“Now, you are pulling your heel from the outside. Now, you are locking that knee. Lock it! Pull. Stretch. Shoulders back! Stretch! Hips forward! Pull. Pull! Pull! PULL!” I felt a drop of moisture race down my cheek as a man in a mini-Speedo gently adjusted me at the hips; I wasn’t sure whether it was a tear or sweat, but either way, I was embarrassed. My head throbbed. The room spun. I desperately patted the ground around me for my bottle of water.

Welcome to yoga class.

To some, this would be torture. But to the men and women who pulled, stretched, grunted, and sweated alongside me — all of whom had paid hundreds of dollars to be there — it’s one of life’s pleasures. The class was part of a two-day seminar on Bikram yoga at the Kripalu Center for Yoga and Health, the country’s largest yoga retreat, perched above Lenox, Massachusetts, in the Berkshires. Bikram, the specifically steamy brand of yoga I sweated through, is just one among seemingly endless varieties. Some focus on meditation and spirituality, others on relaxation; some concentrate on holding poses (called asanas) for long periods of time; some run through poses quickly. All require a mixture of balance, breath, stretching, and mindfulness — an awareness of your body in space.

Celebrities, rock stars, and senior citizens alike are flocking to Bikram yoga. But the same thing that draws the crowds incites the critics, who call the practice dangerous, the founder ridiculous, and the growing interest evidence that it’s simply the latest athletic fad — this year’s Tae-Bo. Because it lacks an overt spiritual element, Bikram takes heat from some within the larger yoga community, who question whether the extreme workout can even be classified as yoga. More important, critics are concerned that Bikram instructors are being churned out too quickly and without proper training, heightening the risk of dangerous injury.

But for devotees, Bikram yoga is a cure-all that inspires cultlike devotion. “This has changed my life,” says Julie Mowschenson, who ran up to talk to me after class. A small-framed, blond 41-year-old, she is a somewhat surprising candidate for the vigorous discipline, because of both her age and size. Nevertheless, she’s hooked. “I haven’t been sick in a year, my PMS is much better, I sleep better, I’m calmer, I’m happier,” exults the Brookline nurse. “I’m in the best shape I’ve ever been in. I love it.”

WHATEVER THE strain, yoga is becoming the trendiest of activities, and — as the New York Times Styles section recently noted — the latest arena in which type-A personalities are rushing to compete. According to polls conducted by Yoga Journal, 18.5 million Americans practiced yoga regularly in 1998, up from 6.2 million just four years before. And those figures don’t even reflect the remarkable growth in the past three years. “When I started a year ago, there were 10 or 12 people in my class,” Mowschenson says of the Bikram class she attends three times a week in Roxbury. “Now, it’s unbelievable. It’s like sardines. You have to get there an hour early just to get your spot.”

But even in the world of type-A yoga-goers, Bikram is hard core. The trendy practice is named after Bikram Choudhury, a self-titled yogi from Beverly Hills via Calcutta, who emphatically claims that doing his — and only his — type of yoga five times a week will lessen the symptoms of chronic illnesses. Jokingly dubbed “Rambo Yoga” or “Bikram’s Torture Chamber,” it is definitely the most aggressive and prescriptive of the various forms. Always done in a room heated to a piping 100 degrees, Bikram yoga moves through 26 poses, each repeated twice — every time in the same order. Completing that routine, its founder promises, will cure ailments as varied as varicose veins, diabetes, and depression. “I will treat you from the bone to the skin, from the head to the toes,” Bikram said multiple times during our phone conversation. “This is a preventative medicine.” And according to Bikram, the heat is a critical component because it allows increased flexibility. “If you took a piece of steel to a blacksmith and asked him to make a sword, he’d heat it up,” he explains.

Whether or not the analogy makes sense to all, a dictatorial environment with shouting instructors, speaker systems, and imperative commands doesn’t leave much room for skeptics. At the Kripalu retreat, the barking woman was none other than Bikram’s wife, Rajashree Choudhury, who as a teenager in Calcutta was the five-time all-India yoga champion. These days, in Beverly Hills, she and her husband preach more than they practice. And people are lining up to listen. Bikram is something of a yoga guru to the stars, counting among his regular clients Michael Jackson, Barbra Streisand, and Candice Bergen. He says he even cured former president Richard Nixon of knee trouble in the early 1970s. “I baked him like a San Francisco Chinatown Peking duck,” he giggles.

