"You ARE the weakest link. Good-bye." That's what I'd like to say to my jelly belly, definitely the weakest part of my body.

But I didn't know just HOW weak my core--the pelvic girdle is--until Bob Esquerre showed me a few core training moves that had me crying "uncle" after a few repetitions.

"Core training is really not new, "Esquerre, 51, explained. "The Chinese said the core held chi, the energy center. The ancient Greeks believed that the seat of all emotion was in the core."

Esquerre has a master's degree in exercise physiology and is known as "the trainer's trainer." He's also nicknamed "Satan" because his training sessions take you to the edge and make you realize that true fitness must start with the internal muscles first, before extending outward.

"Core training is about strengthening not only the abdominal area but the muscles around the sacrum and up the spine," Esquerre says. "I've seen world class athletes burn out because they neglected to emphasize the equation of proximal to distal."

Esquerre, along with other trainers at Michael's Body Scenes use a variety of props including stability balls, medicine balls and Reebok Core boards to help develop the core muscles.

"It's essential to build a strong trunk,' says Lori Sunderman, 34, who says that as a dancer, she's been taught to use her center for balance as well as stability. "We're all feeling the force of gravity and core training helps reverse that pull. We lift up sagging muscles and develop functional fitness.

"Just think of all the day to day movements we do that require strong pelvic muscles," Sunderman says. "Lifting groceries out of the car, diapering babies, vacuuming, doing yard work--all these things are safer and more efficient when you're strong in the center. Plus, you'll avoid many injuries that come from awkward movements. I think it is one of the best anti-aging methods around."

One of Lori's clients, Sandy Lassoff, suffered from lower back problems for months.

"Core training has opened a whole new world for me," she says. "I can sleep at night without pain and my strength has improved 100 percent."

Although core training is so specific that you should have a trainer help you fine tune the movements, Andrea Contant, 37, teaches a Hard Core class Thursdays at noon.

"We use a variety of props to help build strength and flexibility," says Andrea, who credits her own strong core for helping her recuperate after several back surgeries.

"We do a different routine every week," she says, explaining how simple squats become intense when using the stability ball. "You need to suck in your belly button to your spine, lift your pelvic floor upward and tilt it slightly forward to achieve that essential neutral spine."