Rigid washboard abs, one of the most coveted symbols of physical perfection, are out. Core is in. And if you don't know the difference, you could be overlooking one of the best ways to prevent back pain and injury, tone a flabby stomach, improve posture, build better balance and enhance your performance in other sports.

"If the core is stable, any and all movements of the limbs can be stronger and more powerful," said Sara Kooperman, chief executive of SCW Fitness Education, a company that trains fitness instructors.

But for some, including yoga practitioners, the benefits of a strong core run even deeper.

"It's more than just our physical center," renowned yoga instructor Shiva Rea said in the introduction of her "Creative Core Abs" DVD. "It's our inner fire which connects all aspects of ourselves: physical, mental and spiritual."

The body recruits four separate abdominal muscles to support and help move the torso, but most people tend to work just one: the visible and superficial rectus abdominis, better known as the "six-pack" muscle.

The core, on the other hand, is a mostly invisible system of deep muscles, nerves and connective tissue, said Cynthia Neville, director of women's health rehabilitation at the Rehabilitation Institute of Chicago.

The core muscles, which must work together as a team, include the pelvic floor, the transverse abdominis (the ab muscle that acts as a muscular girdle for the organs), the deepest layers of back muscles and the diaphragm.

It addition to linking together the upper and lower body, the core is the foundation for all other athletic motion. If you want to throw a ball, for example, the core should stabilize the pelvis and trunk first, Neville said. The arm uses this support base to generate throwing force. It was this principle that made Nolan Ryan one of the most feared pitchers of all time. It's also made Tiger Woods perhaps the greatest golfer of all time.

If the core muscles aren't the first ones to contract and prepare the body for movement — the result of inactivity from, say, sitting hunched over a computer all day — they need to be retrained.

"When core muscles are strong and toned, they work together to support your spine and to stabilize and preserve your posture, which reduces your risk for back injuries," said wellness expert Jorge Cruise, author of "The 12-Second Sequence" (Crown, $25.95).

When they're weak — a common occurrence after childbirth — it can put you at risk for spinal injuries, back pain, poor posture, urinary incontinence and breathing disorders, Neville said.

In fact, if the core muscles aren't firing right, abdominal work could make matters worse by giving you a protruding belly instead of a flat, sleek tummy.

"This is often the combined result of poor posture, incorrectly performing abdominal crunches and neglecting to train your transverse abdominis, the flat sheet of muscle lying beneath your six-pack muscle," said Anita Bean, author of "Fab Abs" (McGraw Hill, $10.95).

To fix this, Bean recommends gently drawing your belly button toward your spine during all abdominal exercises, flattening rather than crunching your midsection. Specific exercises that target the transverse abdominis, such as the plank, pelvic tilt and ab exercises on an exercise ball, also can help.

"Even when you're not exercising — when you're sitting, standing or walking — try to keep your hips 'neutral' by gently tightening your transverse abdominis muscle," she said.

Most people, however, resist doing core work, in part because they mistakenly think it means endless sit-ups. It's considered grunt work — something that has to be done — and motivation wanes when you can't see the muscle you're working.

Meanwhile, the popular notion that abs should be rock hard can be discouraging, especially because diet is a factor; saturated fats and simple and processed sugars help create love handles.

Classes in pilates or yoga are all effective ways to start core work.

The American Council on Exercise calls the "bicycle maneuver" the most effective abdominal exercise, while fitness equipment that creates instability, including Dyna discs, physio or fit balls and the Bosu ball, also can be effective tools.

CHALLENGE YOUR CORE WITH THIS YOGA MOVE

Forrest Yoga, a style of yoga that promises to help you connect to your core, is known for its focus on the abdominal muscles and for holding poses for extended periods.

The following move — elbow to knee — is a foundation of the practice:

1. Lie on your back and clasp your hands behind your head. Bend your legs, lift your feet and position your knees over your hips. Your feet should be off the floor and slightly lower then the knees.

2. Inhale as you lift your shoulder blades off the floor.

3. Holding your breath, press your low/mid back into the floor and lift your tailbone.

4. Exhaling, reach both elbows toward your left thigh and straighten the right leg. (Both shoulder blades and the low back stay off the floor. The tailbone stays up.) Pull your belly in at the end of the exhale.

5. Inhaling, move the torso back to center and the knees back together. Your head and shoulder blades stay up. Hold your breath.

6. Repeat the movement on the other side. Do between five and eight repetitions, emphasizing good form.