very day, in New York and Paris, Tokyo and Houston, students clad in little more than swimwear grab towels, bottled water and rubber mats and enter a very hot room. As the teacher calls out instructions, they sweat profusely, performing a sequence of 26 yoga postures, repeated in every 90-minute class.

Bikram or "hot" yoga took root in Los Angeles three decades ago, but the technique has spread far beyond coastal cool. The Bikram Yoga College of India in Los Angeles, named for its founder, Bikram (pronounced BEEK-rum) Choudhury, has 314 certified schools worldwide, with 12 studios in the New York area.

As more and more people take up Bikram to lose pounds and gain strength, however, medical professionals are expressing concerns about the demands of yoga contortions performed in extreme heat.

"Heat increases one's metabolic rate, and by warming you up, it allows you to stretch more," said Dr. Robert Gotlin, director of orthopedic and sports rehabilitation at the Beth Israel Medical Center in Manhattan. "But once you stretch a muscle beyond 20 or 25 percent of its resting length, you begin to damage a muscle."

Each week, Dr. Gotlin said, he sees as many as five yoga-related injuries to the knees or the lower back. Postures that require extreme bending of the knees — squats and sitting backward on folded legs, for example — are the most likely to cause tears in knee cartilage. In Bikram yoga, students practice the "toe stand pose," a single-legged squat and the "fixed firm pose," sitting backward with bent knees.

"Basically, the knee is a piece of bone with two strings of muscle on the top and bottom, and you can only tighten those strings so much," Dr. Gotlin said. "The more you flex the knee under load, the more pressure is exerted on the kneecap."

Bikram advocates maintain that the immediate warmth and simple movements at the start of each class are safer than traditional yoga.

"The heat helps people work slowly and safely into the postures and makes injuries infrequent," said Jennifer Lobo, an owner of Bikram Yoga NYC.

But David Bauer, a physical therapist in New York who also teaches yoga, said the enthusiasm and competition among participants could contribute to injuries.

"When you are in a hot studio filled with hard-core Type A personalities, and everyone's adrenaline and endorphins are pumping, you're not feeling any pain," he said, "and it may mask how far you can go."

The mirrored walls in Bikram studios may encourage students to concentrate on outward form, Mr. Bauer said. In contrast, more traditional yoga emphasizes an inward focus on breathing and individual limitations, possibly helping to curb injuries.

"Learning where your body is and what your body can do is what yoga is about, not reaching for an ideal or modeling yourself after a picture in a book," Mr. Bauer said. "If you are just flexible and not strong, at the end of your range you are going to tear a muscle."

Indeed, part of the Bikram yoga philosophy is the push to go a little farther every time a posture is performed. Each pose is done two times per class. Participants arch backward and bend to the side in "the half-moon pose," for example, and then do the movement again, trying to bend the spine even more.

Practitioners maintain that the spinal flexibility and strength cultivated in Bikram yoga can be vital in warding off the effect of aging on posture. Some physical therapists, however, question the value of excessive joint flexibility, saying it can lead to inflammation and pain.

"The extreme range of motion yoga develops does not necessarily have an advantage, and it may be counterproductive," said Dr. Shirley Sahrmann, a professor of physical therapy at the Washington University School of Medicine in St. Louis.

Like dancers, practitioners of yoga cultivate overly flexible spines, which often cause problems in resting posture.

"In my business," Dr. Sahrmann said, "I have more problems with people who have excessive mobility than limited mobility."

The thigh socket, or ball-and-socket joint, at the top of the leg is another overworked joint in yoga. Bikram's "tree pose" requires standing on one leg and drawing the opposite foot to the top of the thigh. The point is to rotate the joint of the drawn-up leg outward as far as possible; but what looks good may not be what is best for the body.

"More is not always better when it comes to joints," said Lee Staebler, a licensed physical therapist on the North Fork of Long Island, who is studying movement impairment syndromes at the State University of New York at Stony Brook.

"Warmer tissues will yield more easily, but stretching beyond optimal limits can compromise joint tissue," Mr. Staebler said.

Ligaments, tough bands of fibrous tissue that connect bones or cartilage at a joint, do not regain their shape once they are stretched out, Mr. Staebler said. A loose joint can be like a loose door hinge that prevents the door from closing tightly.

Still, warnings about torn cartilage or painful wobbly joints are unlikely to keep Bikram devotees out of the saunalike studio they claim to find as pleasant as the beach.

"People either cringe when you describe the heat, or they come and get addicted to it," said Christina Ha, a New York television reporter who first took up Bikram three years ago. On her doctor's advice, Ms. Ha has now stopped doing Birkam because she is pregnant.

Physicians caution that exercising in heat 2 to 7 degrees above the body's core temperature of 98.6 can be dangerous.

Dr. Nieca Goldberg, chief of women's cardiac care at Lenox Hill Hospital in New York, said that because of the stress that extreme heat places on the heart through the demand for increased circulation, people with medical disorders should not do Bikram yoga.

"If you smoke, are overweight or have high blood pressure, this is not the exercise for you," she said.

Some practitioners of Bikram report dizziness, nausea, muscle weakness and cramping. Dehydration is the most probable cause, said Dr. Catherine Compito, an orthopedic surgeon specializing in sports medicine at New York-Presbyterian Hospital.

In extreme cases, losing electrolytes through perspiration can cause cardiac arrhythmia.

"Your body can only tolerate so much fluid loss," Dr. Compito said. She added that in high heat, the normal mechanisms for restoring the body's optimal core temperature cannot function. Evaporation cannot cool the skin. Cool air currents cannot move the hot air away from the body.

Over time, Dr. Compito said, adherents of hot yoga may be able to condition their bodies to work out safely in the heat, but she questioned whether the practice offered any advantages over other types of exercise. For stalwart Bikram devotees, however, she recommended drinking more water than the single bottle most take to class.

"Drinking before, during and after is really the way to go here," Dr. Compito said.

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