It sounds like a yoga class. "And, forward fold. Lift the arms to the sky. Take three breaths. . . . Lift, bend and extend. Now downward dog."

Except a disco beat is playing.

It looks like a yoga class. Thirty students move in slow motion as they kneel on foam mats in a dimly lit ballroom.

Except they are each squeezing a grapefruit-size blue rubber ball between their thighs.

What we have here is not yoga but YogaButt. And it's just one of many yoga classes-with-a-twist at the recent DCAC 2004 International Fitness and Personal Trainer Conference in Reston, where 1,200 fitness instructors and exercise enthusiasts have gathered to flex their ideas as much as their muscles.

Inside another ballroom, more than 100 students lie on yoga mats, legs propped on 29-inch silver exercise balls. This is Louisville instructor Lauren Eirk's Yoga-Pilates-Resist-a-Ball session.

There's also YogaBar for weightlifters; Water Art Yo-Tai Pilates, combining submerged yoga with tai chi and Pilates; Hot Yoga, done in a 105-degree room; and Body Bar Buddha Bar, described as body sculpting meets Cirque du Soleil.

Ripped biceps and steely abs are almost passe at this body-conscious convention. The buzz is about the growing array of yoga-inspired workouts that are reincarnating the ancient Hindu discipline into the rage of the fitness world.

What used to be the domain of the granola-and-Birkenstock fringe has turned into a hypercommercialized industry for the masses. The lotus position has given way to an explosion of fusion classes that hyphenate yoga with every imaginable exercise and body part. The rolled-up mat of the old days has morphed into a multibillion-dollar market of clothing lines, books, videos, music, lessons, props and accessories. It's yogis gone wild at the gym.

Traditional yoga just never went mainstream, says Beth Shaw, inventor of YogaButt -- and YogaAbs, YogaBack, YogaStrength, pre- and postnatal yoga, yoga for seniors and yoga for kids. "It's off-putting to a lot of people, it's strange, it's weird, a lot of people can't grasp it. And, quite frankly, not too many people want to sit around on a floor and meditate and do one pose and then rest for five minutes and then do another pose. That's why we've invented things that are fun."

Down the hallway, Lawrence Biscontini leads 75 students in his Yo-Chi Glow session in a dark ballroom. They wear glow wristlets so yoga positions blend with sweeping tai chi movements to become a sinuous light show.

"The blend justifies the means," says Biscontini, fitness director at the Golden Door spa in San Juan, Puerto Rico, who also is introducing Yo-Cycle, Yog-Opera and Yo-Step at the conference. "I call it cafeteria fitness."

"Fitness" and "yoga" were rarely mentioned in the same breath in 1893 when Calcutta-born yoga scholar Swami Vivekananda addressed a world religions conference in Chicago and yoga gained its first following in America.

Other masters from India visited over the ensuing century. But yoga never grew in popularity until the Vietnam War era, when the counterculture of the '60s embraced Eastern influences, from Beat writer Jack Kerouac's "The Dharma Bums" to the Beatles' flirtation with Maharishi Mahesh Yogi's Transcendental Meditation. Suddenly, yin-and-yang symbols were everywhere, sitar music filled the air and yoga was cool among the rebellious.

The Establishment linked yoga with hippie ashrams and guru worship, however, and yoga couldn't keep pace with another revolution picking up speed -- the Fitness Revolution, says Harvey Lauer, president of American Sports Data, a New York research firm.

Fitness evolved from running in the '70s, to high-impact aerobics in the '80s, to low-impact aerobics and walking in the '90s. As hippies aged and transformed into workaholic, overstressed yuppies seeking self-improvement, yoga started making new inroads.

By the new millennium, a "new, kinder and gentler world of physical fitness" was emphasizing stretching, flexibility, balance and relaxation, says Lauer. Mind-body practices such as tai chi, Pilates and yoga fit the bill.

Next, yoga went Hollywood. In 1998, Madonna released the CD "Ray of Light" with a Sanskrit chant, touting her devotion to yoga. Two years ago, supermodel Christy Turlington appeared in Vogue to introduce her sexy line of yoga clothing. Yoga became a regular mention in celebrity interviews, from Gwyneth Paltrow to Metallica.

