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What Running In the Heat Does to Your Body

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After a slow start, there are finally some truly steamy summer temps on the horizon. And while that’s happy news for your wardrobe and social life (hello, sundresses and al fresco brunches), it’s not so great for your workout—especially if you’re a runner, cyclist, or roller-blader. Suddenly, all that mileage or speed you build up in the cooler months is gone, and you’re gasping after less than a mile. Talk about demoralizing.

The truth is, yes, running in the heat feels harder. Just 30 percent of the energy your body generates actually goes to moving your muscles—the other 70 percent goes to cooling you down, explains sports medicine expert Jon Woo, M.D., a clinical associate professor at the University of Washington School of Medicine. When it’s hot, more blood flow is diverted from your muscles to your skin to help keep you cool.

“Since less blood is available to your muscles, when you run, you’ll find your heart rate is higher and the intensity feels harder,” says Stacy Sims, Ph.D., an exercise physiologist and nutrition scientist. It's not just in your head—you run actually is harder. With less blood driving your leg muscles, your speed takes a hit as well. (What’s the Best Temperature for a Run?) 

And as the old saying goes, “It’s not just the heat—it’s the humidity!” “When humidity is high, sweat on the surface of your skin cannot evaporate as efficiently, and therefore you can’t get rid of heat,” says Dr. Woo. This means even more blood is rerouted from your muscles to your skin.

Another thing to consider: UV rays. “Sunburn exacerbates these issues, as the skin will be hotter from the burn,” says Sims. “Recent research has shown that the skin temperature has a bigger effect on performance than actual core temperature. If you can keep your skin cool, you will keep more blood in central circulation, allowing for better exercise performance even if your core temperature is elevated more than exercise-normal.”

But here’s the good news: It takes just one or two weeks of running (or cycling, or otherwise being active) in the heat to get acclimated to it, says Dr. Woo. “Your body will expect circulating plasma volume and become more efficient in sweating, and, psychologically, you just start dealing with the heat better.” (Your Brain On: Warm Weather.) 

Until then, he suggests cutting the duration and intensity of your exercise by about 50 percent, then gradually dialing it back up to your pre-summer “normal” over the course of seven to 10 days.

You should also aim to hydrate all day (guzzling a glass of water right before you head out the door doesn’t count) and bring water—or, for workouts longer than an hour, a sports drink or electrolyte replacement—along with you when you exercise, suggests Dr. Woo. The colder the water, the better, says Sims. She recommends sticking a few ice cubes in your bottle before you head out. (And brush up on 11 Heat-Related Workout Inflictions to Watch Out For.) 

What you shouldn’t do is pour that ice water over your head, she warns. “The cold will constrict the blood vessels in your head, sending hot blood back to the core,” which could make you overheat faster. Pouring cool (not frigid) water over your head and across your forearms, though, is a great way to beat the heat. Also smart: slathering on a sunscreen designed for sport use to prevent sunburn, says Sims.

Now head out there to beat the heat!

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