Skip to main content

Intervals, Explained

May 13 2021
6 min read
female fitness instructor showing class how to do crunches on workout ball

Whether you’re ready to take your jogging workouts to the next level, improve your pace in an upcoming race...

...accelerate weight loss or maximize the minutes you spend logging miles, adding intervals to your running routine can help you reach your goals. You can do intervals in any form of cardio, but running is one of the easiest and most popular activities for implementing them—just lace up your sneakers and go!

What Are Intervals?

Think about how far you can run at an all-out sprint before you run out of steam, compared to how far you can make it at your everyday jogging pace. The faster you run, the shorter the distance you can go, right? That’s the principle interval training is based on: By alternating between faster and slower paces, you harness the benefits of speed—increased calorie burn and muscle toning, for example—while keeping it up long enough to strengthen your heart and improve your endurance.

Benefits of Interval Workouts

One of the best parts about interval training is that it delivers similar fitness results you get from a steady-paced workout—but in half the time or less. When Canadian researchers had exercisers do three 25-minute interval workouts a week for two weeks, they had similar fitness gains compared to exercisers who logged an hour five days a week for the two weeks. In addition to maximizing your workout minutes, intervals also offer these advantages:

Improve Your Pace. Casual runners who replaced their usual jogging routine with three interval workouts improved their times in a 5K run by 48 seconds despite spending half as much time working out, according to a study in the Journal of Applied Physiology.

Leaner Legs. Your muscles are made up of different types of fibers, called type I and type II muscle fibers. When you go out for a leisurely jog, your type I, or slow twitch, fibers shoulder the effort, since they’re best at providing a sustained effort. By sneaking in sprints, you tap into your type II “fast twitch” fibers, which can only go for shorter stretches. Activating more muscle fibers means burning more calories and stronger, toned legs.

Less Belly Fat. Women who alternated short sprints with slightly longer bouts of recovery lost more than three pounds from their midsections over 15 weeks. Meanwhile, women who worked out twice as long at a continuous pace actually gained a pound of belly fat, found Australian researchers in the International Journal of Obesity.

Interval Caveats. With all these great perks of interval training, you may be thinking there’s no point in doing steady-paced workouts. Not true! Just like strength training your arms on back-to-back days is a no-no, your leg muscles also need a day to recover (with a less-intense jog or a rest day) between interval sessions. In an interval workout, the strengthening happens when muscle fibers are actually broken down, then heal stronger than before during a recovery period. If you don’t give them a down day to rest and rejuvenate, you actually hamper your results—and set yourself up for an overuse injury.

Do a maximum of three interval sessions a week on non-consecutive days. If you work out on additional days, stick to easy running or walking, or cross training. Talk to your exercise physiologist or a running coach about what type of interval regimen makes sense for your unique body and workout goals.

Common Interval Workouts

If you’ve spent much time around so-called “serious” runners, you might have heard them throwing around some pretty strange words to describe their interval workouts. Here’s a guide to some of the most common ones, plus tips on how to try them yourself.

Fartlek: Probably the strangest sounding term for interval training, the word fartlek actually means “speed play” in Swedish. Just like the translation says, the idea is literally to play around with speed during your run (similar to high intensity interval training or HIIT).

Rather than running a set distance or speed, this is a workout you make up on the spot—or even as you go. Speed up and slow down as you feel like it, or choose markers along your route to tell you when to go fast and when to slow down. For example, run fast for the distance of two streetlights, then walk or jog to the next street light and do it again. Or pick random landmarks ahead of you, such as a barking dog or an SUV, to mark when you speed up or slow down.

Tempo: Designed to improve your efficiency, or how fast you can go without becoming short of breath, tempo training is typically done by running at the fastest pace you can keep up for long distances. You should be able to speak in short sentences, but you probably will prefer not to. After warming up at an easy pace for at least five minutes, try picking it up to a tempo pace for 15 to 20 minutes.

Hillwork: An effective workout for building strength in your glutes—as well as improving stamina for your next road race—is to do repetitions of running uphill. After jogging to warm up, find a hill that will take you at least a minute to run up. Run up, then walk or jog slowly down to catch your breath. Aim for five to 10 repetitions.

Tabata: Based on a training model developed by Japanese researchers, Tabata intervals are a way to get a great workout in a very short amount of time: just four minutes. The trade-off? You have to be ready to push yourself hard. After warming up for five minutes, sprint as fast as you can for 20 seconds, then come to a complete stop for just 10 seconds. Repeat that eight times (it will take you four minutes), and you’re done. Continue to walk for two minutes to cool down.