By last summer, 15 million Americans were practicing yoga, 28.5 percent more than the year before, according to a Harris poll conducted for Yoga Journal.

In 1998, Lauer says, the number was just 5.7 million.

With the masses comes big business, of course -- trendy yoga clothing with labels such as L.L. Bean, Old Navy, Nordstrom, Land's End and the Gap, meditative music, books and videos. Yoga researcher Trisha Lamb, associate director of the International Association of Yoga Therapists, in Manton, Calif., estimates that people are spending $20 billion annually on yoga.

YogaFit, the Redondo Beach, Calif., company that invented YogaButt, began in 1994 with Shaw selling T-shirts and books out of her car trunk. Now it's a multimillion-dollar corporation selling dozens of YogaFit products, says chief financial officer Amy McDowell. It has trained 50,000 instructors, licensed studios throughout North America and this summer signed a contract to open 100 studios in Japan.

Basically, yogafuture looks bright.

At Gaiam, one of the nation's biggest makers of yoga products, sales have grown 41 percent over five years -- despite the entry of giants such as Nike and Reebok into the market. From 1998 to 2003, the number of retail locations that carry Gaiam's products jumped from 5,000 to 30,000, according to the Broomfield, Colo., company. Over the past three years, Gaiam sold 1.6 million yoga mats priced at $20 to $30. "That math works out to $30 million to $50 million in mats alone," says marketing director Byron Freney.

According to Barnes & Noble, five or six new yoga books are published practically every month. "Yoga is such a huge category in terms of people's interest," says editor Stephanie Tade at Rodale Press, which last year paid a seven-figure advance for world rights to yoga master B.K.S. Iyengar's new book, "Light on Life" (due out in October 2005).

"Yoga for the Rest of Us" -- targeting the out-of-shape and the elderly -- ranked No. 2 recently on Amazon.com's daily top-selling video list.

But if you still need evidence that yoga has struck a nerve in Middle America: Wal-Mart and Target now carry hefty lines of instructional videos, books and paraphernalia. Wal-Mart's Web site boasts 990 yoga products; Target's has a mind-numbing 4,235.

"Consumer demand for yoga has increased dramatically," says Dayna Macy, communications director at Yoga Journal, the Berkeley, Calif.-based magazine that has covered yoga for 25 years.

Since 1998, Yoga Journal's circulation has more than tripled, from 90,000 to 310,000. In the past year, the magazine's national advertising has increased 35 percent -- including new advertisers such as Target, Kellogg, Ford, Johnson & Johnson, Pfizer and General Mills.

"When you start getting advertisers like that in a yoga publication, there's a reason for it," says Macy. "Yoga is mainstream and they want to reach the demographics of our reader -- female, twenties, thirties and forties, high median household income."

Yoga intersected with Madison Avenue when it expanded from the incense-scented studios to the sweat-scented gyms, around 2000.

Last year, 2.2 million Americans were practicing yoga at commercial health clubs, up from 400,000 in 1998, says Bill Howland, director of public relations and research at the International Health, Racquet & Sportsclub Association, a trade group in Boston. Eighty percent of clubs now offer yoga classes -- twice that of six years ago.

"As little as three years ago, yoga was a very small part of the group exercise market," says Suzanne Olson, a Philadelphia fitness trainer whose company, DCAC, has produced the fitness conference in Reston for 13 years. "Now all the clubs have mind-body programs and they're much larger than other types of group exercise programs like step aerobics or kick-boxing. Now there's a yoga-Pilates studio opening up on every corner."

Yoga is on the schedules at D.C.-area fitness centers in one form or another. Members pay their monthly dues and take as many classes as they want -- a cheaper alternative to the typical $15 per class that yoga studios charge. Fitness First's 14 centers offer 18 types of yoga classes, from traditional beginner yoga to fusion classes such as yogilates. Gold's Gym, which made its name on serious weight training, offers something called Body Flow at its two dozen locations.

