On the muggiest, swampiest days of summer, you’ve no doubt questioned if running shirtless is the best way to beat the heat. But before you start thinking about how to kit up or strip down, you should understand what happens to your body in the heat.
“When you exercise, you use energy that’s stored in your body to perform the work,” explains Samuel N. Cheuvront, Ph.D., a research physiologist who studies fluid balance and endurance exercise. “But only about 20 percent of all the energy we’re generating when we exercise actually goes toward performing the physical work; the rest of it is turned into heat.”
Your body has a limited capacity for storing that heat energy. So when your brain senses your body temperature has increased, it elicits two responses: It shunts more blood flow to the skin, where increased vasodilation—a widening of the blood vessels at the skin’s