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Fartleks vs. Intervals: Which is Better for Speed Training

Speed work is an important part of any runner’s training, whether you’re preparing for a 5k or an ultra marathon. There are a lot of ways to do speed work though and no one-size-fits-all answer. Two of the most popular choices that runners decide to incorporate into their routines are fartlek training and interval training.

Fartlek training involves increasing and decreasing speed while on a continuous run, while interval training involves running at a quick pace for a short period of time and then coming to a full stop for a break.

There are a lot of great reasons to choose either fartlek training or interval training — that’s why they’re both so popular. The one you prefer will depend on your goals, where you are in your training, and the types of running you prefer. Let’s dive into both types of training and consider when you use one or the other.

What is Fartlek Training?

Fartlek training involves going on a continuous run while increasing and decreasing the speed and intensity throughout. The word “Fartlek” is Swedish for “speed play,” so you can think of it as having fun with different speeds as you run. For example, you might decide that you want to go for a 4 mile run. After a warm up, you decide to do 25 minutes of Fartlek work. You’ll do 2 minutes of slow running and 45 seconds of fast running until the 25 minutes are up and you complete whatever is left of your 4 miles at a cool down pace (source).

What is Interval Training?

Interval training involves short bursts of intense and fast running followed by periods of short rest or light work. Interval training was popularized in the 1950s by Olympic champion, Emil Zatopek who emphasized how important it was to engage in intense bursts of running without letting the body fully recover afterwards (source). Even though runners come to a complete stop in between intervals, these breaks aren’t long enough to let the body fully recover from the intensity at which the runner was just running. These short breaks help increase lung capacity, stamina, and the ability to push through. It’s important to strike a balance though so that the breaks aren’t too short as to leave the runner too tired to complete the next interval at high intensity, nor too long as to let the body fully recover.

What are the Benefits of Both Fartlek Training and Interval Training?

Physically, Fartlek training and interval training have a lot of the same benefits and were both popularized for their abilities to improve running performance and endurance. Even without seeing the data, it makes sense. There’s a reason why intense HIIT workouts have taken off and praised for being so effective.

Personally, I would never train for a race of any length without having a speed workout in my regular schedule. While it’s hard to pinpoint any one factor when looking at improvement in running performance I’m pretty confident that my regular speed work is what has help make me a better runner.

Also, as I talk about in my article about speed work for long distance runners, it helps put some power in my fluffy marathon legs. 

Firstly, both types of training improve VO2max, which is our maximum rate of oxygen consumption (source, source). The higher one’s VO2max, the greater their abilities to efficiently utilize oxygen to power the muscles and maintain performance. Basically, VO2max is used as a measure of aerobic endurance abilities.

Secondly, both types of interval training increase lactate threshold (source, source). Lactate threshold is the level at which lactate starts to enter the blood. You know the awful feeling of muscle cramps? You can thank lactic acid for that. Through Fartlek training and interval training, runners can increase the threshold at which lactate starts to build up, which can decrease cramps and increase performance.

Thirdly, both types of training increase ventilatory threshold (source, source). Ventilatory threshold is the point at which breathing rate starts to increase, eventually to the point where breathing becomes labored and challenging. Basically, you can delay the point at which you become breathless when you’re running with some good speed work. 

Speed work tends to have the same benefits, regardless of the exact method you choose. Whether you go with Fartlek or interval training, you’ll have similar results if done correctly. So, why do two separate methods even exist? How are they uniquely different and beneficial? Let’s find out.

What Are the Benefits of Fartlek Training?

One of the best reasons to choose Fartlek training over interval training is that it simulates a real race more so than an interval run does. Of course, you want your pace to stay more consistent in a real race than in a Fartlek run, but a Fartlek gets you out on the roads and simulates the endurance you need to push through a full multi-mile race without stopping. This means that Fartleks have to be more balanced and controlled than intervals. When doing a Fartlek, it just isn’t possible to run at max capacity during your speed chunks and then continue on for more miles without a break. Even when running quickly, you still need to keep your energy under control so that you can finish your run.

The speed chunks in Fartlek workouts are meant to be faster than your normal pace, but not an all-out effort. This is actually good mental training in how to understand your body’s limits and know what pacing is appropriate given the overall length of your run. If you run too fast in your first 2 minute speed chunk, how are you going to finish the rest of your run? Fartleks can help you understand the balance between pushing yourself and conserving your energy.

What Are the Benefits of Interval Training?

Interval training will get you accustomed to fast bursts of speed. How does it feel to run as fast as you can with only a few minutes of rest in between? Mentally, it’s great to know where those extra stores of willpower are hiding and be able to access them when you need a jolt of energy. Feeling tired, yet pushing through to run as fast as you can will take some mental effort and stamina.

Mentally, there’s a big difference between running as fast as you can for 800 meters and then stopping instead of running quickly over the course of a couple of miles. By default, your brain will be more comfortable going at an intense, all put pace because it will know that there’s a full break ahead. It’s hard for the brain to do this when there’s a number more miles to go in a fartlek. By default, you will get more intense speed work from interval training.

Should You Choose Fartlek Training or Interval Training?

Both! Fartlek training and interval training are both great ways to do speed work and have many of similar benefits, physically. My suggestion is to switch between the two depending on your goals. You may prefer more periods of structured interval training when you’re looking to work on your quick take-offs and sudden bursts of speed. You may also prefer intervals when you’re struggling to motivate yourself, as intervals tend to be more structured than fartleks.

On the other hand, you may want to incorporate some fartlek training sessions when you’re looking to work on your mid-run endurance and speed boosts. Mentally, if you struggle with mustering up the motivation to kick it into high gear during the tough part of a run, fartleks can help you learn to push yourself through the pain without stopping.

How Often Should You Do Fartlek Training or Interval Training

The frequency of your fartlek or interval sessions depends on the rest of your running program. Generally, running programs like to incorporate speed training once or twice a week. This isn’t necessarily a case of “the more the better.” As with anything running related, you need a well-balanced plan that prevents injury and fosters your improvement. Speed work is no joke from a physical perspective. It will tax your body and require that you recover properly afterwards. 

Personally, I tend to do one speed workout a week and like to sandwich it in between an easy run and a rest day. Even after doing speed work for quite a while now, I still get pretty sore and need the recovery time. Experiment with your running schedule and see where a speed workout would fit in best. Be careful not to overdo it though and give your body enough time to get used to it.

Both fartlek training and interval training have their merits and their place in a well-rounded running routine. Try them both out and switch between them when you see fit. Either way, you’ll find a lot of gains in your running performance and endurance. You’ll also feel like a superhero when you’re running as fast as you can, even if your spouse says you look like a cute dork.