On a recent Friday morning, a dozen women move fluidly through a series of positions in Margie Weiss's Body Flow class at the Ballston Gold's Gym. Their average age is about 35.

Borrowing from yoga, tai chi and Pilates, and done to easy-listening pop, Body Flow workouts promise to increase strength, endurance and flexibility while reducing stress. In the first four days of August, says Weiss, 444 people took Body Flow classes at the Ballston gym.

"This Body Flow thing is awesome, you just kind of relax and do your thing," says Weiss, 55, mother of Olympian figure skater Michael Weiss and a fitness trainer for 30 years.

Christina Moore works out at this gym five days a week, doing step aerobic classes and Body Flow. "I'm a gym rat, but when you get to my age, you find that doing all that stepping and stuff, you really get kind of sore," says Moore, 54.

People like Moore are the reason yoga is going mainstream, experts say. It's the baby boomers returning to the gym for easier, gentler, low-impact exercise and the hope of staying forever young.

"You have someone turning 50 every eight seconds!" says YogaFit's Beth Shaw, who "at not even 40 yet" has stopped running and doing step aerobics. "I don't do things that are going to pound me. Why? Because I'm interested in longevity and maintaining my joints and staying youthful and supple."

Is fitness yoga just yogalite?

Yoga is a philosophy of life, not just a sweaty workout, some traditional yoga practitioners say.

Para Darin Somma, who teaches a traditional Vinyasa yoga at Capitol Hill Yoga and at 18th and Yoga , is concerned about the "shallowness" of commercialized yoga. He has seen yoga magazines with articles about yoga and sex, yoga and washboard stomachs, yoga and Madonna, he laments. "They had nothing about the deeper study of yoga."

That deeper study is "self-realization," says Yoga Journal's Macy, suggesting that six-pack abs probably don't qualify as spiritual.

Traditional yoga emphasizes relaxation and restoration of the spirit, says Bob Patrick, president of the Mid-Atlantic Yoga Association, in Silver Spring: "This requires that the practice include elements of stillness, easy, relaxed breathing, and attention to what the body is actually doing. . . . This would be difficult to achieve in a program that is exclusively a workout."

Another concern is whether fitness instructors-turned-yoga teachers are qualified. "Are they educated in the yoga tradition or are they just educated in fitness and trying to make yoga fit into their fitness world?" asks Hansa Knox, president of Yoga Alliance, a Reading, Pa.-based organization whose mission is to make yoga teaching a certified profession.

Yoga Alliance doesn't certify YogaFit's weekend-trained instructors who study 18 hours, but it does YogaFit's 200-hour-trained instructors.

At the fitness conference, Richmond yoga teacher Ram Bhagat teaches a class called Soul Yoga "to remind people to connect with the essence of yoga -- to unite the mind, body, spirit and soul." Without that, "people are just getting an appetizer," he says.

Lauren Eirk, the Yoga-Pilates-Resist-a-Ball instructor at the conference, says she has practiced traditional Ashtanga yoga for years, but today: "If I go into a class and say, 'Ardha baddha padma pascimottanasana,' they're going to go 'What?' "

And YogaFit's Shaw, also trained in traditional yoga, has little patience for criticism of the fusion workouts. "Yes, they sneer, they scorn, they snort," she says. "To be honest with you, some of the most rigid people I've seen in my life are yoga people."

Spirituality is an individual thing, she says. "To me one of the most spiritual of experiences is just to be in a state of calm and clarity of the mind. The oneness of spirituality is what we're all looking for, and that we can get through this practice. So is it spiritual? Yes."

In other words, YogaButt, designed to improve the most unenlightened of derrieres, may not be quite what the yoga masters of old India had in mind, but who's to say self-realization can't start with a firm behind?

Margie Weiss's Body Flow class blends yoga, tai chi and Pilates.Yoga never went mainstream, says Beth Shaw, here leading stretching exercises during the DCAC conference. "That's why we've invented things that are fun." Whitney M. Cole, photographed in a mirror, is one of hundreds taking the Body Flow class in Ballston.Shaw's YogaFit company has grown into a multimillion-dollar empire that trains instructors and sells workout videos, DVDs and a line of clothing.