Summer is here and so is the summertime heat. While it is tempting to get your workout in outside (especially if you’re traveling for the 4th of July), you might want to consider hitting an air conditioned gym instead.

A study out of the University of Nebraska at Omaha suggests that working out in the heat isn’t just uncomfortable, but it’s also a waste of time on a cellular level. Dr. Dustin Slivka and his team observed participants working out in three separate temperature-controlled chambers with a goldilocks approach: one was too hot, one too cold, and a third that was just right. Not only did participants perform better in the colder temperatures, but the tissue samples taken from participants who worked out in the heat were very revealing.

“When we had people exercise in a hot environment it really challenged their physiology. That response appears to be very negative. Almost as if they didn’t work out at all.”

Dr. Dustin Slivka, director of the Exercise Physiology Laboratory at UNO

Slivka and his team found that a workout in the heat isn’t as effective for mitochondrial development as a workout at a more comfortable temperature. As you might remember from biology class, mitochondria is the powerhouse of the cell, responsible for respiration and energy production. When mitochondria isn’t functioning efficiently, the benefits of exercise are largely lost.

How do you know when it’s too hot to work out outside? 

Don’t rely on the thermometer alone to determine if it’s too hot to workout outside. Instead, consult a heat index calculator. The heat index is determined using both temperature and humidity, which is a better indicator of how it actually feels outside.

Humidity means more moisture in the air, which means less of your sweat evaporates. With sweat sitting on the skin, your body has a harder time regulating your temperature. Furthermore, when you sweat a lot, blood rushes to your skin to cool it down. This leads to less blood in your muscles, a drop in blood pressure, and can ultimately result in lightheadedness or fainting.

Before you go for that midday run this Fourth of July weekend, consider checking out the heat index first. You can find a heat index reading on almost any weather site, or you can calculate it yourself based on the temperature and humidity. Here’s a chart distributed by the National Weather Service, for reference:

Provided by the National Weather Service

If the heat index puts you in the danger zone, try hopping into an air conditioned gym to work out instead. No matter where you are, you can purchase a day pass gyms near you on the Zeamo app.

Stay hydrated, even if you’re working out inside.

Bottom line? Even if the heat index hasn’t reached danger zone proportions, it’s important to stay hydrated when working out in the summertime (and all the time). Certified strength and conditioning specialist, Rachel Cosgrove, suggests weighing yourself before and after your workout. That way, you can see how much fluid you lost. Drink at least two 8 oz glasses of water for every pound of water weight lost to rehydrate after a workout.

Drink water before your workout, too. Just drink water all the time.

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