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, and exercising in the heat feels even worse than you'd expect from stacking those two factors together. Exercise raises your body temperature, and when you stack that rise in temperature on top of the heat from the weather, it's very possible to reach dangerous levels of body heat. That means your body has to work harder than usual to cool itself down. The ways we cool ourselves also interfere with exercise performance.

For example, your heart is working to supply blood and oxygen to your muscles, and to pump blood to the surface of our skin for cooling. Those tasks are a lot of work—no wonder we feel exhausted in the heat. As a safeguard, 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. It seems our brains slow our bodies down proactively on hot days in order to conserve energy. 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. That's the temperature where serious heat illness can set in, so it makes sense that our bodies will put on the brakes at that point.

In that study, athletes who kept cool with a fancy water-cooling jacket lasted the longest. (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.