I have always assumed that people just think they're working harder because they sweat more in the heat. Until now there has been no study to confirm or deny this supposition.
However, The American Council on Exercise (ACE), recently released the findings of a small study that states there is no additional benefit from a hot yoga class.
Their Chief Science Officer, Dr. Cedric Bryant, said "as far as physical benefits, including muscular strength, endurance, flexibility and balance, you can get those from a standard yoga class. An increase in core temperature would suggest the person is storing heat, and depending on how high, would be at risk for heat injury. We didn't find that."
Another researcher, John P. Porcari, Ph.D., head of the Department of Exercise and Sport Science at the University of Wisconsin-La Crosse, said “I think some people are attracted to hot yoga because it’s easier to go through a greater range of motion when your muscles are warmer."
"Looking at heart rate, they weren't working any harder in the hot yoga class than in the regular yoga class,” he says. “Normally if you go out and walk 3 miles per hour and then you do it again on a day that’s really hot, your heart rate is going to be higher. So, because the heart rate was identical, this tells me that somehow people must have down-regulated how hard they were pushing themselves in the heated environment.”
However, there was a significant difference in RPE between yoga sessions, with the hot yoga session perceived by subjects to be more difficult. On average, they ranked the hot yoga session to be 1.3 RPE units higher than the basic class. “They perceived it to be harder because of the difference in temperature,” says Porcari, “But physiologically they weren’t really working any harder based on their heart rate.”
As with all exercise, it will depend on how much effort you put in as to how much energy you burn during a yoga class, regardless of the temperature. So my advice is to go hard in every class if your body is able and your mind is willing.
This study did not include a temperature as high as that used in a Bikram yoga class, so the jury is still out on whether extremely high temperatures make a difference to physical benefits, so I guess we'll wait to see what research will be done.
There may still be room to argue the results because the study size was fairly small, and I wasn't there to check the testing parameters myself, but until proved otherwise I'll stick to saying that all yoga is awesome for you, so get yourself down to a yoga class in our studio :)
You can download the full study report from the American Council on Exercise.