The injured say overcrowded classes and disengaged instructors are to blame, but yoga schools say people need to listen to their bodies and not push themselves beyond their limits. Medical experts say the heat encourages some to go too far.
It's East vs. West, as the traditions of a 4,000-year-old discipline are being co-opted by a fast, competitive culture.
Something has to give, and for some people it's their backs.
"I've been in Boulder for 15 to 16 years, and I hadn't seen yoga injuries until the last three or four," said Mark Plaatjes, a physical therapist, former competitive runner and a partner in the Boulder Running Co. "My clients say, 'I was doing yoga and I was in a pose and I came out of the pose and I was hurting.'"
Plaatjes says most of the yoga injuries he treats -- to the low back, hamstring, knee and sacroiliac joint -- result from people overdoing it in a very hot yoga room.
"In that situation, you're taking your tissue past what it can normally do," he said. "I don't discourage doing yoga. I think it's got lots of good advantages. I just tell people to modify it."
For decades, yoga bubbled along under the surface in this country, its various styles practiced by a devoted minority. In the 1970s and '80s, a few celebrities went to the mat, raising yoga's profile a notch.
In the past six or seven years, yoga has become a trendy exercise regimen, with studios popping up all over suburbia and classes filling up at rec centers and health clubs. The meditative side has been de-emphasized in favor of the physical, and much of that has evolved into more of an aerobic workout than a slow stretch.
The athletic style has endless variations and goes by many names: Power Yoga, Aerobic Yoga, Flowing Yoga, Hot Yoga, Bikram. (Other styles, including Iyengar, are not done in a hot room.)
One of the original types of hot yoga, Bikram is a tightly controlled series of 26 postures developed and marketed by 57-year-old Bikram Choudury, a yoga teacher from India who started his empire in Beverly Hills in the 1970s with a celebrity clientele. He has since expanded to 314 certified schools worldwide.
To a novice, the superheated studios used by Bikram and other "hot yoga" proponents can seem stifling. To an enthusiast, it's a blessing, because as muscles relax, the sweat pours and poses flow more easily.
"The hot room can get you into a position that your muscles have never been in before. If you're not careful, you can get into a range of motion that's too much," said Dr. Venu Akuthota, a physical and rehab physician at Denver's University Hospital, where he is director of the Spine Center and a doctor at the CU Runner's Clinic.
Akuthota is from India, the birthplace of yoga, and practices a more meditative style. He said he has seen "quite a few" patients with yoga injuries.
"Yoga can be therapeutic, but people get competitive with each other, and that plays into it," he said. "It kind of relates to the kind of patients that use yoga. Fit people, baby boomers."
"We see people practice too aggressively all the time," said Marissa McCroskey, a certified Bikram instructor at Yoga on Sixth in Denver.
"We say, 'Don't go there,' and they go there. We constantly tell people, if you feel sharp pain or pressure, please back off. Your body is telling you it is not ready."
But some practitioners lay part of the blame for injuries on poor instruction. Laura Bottaro, 32, an avid runner, strained her hip doing power yoga. "I heard from fellow runners (yoga) was a good way to avoid small injuries. But there was no instruction."
Now Bottaro does Ashtanga yoga, where she says her teachers place great emphasis on individual instruction and placement.
Joelle Coakley of Denver, 35, is a triathlete, adventure racer, telemark ski instructor and certified personal trainer. She did Power Yoga for a while and then strained her back. Now she practices Iyengar, a style that emphasizes alignment and form over power.
"I think the heat is deceiving because people think, 'Oh, my muscles are warm.' I overextended in the heat. In most cases, instructors were not correcting bad form. I saw people just powering through asanas (poses) and burning calories, which I believe is why most people do power yoga."
Radha Garcia, director of Bikram's Yoga College of India in Boulder, agrees that Type A personalities can work at cross-purposes to yoga.
"If we see overachievers in class and people who are muscling themselves into postures, we can work around that by stopping and getting them back to the breath. I've had a few people over the past five years say they got hurt, but they always pushed themselves too hard. They were being too competitive."
Dr. Sonja Stilp, a physician at Orthopedic Associates in Aspen, has done Bikram yoga herself and also has seen an increase in the number of Bikram-related strain injuries.
Stilp said weak but flexible people are more at risk for injury than strong, stiffer people.
"The weak cannot control their movements, or they have poor control," she said. "Sometimes tight muscles are actually protective."
Dr. Deborah Saint-Phard, director of the Women's Sports Medicine program at CU and a team physician for the Buffs, advises balancing flexibility with strength, a primary tenet of yoga philosophy but one that many people ignore.
"If you stretch a muscle out too far, you weaken that muscle and make it more susceptible to strain," she said. "It's possible to be too flexible. Work on strengthening as well."
Right after hot yoga, Saint-Phard recommends avoiding physically demanding activities such as running or lifting weights.