Interval sessions can sometimes feel monotonous for trail runners. Fortunately, there is a more enjoyable alternative that is just as effective. It's called fartlek — a form of interval training based on the runner's feel. Find out how to use fartlek in trail.
What is fartlek?
Fartlek was developed by Swedish athlete Gösta Holmér in the 1930s. It comes from the Swedish words "fart" (speed) and "lek" (play). It therefore translates as "speed play".
This training method is built on pace variation. The principle is to alternate fast and slow phases, without following a strict plan or depending on a stopwatch. The runner adapts their effort according to how they feel or the terrain. The structure therefore differs from that of a classic interval session.
Originally designed for cross-country and middle-distance runners, this method is now hugely popular among trail runners. It is an excellent way to train for trail in the city or out in nature, since visual markers (lamp posts, trees, bends…) can serve as reference points. Fartlek effectively replicates the demands encountered on the trails, where changes in gradient and terrain require constant pace adjustment.
The benefits of fartlek for the trail runner
Fartlek in trail has numerous benefits. First, it helps develop adaptability. Without the pressure of a stopwatch, you focus more on how you feel. You get used to constantly adjusting your effort and changing pace according to the terrain. The body learns to calibrate its pace according to elevation change, which is essential in trail.
Fartlek also encourages self-awareness and autonomy in effort management. Unlike traditional interval training, you learn to run without fixed markers and to adapt your decisions to how you feel in the moment. This builds confidence, focus and race-day responsiveness.
Finally, from a physiological standpoint, soft, varied terrain is less damaging to tendons and joints than track or tarmac. Fartlek develops aerobic power while offering more freedom than highly structured interval training. It also improves cardiovascular endurance, the ability to sustain high intensities, running economy and recovery after a change of pace.
How do you build a fartlek session for trail?
Instinct fartlek
Several types of fartlek session can help you improve in trail. Instinct fartlek is the most free-form. You choose all the session parameters yourself: intensity, effort duration, recovery duration… The aim is to alternate paces according to your sensations and the features of the course.
During these sessions, the ideal approach is to set yourself visual markers. For example, you can accelerate to the next big oak tree, recover to a stream, push again to a hiking sign, and jog to the next junction. Also rely on your feel. Accelerate when you feel strong, drop back to an easy pace when the effort gets tough, and alternate freely like this for 30 to 45 minutes.
Structured fartlek
Structured fartlek is perfect for building endurance and threshold fitness. It is built around precise time intervals but remains more flexible than a classic interval session. It allows for more targeted work than instinct fartlek, but requires more planning since the exact session content must be defined before you start.
You can use pyramid formats of 1-2-3-2-1 minutes of hard effort, separated by 1 to 2 minutes of active recovery. Block formats are also very popular. For example, chain 10 one-minute repetitions at a strong pace followed by one minute of active recovery on technical trails.
Terrain fartlek
Another type of session is the terrain fartlek. Highly effective for trail, it involves chaining repetitions over a hilly course or on a pre-selected climb. Here, the terrain dictates the intensity.
You need to push on the climbs, maintain a sustained effort on the flatter sections and recover on the descents. On steeper slopes, it can also be worth hiking on trail to keep intensity high without going into the red. You can therefore run fast on the gentle uphill gradients, power-hike on the steepest climbs, accelerate again at the top and then recover on the descent.
What mistakes should you avoid?
Fartlek in trail is highly effective, provided you respect a few key rules. Going out too fast is one of the most common mistakes. If you sprint flat out from the very first block, you risk blowing up after just 15 minutes. Build up progressively so you can chain together multiple pace changes throughout the session.
Skipping the warm-up is equally inadvisable. Fartlek involves phases of sustained effort. To prepare the body and reduce the risk of injury, a progressive warm-up of 10 to 20 minutes is recommended. For the best training conditions, make sure you also have quality trail clothing and trail shoes.
Just like the warm-up, recovery is essential during any training session. Skipping it prevents your heart rate from coming down properly. After your session, allow a few minutes of cool-down to aid recovery and reduce the risk of muscle soreness.
Fartlek in trail is an excellent method, as long as you know how to use it. If you want to vary your training and break free from track interval sessions, it is the ideal alternative for making lasting progress.
Photo credits: Delphine Daniélou
