April 24, 2015
An American Council on Exercise (ACE)-sponsored study was published this week on the heart rate and core temperature responses to Bikram yoga that is already generating some buzz among yogis devoted to this yoga style. The study, conducted by researchers at the University of Wisconsin–La Crosse’s Department of Exercise and Sport Science, reported core temperatures reaching 103 and even 104 degrees F, values outside of the safe range that could lead to life-threatening consequences.
Although not mentioned in the article, there are two other studies that also quantified the thermal responses to Bikram yoga. One study published in Alternative Therapies conducted at San Diego State University showed an average temperature increase of 1 degree above baseline for novice and 1.8 degrees for advanced practitioners who had completed a minimum of 20 classes. No individual maximum temperature values were reported in this study. It is also important to note that the researchers chose to measure oral temperature, which is less valid that than the core temperature measurement used in the ACE-sponsored study.
Another unpublished study funded by Pure Action completed at Colorado State University in Dr. Brian Tracy’s laboratory, measured the core temperature responses to the entire Bikram yoga session using the same measurement technique as the ACE-sponsored study, the ingested core body temperature sensor. While the full manuscript is not yet available, the results showed that the core temperature responses reached a maximum of 100.8 degrees F, which is a perfectly safe rise in core temperature.
The variation in the study results could be due to discrepancies in study design. The study at Colorado State University was conducted only in young adults between the ages of 20 and 40 while the ACE study included adults from the ages of 28 to 67. Another difference between these two studies is that Dr. Tracy’s laboratory performed the experiments in a tightly temperature- and humidity-controlled environmental chamber while the University of Wisconsin study was performed at Bikram yoga studios. Given that the specific temperatures were not recorded, we cannot definitively conclude that the core temperature responses were not a result of potentially higher than stipulated temperatures or relative humidity as are sometimes the experienced in some studios across the country.
The difference in age groups between the two studies is of key importance given the propensity for older adults to become less efficient at thermoregulation. This group should be approached with caution in any Bikram studio given the increased likelihood of heat exhaustion or stroke in this population.
With that being said, Emily Quandt, who led the ACE-sponsored study in Wisconsin, brought up some recommendations for Bikram yoga instructors who lead these heated classes daily. Some of these points warrant discussion among yoga professionals. She says to “focus on hydration,” a point expanded upon by Dr. Cedric X. Bryant, Ph.D., Chief Science Officer at ACE, who adds ““Bikram teachers should recognize that participants’ thermoregulatory systems will be challenged in this environment.”
The combination of heat and humidity pose a greater challenge than heat alone because it compromises the body’s primary means of cooling during exercise: the evaporation of sweat. It is not the act of sweating which cools the body; it is the evaporation of sweat from the surface of the skin which cools the underlying blood which then returns to the core of the body to prevent unsafe rises in core temperature. Humidity lessens the effectiveness of this mechanism because evaporation is less likely to occur in humid conditions.
Dehydration leads to reductions in blood volume. When blood volume is reduced, the body is less efficient at cooling, which can also increase the risk of heat exhaustion or stroke. These concerns are not limited to Bikram yoga, but are applicable to all forms of exercise in the heat. A runner might have the same complications when running outside on a hot, humid day. A key difference there is that with running, there is more convection or movement of air across the skin’s surface, which facilitates cooling despite the risky conditions. There is much less of this occurring in the yoga room given the frequent breaks between postures and savasanas.
The other two recommendations of shortening the class and decreasing the temperature will likely not be accepted across the broad spectrum of studios worldwide as devotees of this practice are mostly staunch supporters of the 90-minute 105-degree stipulations. While some studios have adapted classes to 60 minutes, most stick with Bikram yoga in its truest, original form. Familiarity with the signs, symptoms and treatment techniques for heat exhaustion that are outlined in the ACE article and stroke as well as having emergency procedures are vital and should be emphasized among all hot yoga instructors. With hundreds of studios all over the world, Bikram yoga isn’t likely going anywhere soon, so just remember to practice safely.
Dr. Stacy D. Hunter, PhD