When kids run, they don’t track their heart rates or pay attention to their paces. They just zoom along however fast as they like, and they often laugh while they do it. That kind of joy in running can be rare to capture as an adult. But there’s one workout that brings a bit of that fun back: the Fartlek.

Sure, the term may sound a little silly. For that, you can blame the Swedes—Fartlek translates as “speed play” or “speed game” in Swedish. “I like to remind my athletes that [Fartleks are] just basically playing with speed in a way that feels good to them on the day,” says USATF-, RRCA-, and UESCA-certified running coach Amie Dworecki, CPT. “It does involve some faster running, but running in a way that’s fun.”

If this sounds like something your training plan could use more of, here’s what you need to know about Fartlek training, why it could benefit your running, and how to get the most out of this workout.

What is a Fartlek?

The premise of Fartlek training is fairly simple. “It’s basically short bursts of speed when you’re running mixed in with your regular running pace,” says USATF- and RRCA-certified running coach Marnie Kunz, CPT. Those bursts can be as random as sprinting to the next stop sign every time you see a red car. Or you can have more structured sets of one minute quicker, then three minutes easier, for example. Either way, it’s continuous running, so you return to your relaxed, conversational base pace after each burst of speed.

The idea behind the workout came from Swedish national cross-country coach Gösta Holmér in the 1930s. He wanted to make his struggling team more competitive by combining speed work with endurance training. It worked: His runners began breaking records in multiple events—and the rest of the world began taking notice. Now, Fartleks are a staple part of many running programs.

The difference between intervals and Fartlek training

Both Fartlek and interval workouts are forms of speed training where you push the pace for a while, then let your body recover somewhat before pushing again. But while intervals are precisely measured, and often done on a track, Fartleks are more informal. You can do them anywhere, and the bursts of speed may vary in distance or intensity based on how you’re feeling.

Also, even when you pick up the pace, you aren’t pushing so hard that you need to stop running after a faster burst, says Dworecki. “You don’t want to hit that 10 out of 10 to where you feel like you have to walk afterward.” While after an interval, you may take a break to huff and puff on the side of the track with your hands on your knees, the point of a Fartlek is to keep running the whole time to build your endurance while also working on your speed.

These workouts end up being slightly easier on the body because you aren’t pushing quite as hard as you may during intervals. That’s why Dworecki often recommends Fartleks to runners as a precursor to interval training. “It introduces your body into speed work,” she says. That can be helpful both for beginners who are newer to running, or runners who are in the earlier stages of a training cycle.

How Fartleks can improve your running

Fartleks have several benefits if you work them into your training strategically.

1. They can help protect your body against injury

Because Fartleks can be used as a way to ease into speed work, Dworecki says they can help to keep your body healthy. “It helps you prevent injuries that might come up if you just jumped into interval training,” she says. For the same reason, she often gives her athletes Fartleks during the week after they’ve had an especially difficult long run when she’s afraid they may not be recovered enough to do harder intervals or tempo runs.

2. They can make you faster

Fartleks are a great way to improve your VO2 max (the gold standard for measuring cardio fitness) and increase your lactate threshold (the point at which lactate accumulates faster than your body can handle, leading to fatigue), according to the National Strength and Conditioning Association. One small 2020 study found that regular Fartlek training can also significantly improve muscular endurance¹.

What makes Fartleks unique is that they keep your heart rate high, rather than letting it come all the way down between intervals. This trains your body (and mind) to push hard even as you get tired. “Your body adapts to running faster even when you’re fatigued because you’re running throughout the workout—you don’t get a full recovery,” says Kunz. This can be especially helpful practice for races, where you may want to speed up for a bit to pass other runners, and then still keep running to the finish line.

3. They can help you improve your running form

Spending some time running faster can make you run more efficiently. “You’re improving your biomechanics,” says Kunz. For example, to speed up, most of us naturally pick up our knees more, lean forward, and increase our cadence. All of these add up to a better running form.

4. They can help you rediscover the fun of running

Intervals typically feel like hard work. But a Fartlek can make running feel more like play. “This is a workout that really should leave you feeling good and invigorated,” says Dworecki. And for runners who normally only do steady-paced runs, a Fartlek can switch things up in an unintimidating way.

How do I start Fartlek training?

For beginners, both coaches recommend starting with Fartleks by adding three or so faster segments into your regular run. Each burst should be about 100 meters or a minute long. Then give yourself three to five minutes to recover in between. Don’t worry if that feels too easy. “If you feel like you can do more, then the next time do a little bit more. But it’s always better to feel good at the end of your workout rather than risking injury,” Dworecki says.

Advanced runners can work about eight or so faster segments into a Fartlek run. Dworecki suggests keeping each between one and two minutes and doing them at whatever pace feels good to you, with one minute of recovery in between.

No matter your level, remember the basics: Make sure to start with a solid warmup of pre-run dynamic stretches then at least 10 minutes or one mile of easy running, says Kunz. After the speedy stuff is done, finish off with another 10-minute cooldown of easy running.

How to make the most out of Fartlek training

Even though Fartleks are a more casual approach to speed training, they still take a toll on your body. Use them conservatively, warns Dworecki. “It’s not a workout you want to do every day because you can overdo it,” she says. Any kind of speed work—whether that’s Fartleks, intervals, or tempo runs—should only be done about once a week, or twice at most if you’re a more experienced runner.

And remember: Don’t take your Fartleks too seriously. These workouts should add a sprinkle of playfulness to your running routine. “You don’t want to go too strict on it,” says Dworecki. Instead, just have some fun seeing how fast you can go.