Chances are, you’ve heard of high-intensity interval training, or maybe have seen it referred to as its acronym, HIIT. It’s grown in popularity in recent years as more and more scientific research has been published proving the effectiveness of HIIT workouts. It was voted one of the top fitness trends of 2020 by fitness professionals around the world. If you want to incorporate interval training into your workout routine, here are the principles to stick to.

Intensity

The intensity aspect of HIIT is what makes it the most challenging part, and might even scare people away from trying it. During active periods of a HIIT workout, you have to push yourself to your maximum limit in order to yield the most gains. Although you’re in for a challenge, the intensity and fast-paced nature of HIIT workouts can make them more fun than running on a treadmill for an hour.

You can gauge the intensity level of an exercise by referring to the Rated Perceived Exhaustion Scale (RPE). The RPE ranges from 0 (nothing at all) to 10 (very, very heavy), and you should aim to push yourself to about a 9 during HIIT workouts. Another way to measure the intensity of your workout is through your heart rate, which a FitBit or Apple Watch can help you determine. Aim to get to 80 to 95% of your estimated maximum heart rate.

Why are HIIT workouts so intense?

The intensity of HIIT requires your body to recruit more of your muscles. You have two main types of skeletal muscle fibers: slow-twitch and fast-twitch. Slow-twitch muscle fibers contain more mitochondria and myoglobin and are surrounded by more capillaries, which means they have more blood, and thus oxygen, flow. Because of that, they are fatigue resistant and can sustain lower intensity exercises for longer periods. Fast-twitch muscle fibers contain fewer mitochondria and myoglobin and are surrounded by fewer capillaries. They fatigue quickly and take longer to recover because there’s less blood flow to these muscles. However, they can sustain quick, powerful movements, like those that you might perform in a HIIT workout.

HIIT harnesses your fast-twitch muscle fibers, and the intensity helps you achieve muscle fatigue. This heightens your body’s excess post-exercise oxygen consumption (EPOC) rate. The more intense the workout, the greater your body’s oxygen debt is — as a result, you’re consuming oxygen at an elevated rate, which means you’re still expending energy and burning calories. Believe it or not, you’ll still burn calories for up to 24 hours after a HIIT workout session.

Duration

HIIT is so popular nowadays because although it’s challenging, it doesn’t take a lot of time. If you have a busy schedule, HIIT is a great way to burn calories in a short period of time. A typical HIIT workout lasts from 20 to 30 minutes, although some sources disagree. If your workout lasts an hour, chances are you’re not pushing yourself hard enough. The New York Times helped to popularize interval training in a 2013 article about a “science-based” seven-minute workout. Because HIIT workouts can vary in length, it’s even easier to fit them into your schedule depending on how much spare time you have.

Recovery

During the Workout

The work to recovery ratio is the other key to a successful HIIT workout. Because of how hard you’re working your muscles and cardiovascular system, rest intervals are necessary to keep you going throughout the entire session. The ratio of intensity to recovery varies, as does the duration of the whole workout.

For example, you might sprint your hardest for 30 seconds, then walk for 30 seconds (a 1:1 ratio). Or, you might hold a plank for 20 seconds and rest for 10 seconds (2:1). The recovery period usually takes up around half of the time in your workout, although it might not seem like it.

If you’re doing a 30-minute sprint interval workout, you might only spend 15 minutes of that time actually sprinting. The other 15 minutes is allotted to recovery — but it’s impossible to run at full speed for 15 minutes straight, which is why the recovery period is so essential to helping us push past our limits.

After the Workout

Recovering immediately after a HIIT workout involves a cool down, some stretching, and drinking lots and lots of water. You’ve just done what’s equivalent to an hour-long run (give or take), and your resources are depleted. Pat yourself on the back after a workout, and make sure to drink plenty of water, eat a protein-filled meal, and allow yourself to rest.

Allowing your body to fully recover from a HIIT workout is as important as doing the workout itself. As we stated before, the EPOC period can last up to 24 hours, or however long it takes for your muscles to regain their oxygen stores. There’s no point in doing intense HIIT workouts two days in a row when your body is still burning calories from the first workout the day before. Plus, your muscles need time to recover. When you do an intense workout, you create microscopic tears in your muscles. This activates your satellite cells, which help you repair and build your muscle fibers so that you heal the tears and gain more muscle mass.

How Often Should You Do HIIT Workouts?

Because of how intense and taxing on your body they are, we recommend limiting your interval training to two or three times per week so you have plenty of time to recover. On “off days,” you can still move your body. Strength training, steady-state cardio (i.e. running, swimming), or yoga are great options to incorporate into your workout routine. Moving your body even a little, like going on a walk around the neighborhood, can help you feel less sore and stiff after a workout.

While the main principles of HIIT are intensity, duration, and recovery, a HIIT workout can look like many different things. That’s one of the greatest parts of HIIT — you can incorporate so many different types of core exercises into a HIIT format.