals go pay a stiff price. Slackened abdominals throw posture off and contribute to back problems, according to athletic trainers.

Weak stomach muscles can also lead to sports injuries as the body strains to compensate for what the abdomen can’t do.

The only way to strengthen stomach muscles is with exercise--sit-ups, leg lifts and stomach curls.

“They’re painful, boring and just not a lot of fun to do,” said Byron Hansen, director of Professional Sports Care in Pasadena, and an athletic trainer for the USC football team.

Conditioning slack abdominals should be done slowly.

A beginning series might include 15 sit-ups followed by leg lifts and then stomach crunches. In this exercise, the legs are crossed, bent slightly and held perpendicular to the body. The upper body is then raised to meet the legs.

Trainers advise resting for a couple of minutes before repeating the series.

“It’s always better to do a few good exercises rather than a lot of bad ones,” said Robert Panariello of the Sports Medicine, Performance and Research Center at the Hospital for Special Surgery in New York.

Among the other tips:

* Be sure to warm up abdominal muscles before exercising. Start by lying on the back, legs straight, then bring one leg at a time to the chest and hold for a five-second count. Repeat about five times on each side.

* Always keep legs bent while doing sit-ups to reduce back strain. Many trainers recommend that you anchor your feet under a chair or have your feet held down by a partner. This helps isolates the abdominal muscles.

* If you have back problems, avoid any abdominal exercises that require twisting, which may place extra strain on the back. Abdominals are one of the few muscles in the body for which going for the burn is the way to tell that real progress is being made.

As Barnes said: “It’s imperative with the abdominals to feel something.”