This Is What Happens to Your Body When You Do a High-Intensity Workout

HIIT has been lauded for its calorie-burning abilities—but how does it work?

Among today's fitness folk, high-intensity interval training (HIIT) is all the rage. The workout—which was found to be one of the top fitness trends in a 2017 American College of Sports Medicine survey—involves short intense bursts of movement followed by active recovery periods. When done correctly, HIIT can burn more fat and calories in a shorter amount of time than many other cardio workouts.

But what's going on inside the body that makes high-intensity exercise so special? Keep reading to learn more about what happens to your mind and muscles when you perform a high-intensity interval workout.

Your brain releases feel-good chemicals.

"My favorite thing about HIIT training is what happens in your mind," explains Eric the Trainer, a certified personal trainer and the host of Celebrity Sweat. So what exactly goes on in your brain? Well, when you work out, your brain is triggered to release feel-good hormones called endorphins. And if you're looking for a mood boost, then HIIT is the way to go: One 2017 study published in the journal Neuropsychopharmacology found that 60 minutes of high-intensity interval exercise is more conducive to releasing endorphins than 60 minutes of a moderate-intensity gym sesh.

Your brain's capacity for learning and remembering increases.

The hippocampus is the area of the brain that is responsible for things like memory and learning. And while you might not immediately associate toning your muscles with improving your memory, one 2015 study published in the British Journal of Sports Medicine found that when older women performed aerobic exercise at the gym twice a week for six months, the size of their hippocampus increased. Defined muscles mean sharper brains—so hit the gym, people!

Your circulation improves.

"The expansion and contraction of muscles that occurs while performing exercise [boosts] circulation, allowing nutrient-rich blood to reach nearly every part of the body," says Eric. Indeed, one 2003 review published in Circulation concluded that working out regularly "improves the capacity of the blood vessels to dilate… [and was] consistent with better vascular wall function and an improved ability to provide oxygen to the muscles during exercise." By improving your circulation, you'll be able to help keep your body in optimal condition.

Your core temperature rises.

When you do a high-intensity workout, all the energy you're exerting produces heat inside the body. As a result, your core temperature will rise. That has other effects besides simply warming you up. Movement specialist Jacob Andreae explains on his blog that "blood vessels that are close to the surface dilate, allowing greater blood flow, which can give the skin a reddish appearance." Also, as you might've already noticed, your body will respond to these increased internal temperatures by sweating—a lot.

Your muscles tear.

When you do a high-intensity workout, you create little tears in your muscle fibers—and that's actually beneficial. Think about it like this: When your muscles tear and rebuild, they become stronger.

"White blood cells start to repair the damaged muscle after about 12 to 24 hours and they release a number of chemicals, which are likely to be involved in the generation of local muscle pain," Dr. Mark Tarnopolsky, a neuromuscular disorder specialist at the McMaster University Medical Center, explained to the New York Times.

Yep, that means it's these microtears that cause soreness. The good news? The more high-intensity workouts you do, the more accustomed your muscle fibers will become to the workouts. Ironically, more gym sessions mean less soreness.

Your heart rate increases.

One of the easiest ways to tell whether you're having a good HIIT session is by how fast your heart is beating. Your muscles require more oxygen during a high-intensity workout, so your heart has to work overtime to provide the necessary blood. While this pumping sensation can sometimes feel uncomfortable, becoming a regular at the gym can help you lower your resting heart rate and therefore put less pressure on your most vital organ.

You use up a lot of oxygen.

When you go for a slow jog or take a yoga class, your lungs are able to keep up with your body's demand for oxygen relatively easily. During a high-intensity workout, however, oxygen stores can deplete quickly—and that's exactly what you want to happen. When your body runs out of oxygen, it has to work hard to rebuild its supply. That results in more calories burned for a longer post-workout period.

For instance, one 2011 study published in the journal Medicine & Science in Sports & Exercise found that when subjects rode a stationary bicycle at a high intensity for 45 minutes, they burned an average 420 calories during the workout session. Over the next 14 hours, they burned an extra 190 calories, meaning the after-burn effect of their workout increased the total calories burned by 37 percent. Burning calories during your post-gym Netflix binge? Score! And for a simple way to reward yourself for your workout, check out these 100 Amazing Summer Buys Under $100.

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