The Choudhurys opened a school in 1974, the Yoga College of India, where they currently lead two-month-long instructor-training sessions three times a year — for $5000 per student. The last round graduated about 200 fresh new Bikram instructors, with specific directions to go forth and conquer, and open their own Bikram Yoga College of India studios, with financial assistance if needed. “All these people are graduating and going to their town and opening schools,” Bikram says. “They are making between $30,000 and $50,000 within a month.” The next instructor-training session, which starts in April, is nearly full, with 300 people already registered.

Beyond the classes, there’s a book, a video, and audio tapes. In fact, Bikram is very much a brand name. “We really want everyone to speak the same language,” notes Rajashree. And that language is spreading: today, there are more than 120 Bikram-yoga studios in the United States, and 10 internationally. Boston alone boasts three separate studios. And the weekend-long seminar at Kripalu was so overbooked there was a four-page waiting list of people pining to attend.

The self-proclaimed yogi certainly doesn’t downplay the financial success his spiritual practice brings (though his visible appreciation for Rolls Royces and Rolexes makes him an easy target for sneering). And he doesn’t mind being thought of as a guru, either. When asked how he wants to be identified, Bikram responded: “You are asking me to be humble. I don’t know. I was born with supernatural power. I see things. When I see you I will know you more than God knows you, more than your mother knows you. And I’m never wrong once in my life. What do you call that?” He finished with a suggestion. “God-gifted healer?”

When Bikram’s name came up at the Kripalu seminar, people’s eyes sparkled, and the many Bikram instructors in attendance threw around personal anecdotes. “He’s incredible,” sighed Sydney Saunders, who runs a Bikram studio with Jonathan Burbank on Lincoln Street in Boston. “If someone doesn’t get along with him, it’s because of them.”

THE DAY after my bout with Bikram’s torture chamber, I was in bed with a raging headache and the chills. My body hurt more than it had after I’d finished running a marathon a few months before.

The Bikram people warned me this would happen. “That’s completely normal,” said Saunders, over lunch in between sessions, when I mentioned other people’s anecdotes of vomiting, passing out, and general flu-like symptoms. Bikram tells me that Kareem Abdul-Jabbar and Dan Marino were similarly floored by the experience. “They collapse in my class in 10 minutes. They get up, go out, and throw up. They find out they’re nothing but a piece of junk,” he says, with no small amount of pride.

Some medical professionals are visibly concerned that those “normal” aftereffects are dangerous signs of dehydration brought on by unnecessarily excessive heat exposure. “Throbbing headache, dizziness, nausea, lightheadedness — that means you’re not getting enough blood flow to the brain,” says Lawrence Armstrong, an associate professor of exercise and environmental physiology at the University of Connecticut, who specializes in heat illnesses. “That means you should stop.”

And despite Bikram’s assertion that the heat is necessary to “bake” muscles, ligaments, and tendons internally, Armstrong says that’s bunk: “You could conduct yoga in a 100-degree room or a 70-degree room and get the same gains. I’m skeptical that a hot air temperature is going to make the muscles, ligaments, and tendons warm. The body maintains its temperature at 98.6 degrees, and to warm muscles and ligaments takes a lengthy heating session or exercise which produces internal heat and increased blood flow. If a body maintains 98.6 degrees — which it does by sweating — the muscles are not warmer.”

Among the exhaustive list of diseases Bikram claims to have cured is multiple sclerosis (MS), a degenerative neurological disorder. But Judy Rosenbaum, the program manager at the Central New England chapter of the National MS Society, says that without question, heating up the body — especially as high as 100 degrees — is one of the worst things you can do for patients. Although other forms of yoga have been shown to lessen the severity of MS symptoms, she cautions that heat often brings on serious reactions. “With MS patients, you need to emphasize cooling down,” she says. “A significant number of people with MS have trouble with heat.”

And though most people cite weight loss as their favorite result of Bikram yoga, Armstrong says that any dips on the scale are simply the result of water loss from sweating; the weight is likely to come back with the next meal.

