A new UConn study shows that a standard heat tolerance test does not work equally well to measure tolerance in males and females
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A new study by researchers in the College of Agriculture, Health and Natural Resources (CAHNR) has demonstrated that a standard heat tolerance test does not work equally well to measure tolerance in males and females.
This research was published in Physiological Reports. This work was led by Jacob Bowie, Ph.D., a UConn postdoctoral fellow in the laboratory of Elaine Choung-Hee Lee, Ph.D., professor of kinesiology, and in collaboration with Douglas Casa, Ph.D., Board of Trustees Distinguished Professor of Kinesiology and CEO of the Korey Stringer Institute.
Heat tolerance is a measure of how well someone tolerates exercise-heat stress. Researchers and practitioners use changes to heart rate and internal temperature over time to determine when someone has become acclimated.
Heat tolerance tests involve participants exercising in high heat and humidity over days to weeks. When a person meets a specific threshold for temperature and heart rate, they are considered acclimated.
These tests are often implemented in military and athletic populations to determine when it is safe for someone to return to duty or play following an exertional heat illness.
Heat tolerance tests are used across the board for females and males. However, heat tolerance tests were developed based on the male body and there had been little research on how well these tests actually measure female heat acclimation.
"When these tests were developed only using male data, we could expect the results to be biased towards men in terms of their interpretation, and that's what we found," Bowie says.
In this study, the researchers used a heat tolerance test that has been widely used for military and athletic assessments. The test consists of walking on a treadmill at a slight incline for two hours in 40 degree Celsius (104 degrees Fahrenheit) temperature with 40% humidity.
By the test's standards, a person is considered heat tolerant if their core temperature stays below 38.5 degrees Celsius (101.3 degrees Fahrenheit) and their heart rate below 150 beats per minute for the duration of the test.
The researchers found that female participants started the test at a higher temperature and heart rate. But they also stabilized at a higher temperature as they acclimated to the heat.
"The average for females is actually above that threshold," Bowie says. "In the broader picture, when we don't account for those sex-based differences, we could be limiting participation in military training or missions, or we could be tagging athletes as not ready to return to play but they actually are."
While there are no health risks, an overly conservative heat tolerance standard for female athletes, for example, could lead to excess heat acclimation training sessions for professional athletes, such as those going to the Olympics, who are on very tight schedules.
"In one way it's great because we're protecting their safety to prevent exertional heat illness," Lee says. "But they're very limited in their training schedule leading up to competition."
This project is part of a larger U.S. Department of Defense grant looking at how to better determine when someone is acclimated to the heat and hence ready to go on the field, into active duty, or to work.
"We're answering fundamental questions about how you measure tolerance," Lee says. "There are lots of gaps that we're trying to fill."
Bowie is now working on a study using a heat tolerance test that requires participants to run on a treadmill until they reach an internal temperature of 39.5 Celsius (103.1 Fahrenheit). This temperature is a critical threshold; beyond it there are serious complications including the body losing its ability to cool effectively, denatured proteins, and damage to neurons.
This work will help develop better standards for athletes.
"Those [tests] are more akin to the environment we can expect an athlete to be in," Bowie says. "We don't expect an athlete to be mowing the lawn. We expect them to be at a high level of exertion for a sustained period of time."
This work relates to CAHNR's Strategic Vision area focused on Enhancing Health.
