by Steve Cline

No matter what your activity — even standing or sitting with good posture — you use your core muscles. And yet, vital as these muscles are, most of us aren't sure what the term means.

"A lot of people, when they hear the word 'core,' think only in terms of the stomach, the abdominal muscles," says April Swales, a physical trainer at the Cooper Fitness Center in Dallas. "But when you're talking about the core, you can't just isolate one part of the whole system."

Sit-ups alone won't work the 29 muscles in the stomach, back, hips and pelvis that stabilize the spine and make up the "core." That's why more and more health clubs offer core fitness classes that use equipment meant to strengthen these oft-ignored muscles. An American Council on Exercise survey found the number of clubs that offer core-conditioning classes more than doubled in one year to reach 64 percent.

The muscle most people identify with the core is the rectus abdominus at the front of the stomach. People with prominent rectus abdominus muscles are said to have "six-pack abs." Other core muscles in the stomach area include the transverse abdominals, also called "nature's weight belt," and the obliques.Working with the muscles of the back, hips and pelvis, these muscles provide power for your body.

Studies have shown that milliseconds before you flex your arm,muscles in the core act to stabilize the body, according to Corey Stroderd, a Southern California physical therapist. "All movement starts from the core," he says. "It doesn't matter how strong your arms and legs are if you don't have a strong core."

When the core muscles are weak or out of balance, other muscles try to pick up the slack. That can lead to instability, poor posture and injury, chiefly in the lower back.

The exercises in a core fitness class are familiar, but with a twist. The classes add special equipment that makes it easier to work all the core muscle groups. These tools seek to get the core muscles going by creating an unstable environment. For example, doing a sit-up on a stability ball forces many core muscles to make small contractions to keep you level. Proponents say these constant adjustments in all directions work the core more than straight-line movements, such as a sit-up on the floor.

"In everyday life, we don't move in a straight line,'' Mr. Stroderd says. "We are constantly twisting and bending."

Not all equipment that claims to help the core muscles works, though. Be wary of gadgets that claim to make an abdominal workout easy or promise a flatter stomach in 30 days, Ms. Swales says. "The bottom line is if you can't get on the floor and do the exercise properly, then those machines aren't going to help you."

In fact, core training likely won't give you a washboard stomach.Well-defined abdominal muscles show "a low amount of body fat" and not necessarily strength, Mr. Stroderd says.

But core training can do far more for you than give you great abs. A stronger core can advance almost any physical activity — even those as routine as mopping the floor or picking up a child.

"People come in who suffer from back pain or they can't perform normal functions, and their quality of life is diminished," Ms. Swales says. "They really appreciate what core exercise can do for them because they see their body get stronger and move the way it is supposed to move."

Core Fitness Exercise Gear
Stability ball: Physical therapists developed these inflatable, heavy-duty rubber spheres (also called Swiss balls or exercise balls) to aid rehab. You can use varying sizes for exercises done while sitting or lying down.

BOSU ball: This looks like the top third of a stability ball with a solid base. You can use a BOSU (both sides up) ball with the base down or, for a real challenge, with the rounded side down.

Wobble board: This platform has a rubber half-sphere attached to the bottom. It makes standing exercises, or just standing, harder by making the core muscles adjust for balance.

Foam roller: You use these foam tubes (which look like pool toys) to do standing exercises. They can be cut in half to provide more stability.

Dyna disc: These inflatable, heavy-duty rubber discs are thick in the center and thinner at the edges. Used for standing or sitting exercises, they're more tricky than BOSU balls.

Core board: An oval platform with four legs that let it wobble and twist. Used for standing exercises, it provides more stability than a BOSU ball or dyna disc.

INTEGRIS Health 3300 N.W. Expressway, Oklahoma City, OK 73112 (405) 949-3011 | HealthLine: (888) 951-2277