Bikram and Rajashree are seeking a grant to do medical research on diseases so that they can eventually prescribe poses for specific conditions. But Todd Jones, an editor at Yoga Journal, questions whether such prescriptiveness — what he calls the “take three asanas and call me in the morning” approach — is really the purpose of yoga. “The important thing to remember is that yoga is holistic,” he says, “and any holistic approach can help heal, not necessarily cure.”

WITH HIS successful school, videos, tapes, and packed classes, Bikram is clearly delivering a welcome message to the masses. But he’s not the only one cashing in on a blend of Western athleticism and Eastern practice. Baron Baptiste’s Power Yoga Institute, in Cambridge’s Porter Square, also features a curious mix of high-octane physicality and pseudo-spirituality. And Baptiste claims he can make a yoga teacher out of you in just eight days, at his “boot camp” in the Yucatan.

At a recent class at the Porter Square studio, a crew of hyper-fit, Patagonia-clad twenty- and thirtysomething beautiful people began lining up in the stairwell a half-hour before class. A few minutes before 11 a.m., we raced into the humid, unbearably hot room to the pumping tunes of Van Morrison and ethno-techno. By the time everyone had claimed a spot, there was less than a foot separating each half-clad person from his or her neighbor. At one point during the 90-minute session, our intimidatingly beefy instructor, Rolf — who’s rumored to have been a drill instructor before his yoga days — yelled at us to “awaken our souls.” He ordered us to belt out a series of full-throated, high-decibel ommms, and he laughed in glee as a woman let out an exhausted, defeated “Jesus!” Then he asked, “How do you want your abs: rare, medium, well-done?” In response, the 70 or so glistening people chanted, “Well-done, well-done!” I couldn’t decide whether to laugh or be scared.

The Power Yoga class certainly tried to weave in some spirituality with the sweat. A Hopi Indian reading gave us a momentary rest from the grueling, fast-paced race through basic poses. We finished off with our hands in praying position, calling out a vigorous “Namaste!” — whatever that means. And there were those ommms. But it was difficult to take the spiritual elements seriously — especially as I mindfully wiped my hairy neighbor’s sweat off my thigh.

SPIRITUAL OR not, this type of yoga has a certain athletic appeal. When you walk out of class dripping, it’s hard not to feel that you’ve done something. So it’s not so surprising that the fitness-crazed folks at both Bikram’s and Baptiste’s classes look as if they’d be equally at home scaling a rock-climbing wall, mounting a bike in a Spinning class, or taking someone out in kickboxing. Radical yoga is the fitting next step for a population of people hungry for the newest extreme sport — and looking for a little spirituality while they’re at it.

But many yoga insiders question whether these classes can even be classified as yoga. “It’s a very modern take on yoga,” sneers Wendy Green, a 30-year yoga veteran who teaches in New York and consults with corporations on how to bring yoga into the workplace. “It’s the antithesis of yoga,” she repeats a few times. “I would say that the workout should be the byproduct of yoga. I don’t think you go to yoga for the workout.”

Jones, the Yoga Journal editor, confirms that “a lot of people are coming to it as an adjunct to a fitness program.” But he warns that “yoga is like any other challenging and demanding physical discipline.... Most people who have practiced power yoga for a long time have had injuries because they’ve pushed over instead of to the edge.”

Professional yoga practitioners stress that no one type is best for everyone. “What’s appealing might manifest itself one way in your 20s and one way in your 50s,” says Judith Lasater, a yoga instructor of 30 years who is a founder of Yoga Journal and the author of the forthcoming Living Your Yoga: Finding the Spiritual in Everyday Life (Rodmell Press). “I encourage people to look around and find something that soothes them and meets their needs.”

Bikram’s devotees aren’t so interested in soothing, though. They advise me not only to push myself, but to come back for more immediately. “We encourage people to come back the next day,” says Boston Bikram instructor Saunders. “It’s hard, but you’ll feel better.”

Sweaty and sore, thirsty and tired, I found that hard to believe. But as I looked at the lines snaking out the studios’ doors, surveyed smiles in the cramped classes, and listened to the fervent personal anecdotes, one thing became amply clear: something in Bikram’s recipe — an essence perhaps more hot than sweet — compels enthusiasts to eat it up. What that is, we’re still trying to figure out.

Nina Willdorf can be reached at nwilldorf[a]phx.com.