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Google “hot yoga,” and the word “danger” appears just as much as “safe” and “benefits.” But here’s the problem: True laboratory studies hadn’t been done to analyze the effects of hot yoga. That is, until recent years.
Brian L. Tracy, Ph.D. and associate professor (whose expertise lies in neuromuscular physiology) at Colorado State University in Fort Collins, Colo., has brought Bikram yoga into the lab — twice — since 2006, when a master’s student formed the idea to get Bikram yoga under the microscope. Their goal: to explore the yoga’s effects on strength, steadiness and balance. Participants practiced three classes a week for eight weeks in a room heated to 105°F with 40 percent humidity (the standard for a Bikram class).
Two years later, Dr. Tracy and members of the Neuromuscular Research lab published, “Yoga as steadiness training: effects on motor variability in young adults,” as the first-ever peer-reviewed scientific paper done on this type of yoga. The conclusion? Yoga substantially improved balance and moderately improved leg strength leg muscle control.
In 2013, Dr. Tracy published the second part of study, titled “Bikram yoga training and physical fitness in healthy young adults,” which focused on the short-term physical fitness effects: increased lower back and hamstring flexibility, deadlift strength, shoulder flexibility and slightly decreased body fat.
Now, Dr. Tracy has completed another study, which sought to establish the caloric expenditure of Bikram yoga. It measured three main things: metabolic rate, core temperature and heart rate. Nineteen experienced Bikram yoga practictioners (each had taken two classes per week for at least one year) from ages 19 to 40 took a single class in an environmental chamber at CSU heated to 105°F with 40 percent humidity. Each class was performed individually, while listening to recorded instructions. The participants were told to “give a good, solid effort,” said Dr. Tracy, and to perform the postures to the best of their ability.
The study’s conclusions actually dispel several myths floating around the Internet.
Fallacy No. 1: It’s not uncommon to hear whispers of burning 700 to 1,000 calories in a single class. However, using portable analyzers (those crazy face masks) to measure energy expenditure (oxygen consumption) by collecting expired gases, Dr. Tracy’s team found that, on average, the men burned 460 calories, while the women burned 333 calories. The Internet rumors of more calories burned are usually estimates based on heart rate. However, heart rate measurements are skewed because of the heat, which causes the heart rate to quicken more than if practicing in room temperature. This study focused on the metabolic rate to accurately measure the actual calories burned. In the first 55 minutes, during 13 standing postures, the metabolic rate quadrupled. In the second 45 minutes, during 13 floor postures, the rate dropped to about 3.5 from 4. And if the 333 to 460 calories burned still seem low, remember that these numbers came from experienced Bikram yogis; the number could likely be higher for “a pretty big person performing the yoga at a very high intensity,” said Dr. Tracy.
To put these numbers in layman’s terms, the participants’ metabolic rate was comparable to the average person walking at 3.5 miles per hour on level ground — or a moderately brisk walk, says Dr. Tracy.
Another myth is that practicing in a 105°F room for 90 minutes can raise the body temperature to risky levels. Participants each swallowed ingestible pills that measured core temperatures and transmitted data from the gut. Normal body temperature is 98.6°F. The average body temperature for participants slowly increased during the standing series, and then plateaued at an average of 100.3°F, which “is not a dangerous elevation of core temperature,” Dr. Tracy said. According to the Mayo Clinic, body temperature elevations aren’t serious until they reach 103°F.
Thirdly, participants wore a standard chest strap during the 90-minute session to record their heart rates. During the first 55 minutes of the class, where practitioners carry out standing postures, the average heart rate was elevated to 146 beats per minute (bpm). During the last 45 minutes, where participants practice postures on the floor, the average heart rate fell to 123 bpm. “The heart rate was higher than you’d expect than if a person was doing yoga in a room with no heat,” Dr. Tracy said. However, “it’s extremely well-known that if a person exercises in the heat, the heart rate is higher than if that person was exercising at room temperature. Not a surprise.”
Dr. Tracy’s team also recorded water consumption and water loss throughout the class. The eight men drank more water and perspired more than the 11 women: On average, they consumed 0.5 liters of water and lost about 2.5 percent of their body weight. The 11 women, on the other hand, drank only about 0.1 liters and lost about 1.9 percent of their body weight in water loss.
Overall, Dr. Tracy and his team found that, for this particular group, Bikram yoga produced a moderate metabolic response, robust heart rate, and substantially (but not dangerously so) elevated core temperature. Because this is the first study done of its kind, with only 19 participants, its findings are limited. Dr. Tracy acknowledged that the responses they found could be “considerably different for less experienced practitioners, exceptionally highly motivated participants who perform the yoga with different form, or people with a different health status.”