At some point this year, I noticed that I could not bend over and touch my fingertips to the ground. The combination of running the marathon, completing a century (riding 100 miles on a bicycle) and the subsequent nursing of sore knees, tight quads and stiff hips had taken its toll: what little flexibility I was born with had vanished.

Last year's New Year's resolution to cut back on running and spend more time stretching went the way of most New Year's resolutions (ignored until forgotten), and the thoughts of struggling through an organized stretch class filled with limber-bodied budding ballerineas was too intimidating to consider.

So I did what any cardio-addicted athlete would do who needs to sweat to feel she has worked out: I signed up for the hottest exercise class in town. Hottest literally, because Bikram yoga (less formally known as hot yoga) is conducted in a room heated to 100 degrees.

During the 90-minute class at the Yoga College of India, where I practiced 26 Hatha yoga poses, including the tree, the mountain and the triangle, I still managed to be the least limber in class. But who cares when you are touching your nose to your knees (and wiping off the sweat pouring down your face) for the first time in your life?

Hot yoga, a strenuous version of the suddenly trendy 5,000-year-old discipline based on breathing and stretching techniques, was developed in the early 1970's by Bikram Choudhury as a way to heal the body after injury or illness. (Mr. Choudhury had been an Olympic weight lifter until he suffered a career-ending injury.) The yoga poses he chose were designed to enhance strength, flexibility and balance, a combination, he said in an interview from Beverly Hills, Calif., that is "essential in reducing the negative aspects of stress, illness, injury and aging on the body."

The first studio, Bikram's Yoga College of India, opened in Beverly Hills in 1973, and celebrities like Madonna, Michael Jackson and Robert Downey Jr. have all come to sweat. Today, gyms around the country are turning up the heat and turning themselves into hot yoga studios faster than they can say om. There are more than 50 Bikram yoga "colleges" in the United States, and some 500 Bikram-certified instructors. In the New York City area, several hot yoga studios have opened in the last year, including the Bikram Yoga College on Eight Avenue between 48th and 49th streets.

What sets this style of yoga apart from others is, of course, the heat. Warmed-up muscles make the body more pliable, enabling a deeper stretch. "The heat speeds the blood supply into the muscles, tendons and ligaments," explained Toni Goodrich, owner of the five-year-old Greenwich Yoga Center in Connecticut and former student of Mr. Choudhury in Los Angeles.

This is no class for wimps. Even without the heat, Bikram yoga is challenging. Holding 26 different body positions for 10 to 60 seconds at a time involves strength, stamina and intense concentration. "The first class you take, you're going to feel like throwing up," said a fellow student, Christina Magliocco von Oist. "But soon, you get used to the heat, and the yoga gets easier the more you do it."

She was right on both counts. The first time I took hot yoga, I did feel ill!--not from the class, which had not even started, but from the smell that hit me when I stepped out of the elevator into the fourth-floor reception area. Think of a locker room that has never been cleaned or a musty summer house that has never been aired. Then put the locker room inside the summer house.

Odor aside, the class, taught by a co-owner, Donna Rubin, was just what I had hoped: not so difficult that I got discouraged but hard enough that the athlete in me felt challenged until the very last minute, when we lay on our backs and rested.

Throughout the 90 minutes, Ms. Rubin carefully instructed us, demonstrating each move. If we could not perfect the initial pose, for example, if our standing leg was not perfectly straight for the "standing head to knee" pose, she would not let us move on to a more advanced position.

"There's always a modified version, should a beginner have difficulty," she explained after the class. "It's important to go at your own pace. As your body improves, the posture will improve. Go to where you feel the stretch, but don't push beyond your capability, and don't compare yourself to anyone around you. That way, you'll never get injured."

The poses she led me through were simple, a sort of yoga for dummies. In the "awkward," or chair pose, for instance, I squatted as if sitting in a chair and held that position for 20 seconds. Then I took a brief break before practicing it one more time.

As the 90 minutes progressed, I grew more and more fatigued, probably from the Calcutta conditions: you start sweating profusely the minute you walk into the enclosed room with its three whirring wall heaters. I wanted to yank open one of the windows facing the street.

I became irritable after one particularly long-held pose in which my sweat-drenched kept sliding off my sweat-drenched thigh. A washcloth, handed out at the beginning of class to grasp slippery body parts, was drenched and useless in minutes, and the beet-red color of several classmates' faces was alarming. But Ms. Rubin kept encouraging me throughout.

"I promise," she said, "the first time is the hardest. The next time you come, you won't even notice the heat." She also reminded us to drink water between the poses and to leave the room if we felt too hot. By the end of the class I felt as if I had run a half-marathon in mid-August. That night I slept like a rock, every inch of me stretched, strengthened, and thoroughly fatigued.

Can exercising in man-made heat be dangerous? "Probably not," said Dr. Melvin Williams, an exercise physiologist at Old Dominion University and the author of "Lifetime Fitness and Wellness, (William C. Brown, 1992). "When muscles get warm, they're more pliable and can be stretched more easily, which is why you stretch better after a run than before. In heat, there's probably less chance of an injury."

And Marika Molnar, a physical therapist with the New York City Ballet, said: "You could make the argument that all yoga and stretching should be conducted in heated rooms. When you're stretching muscles to any extreme range of motion, it would be nice if the heat of your body and the heat of the room were similar. The heat allows the muscles and connective tissues to relax and resist the tension of the cold. Dancers always tell me how much more range of motion they have in the summer, because studios aren't air-conditioned."

Could I get hurt if the heat is artificially creating flexibility where there isn't any? "You can't fabricate flexibility," said Ms. Goodrich. "Each person has a range of motion that they begin with and that they can attain. The heat just aids the process."

"Bikram yoga is not about being a noodle," she emphasized. "It's about being strong." And after a hot yoga class, very sweaty, too.

Repeat the entire pose to the opposite side.
(Source: Bikram's Beginning Yoga Class, by Bikram Choudhury)