Combine cycling or pilates or another exercise with a hot room and you have "hot fitness." Rooms can range in temperature from 82 to 95 degrees, depending on the activity. The trend started with hot yoga and has spread.
"When you take a heated spinning class, you feel like you get so much more out of it. You sweat; your blood is pumping," said Jen Ellenburg, who swears by her hot cycling class. "It's amazing!"
Mimi Benz is certified in indoor cycling and owns the studio Ellenburg goes to. She says there are many benefits to spinning indoors in increased heat.
"I mean, obviously, your heart rate's going to increase because it's a heated environment, which yields more of a caloric burn," said Benz.
Benz says she hasn't seen any increased risk over what she'd see in a traditional class, such as potential dizziness or overheating if there isn't enough hydration. But, not everyone is okay with turning up the heat.
"Taking exercise programs and putting them into a gym in a hot environment to me is kind of a scary proposition," said Walter Thompson, Ph.D., a Fellow with the American College of Sports Medicine.
Thompson helped develope the ACSM's standards and guidelines for health and fitness facilities, including temperature.
"The standard is about 72 degrees Fahrenheit uh and about 40 percent humidity," he said.
Variation from that, according to Thompson, could have a physical impact.
"Well the interesting question is what kind of physiological reaction do people have in the heat? Uh and that is an increased body temperature, an increased heart rate, an increased blood pressure," said Thompson.
The additional burn may be fine for trained athletes and young, healthy people. But, he is concerned about the potential stress on those who may not be acclimated to the increased temperatures or who may have other risk factors like cardiovascular or respiratory problems.
Proponents of hot yoga and pilates say preparation is key for any form of exercise.
"You put on a pair of shoes and run twenty six miles and you're out of shape, you put yourself into danger, don't ya? You've got to be in shape whenever you do any kind of exercise and you need to control your own personal effort," said King Rollins, Owner, Hot Yoga and Pilates Studio.
Both facilities we visited said their instructors are trained to watch for signs of distress and are CPR certified, and neither has seen any serious heat-related injuries. But, Thompson still urges caution.
Thompson warned, "The question is whether or not the average gym goer, the average client in a gym should participate in these heated environments? And my answer still is 'No!'"
Ellenburg says the heat took some getting used to, but she is hooked and has no plans to cool down.
"I'm addicted to that feeling of sweating," she said. "It makes you feel so pure."
There are also claims about detoxing benefits from these heated classes. Thompson says he hasn't seen any science to back that up, although he says muscles may loosen or stretch more easily in a heated environment.
Thompson says until now there has been no large scale certification standards for fitness facilities. The American College of Sports Medicine is changing that. The new standards meet criteria by NSF International, an accredited third party body. The certification will hopefully be available in the first half of 2014.
We also contacted the American Council on Exercise for their take on the trend, but did not receive any comments.