Consider this your bible to all things high-intensity interval training, including what HIIT is, its key benefits, and the best HIIT workouts to mix into your fitness routine.
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If you're a fitness newbie, you've probably seen the term "HIIT" written in the description of an online workout or heard trainers briefly mention it, without providing any details, during your in-studio classes. In turn, you might be feeling totally clueless, wondering what this strange acronym means and why everyone swears by the hurts-so-good workout style.

To get answers to all of your burning Qs, turn to this HIIT workout guide, which explains "what is a HIIT workout?" and breaks down some of the biggest benefits of HIIT. Plus, you'll find a round-up of some of the best HIIT workouts, including equipment-free options and strength-building sweat sessions that will leave your muscles shaking.

What Is HIIT?

Though the exact definition varies from trainer to trainer, HIIT — which stands for high-intensity interval training — generally refers to a style of training that involves alternating between bursts of vigorous exercise and short periods of active recovery moves or full-on rest, says Chris Ryan, C.S.C.S., a certified personal trainer with MIRROR and a lululemon ambassador. "You're looking at doing some sort of exercise and then switching it up to keep the heart rate high," he explains. "You're also keeping the workout itself to a limited amount of time, so you're not looking to work out for two hours." For example, you might do 45 seconds of an intense exercise such as thrusters, followed by 45 seconds of toe-touch jacks (tapping your foot out to your side rather than full jumping jacks) or just full rest, he says. Power through that circuit three times, repeat the process with two other exercise circuits, and you've got a 15-minute HIIT workout that'll leave you dripping in sweat. 

The easiest way to ensure your HIIT workout is high-intensity, as advertised, is to consider your rating of perceived exertion. Your RPE is essentially a self-administered rating of how hard you feel like you're working (based on breathing rate, heart rate, sweat, and muscle fatigue) on a scale of 1 to 10, according to the Centers for Disease Control and Prevention. An RPE of 5 may feel like moderate intensity, while an RPE of 9 may feel like you're going all-out, adds Ryan.

But if you want to get more technical, look at your heart rate. For physical activity to be considered "vigorous-intensity," your heart rate should be between 77 to 93 percent of your maximum heart rate, according to the CDC. (To figure out your estimated maximum heart rate, just subtract your age from 220.) Throughout a HIIT workout, your heart rate might be around 130 to 160 beats per minute, and during particularly challenging portions, it might jump as high as 170 or 180, says Ryan. Do a little math wizardry, and that means a 30-year-old with a heart rate anywhere from 146 to 177 during a HIIT workout is making the most of their sweat sesh. (BTW, the talk test can also be helpful to understand your workout's intensity.)

The Benefits of HIIT Workouts

They improve your cardiovascular health and VO2 max.

All the plank jacks, high knees, and butt kicks involved in HIIT workouts can do your ticker some good — and help you meet your recommended activity quota for the week. ICYDK, the U.S. Department of Health and Human Services recommends adults power through at least 150 minutes of moderate-intensity physical activity or 75 minutes of vigorous-intensity activity weekly. Just a single session of moderate-to-vigorous physical activity can reduce blood pressure, and folks who stick to that moderate-intensity quota have significantly less risk of developing cardiovascular disease, according to the HHS. And luckily, HIIT falls into the "vigorous-intensity" category, so you're likely to reap those benefits.

What's more, regularly challenging your body to a cardio-heavy workout like HIIT can help make daily activities much less strenuous. "The more your heart and your lungs are so-called stressed, the better off you're going to be walking up a flight of subway stairs or carrying grocery bags," says Ryan. "Your everyday life is just going to be that much better because you're not going to exert yourself so hard for an 'easy' activity." 

Another cardiovascular benefit of HIIT: Your endurance workouts might feel a bit less taxing. When compared to long, slow training intensities, research shows high-intensity aerobic interval training "significantly increases" VO2 max — the maximum amount of oxygen your body can utilize during intense exercise. The higher your VO2 max, the more energy your body can use, and the longer you can exercise, according to UC Davis Health

You'll continue to burn calories after your workout ends.

Thanks to excess post-exercise oxygen consumption, or EPOC, your body will continue to use calories well after you've finished your HIIT workout. "It's the gift that keeps on giving," says Ryan. Post-sweat sesh, your body is working hard to restore itself to homeostasis — your body's normal, resting metabolic state — and it needs to take in more oxygen than it did before you exercised in order to do so, according to the University of New Mexico. Since you're consuming oxygen at a higher rate to replenish the adenosine triphosphate (ATP) that was used as fuel during your workout, reoxygenate your blood, and cool down your core temperature, you're also using more energy (read: burning more calories). And a HIIT workout is basically a recipe for EPOC success: Research suggests the extent of EPOC increases as workout intensity does, and a small 2019 study found that folks who performed interval exercises had greater EPOC than those who did steady-state exercise. 

You'll see muscle gains.

Though they may seem cardio-centric, HIIT workouts can simultaneously serve as strength training, depending on how they're programmed, says Ryan. For example, you can get your heart rate up and strengthen your arms by adding a light dumbbell and a press to a standard jumping jack, or holding weights while performing punches, says Ryan.

Equipment isn't required.

Even if you're traveling and don't have access to equipment, you can still power through an intense sweat and score some of the health benefits of HIIT. Exhibit A: A small 2021 study found that young, healthy adults who completed three equipment-free, bodyweight-based HIIT workouts a week improved their cardiorespiratory fitness after just four weeks. That said, HIIT workouts don't have to include just bodyweight moves, and you can try this style of workout while strength training, cycling, walking, swimming, aqua training, elliptical cross-training, and more, according to the American College of Sports Medicine.

8 More Reasons to Do High-Intensity Interval Training

The Best HIIT Workouts

To boost your heart health and muscle gains, consider incorporating these best HIIT workouts into your routine. Whether you're a total beginner or need a workout that's easy on the joints, you've got options.

Feel like going rogue? Create your own high-intensity workout by choosing a handful of these trainer-approved HIIT exercises, then perform each for 45 seconds, followed by 45 seconds of active recovery or rest, for three rounds. Be warned: You'll feel sore AF later today but superhero-level strong tomorrow.