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Nobody likes to feel sluggish and sweaty, so when the sun is set to “broil,” we understand that you’d rather take your workout to an air-conditioned gym. But if you tough it out in the heat, not only will the workouts get easier, you’ll also have better endurance once the mercury drops again.

Hot workouts can be dangerous, so I trust that you know the common sense advice about running in the heat. Among the most important: Drink to thirst (or a little bit more), and stop and get help if you start feeling symptoms of heat illness like nausea, dizziness, or weakness. And while it’s great to work on your ability to run in the heat, don’t be stupid about it—stay inside if the temperature is hotter than you can handle, and stay aware of smog and ozone levels (which get worse on hot days) if you live in an urban area.

Why exercising in the heat feels so miserable

Running is miserable and heat is miserable, therefore running in the heat is miserable. But there’s more to it than that: Exercising in the heat feels even worse than it should.

Your brain perceives effort differently in the heat, so even before you overheat, you feel sluggish. In a study published in the European Journal of Physiology, cyclists who worked out in a lab with a temperature of 95 degrees were slower than when they did the same time trial at 59 degrees. That makes sense, but here’s the weird part: They didn’t overheat and then slow down. They were slower from the start. Our brains slow our bodies down, it seems, proactively trying to conserve energy based on the fact that it’s a hot day.

As a workout continues, our bodies heat up. In another study, published in the Journal of Applied Physiology, when asked to cycle to exhaustion, participants pooped out when their core temperatures reached 104 degrees—no matter what temperature they started at. The athletes that took the longest to reach that temperature were the ones who wore a fancy water-cooling jacket. You can mimic this effect in your own workouts by drinking ice-cold beverages and pouring water over your head. The longer you can keep your body cool, the longer you can keep up a hard effort.

But cooling your body isn’t a complete solution. Dumping a cup of ice water over your head or putting one into your belly only provides momentary relief, and water-cooling jackets aren’t practical outside of physiology labs. So let’s look at what happens in real world conditions.

Your body tries to cool off, in part, by sweating. When moisture evaporates from your skin, it takes some body heat with it. In humid weather, though, sweat doesn’t evaporate as easily because the air is already full of water vapor. So when we’re talking about “heat,” we really mean something more like “perceived heat,” which is a combination of heat and humidity. This heat index chart shows the relationship:

Chart showing likelihood of heat disorders with prolonged exposure or strenuous activity. Temperature from 80 to 110 degrees F is along the top, relative humidity from 40% to 100% is along the side. You would reach

Graphic: NOAA

You’ll run slower in the heat (and humidity). While you can find charts like this one that predict exactly how much slower you’ll be, the truth is it depends on whether you’re used to the heat, and on your body size.

That’s right, not your fitness level, but your actual physical size. People who are larger have more muscle, fat, or both. Muscle generates heat, and fat acts as an insulator. On the other hand, smaller folks generate less heat, but have more skin through which to dissipate that heat—the ol’ surface area to volume ratio. This is why petite runners place better in races on hot days.

Some people think being more fit makes you better at dealing with heat, but the opposite is true: The fitter you are, the more body heat you produce, just because you’re so good at working hard. Short of changing your body shape (which is possible, but hardly a short-term fix), what can you do to tolerate exercising in the heat better? The answer is simple: Spend more time exercising in the heat.

Survive a hot weather run with these:

How to adapt to exercising in the heat

Running in the heat makes you better at running in the heat—and it makes you better, period.

Say you do all your workouts outdoors this summer, while your equally fit twin does identical workouts on a treadmill in an air-conditioned gym. Who do you think will finish first in a 5K on a hot weekend in August? That’s right, you will.

But even if the weather is unseasonably cool on that August day, your heat training will still help you beat your twin. Part of the magic of heat training is that it increases the amount of blood in your veins (the better to put it towards your skin for cooling, while still having enough to fuel your muscles). The effect has been compared to a mild, totally legal version of blood doping.

Here’s the bad news: heat adaptation takes work. It’s not enough to sit around in the air conditioning all summer, only venturing outside for occasional workouts. A study published in the European Journal of Applied Physiology found that people who made no particular effort to exercise in the heat didn’t have any better heat tolerance in the fall than in the spring. If you want the advantages of heat training, you have to work for them.

This US Army training protocol provides a good road map for adapting yourself to the heat: Spend at least two hours in the heat each day, it says, and include cardiovascular exercise (like running, cycling, or anything that gets your heart rate up) as part of that. If you can’t handle two hours without feeling symptoms like nausea or dizziness, do what you can and view the two-hour benchmark as a goal to work toward.

You can expect to be better adapted to the heat after about two weeks of spending two hours per day in the heat, although you may be able to start seeing results in just a few days.

Don’t expect to be able to do the same workouts at first. Remember that your body is still trying to convince you that you are super tired and need to slow down. Safety guidelines for workers provide a reality check here. On their first day in the heat, the Occupational Safety and Health Administration (OSHA) recommends giving workers just 20% of their usual workload. Within a week, they should slowly ramp up to 100%.

Also, to stay adapted to the heat, you have to keep spending time in it. You can take a few days off, but if you slack off for a week, you’ll start to lose your hot weather superpowers: about 75% loss after three weeks, according to the Army’s estimates.

To keep up your heat training in cool weather, you can try wearing long sleeves and tights, like elite runner Kara Goucher did when training for a world championship race in muggy Osaka. (She won a bronze medal, the first American ever to do so). She also spent a few weeks in Osaka before the race began; traveling to experience the heat might be an option worth considering if you’re a dedicated athlete with vacation time to burn.

You can also try the opposite of our keep-cool advice, choosing to run at the hottest time of day on asphalt roads with no shade. Stay safe, and enjoy your new superpowers.

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