FARTLEK TRAIL RUNNING: WHAT IT IS AND HOW TO TRAIN WITH IT, BY MAYAYO.

Posted by sergio mayayo on February 2, 2026 in fartlek, fartlek trail running, trail running, trail running spain | 1 Comment

FARTLEK TRAIL RUNNING: Train your mountain races with speed play. Our TRAIL RUNNING TRAINING section looks today at how to apply this “speed play” created a hundred years ago in Sweden to improve our mountain races today.

We kick off with a veteran video of Mikel bringing humor to fartlek, then continue by analyzing what it is and how to apply it today, with Mayayo.

Read this in spanish at CARRERASDEMONTANA.COM

FARTLEK TRAIL RUNNING:

Train your mountain races with speed play

Fartlek training was born for cross-country running in 1930. This training model is about to celebrate its first hundred years, as it was developed in the 1930s by Swedish coach Gösta Holmér to improve the speed and endurance of his cross-country team.

As an athlete he had won a bronze medal at the Stockholm 1912 Olympic Games, and when he moved into coaching cross-country he created fartlek as a flexible alternative to rigid interval training, involving running at varied paces in natural, forest environments. In doing so, he revolutionized Swedish athletics, with several world records in the early 1940s.

FARTLEK: WHAT IT IS

The original idea was born to break two chains suffered by cross-country runners in training and which, over the years, I think have also crept into trail running: the always-the-same steady run for recreational runners, and the perfectly measured interval for elites. Fartlek proposes the exact opposite: alternating pace changes within a continuous outing, using terrain, wind, body, and mind as a metronome. You decide when to push, how much, and how you return to calm… but you decide with intention, not on a whim. And all of it with a focus on having fun running, playing with effort and terrain.

Gosta Holmer

Let’s not confuse fartlek with “running like a madman”.

In the hands of a veteran runner, fartlek is used as a precision tool: you cut where it matters, not where it makes you bleed. You change intensity to touch different energy systems without breaking the continuity of the running stride. (See below: “Metabolic Flexibility”.)

Applying that principle to our mountain races, I’m sure you’ll agree we have an indecent advantage: the mountains already come with the switches installed. A hill, a broken trail, a false flat that bites, a technical descent that demands quick feet… The way I see it, the mountain asks for changes; fartlek teaches you to turn them into training.

From Irún to Cercedilla.

In fact, whenever we go running around Cercedilla with my friend Alejandro, he gives me a practical lesson, throwing in a few surges purely for pleasure, just to have fun. It’s been a long time since I’ve accepted any of those trap-bets like… “Let’s see who tops that hill first” or “Bet you can’t finish this descent at the same time as me.” When he accelerates, I let him go, with healthy envy, I confess. Shortly after, we regroup and continue the route.

As applied by Mikel from Irún or Alejandro in Cercedilla, trail running fartlek is a flexible form of interval training. At times you brush or exceed your “threshold” zone (that pace you could sustain for a good while, but not all morning), and in between you recover without stopping, returning to a comfortable intensity. You don’t need a track or a watch barking orders at you.

You need judgment: knowing what you want to stimulate today and what you’re willing to pay tomorrow. Because at a certain sporting age, the best plan is not the most rigid, but the one you survive week after week without getting injured or hating the sport. And fartlek, well done, is the art of pushing without breaking.

FARTLEK TRAIL RUNNING: WHY IT WORKS

It works because it resembles racing, but without the emotional tax of a bib number. In mountain races you rarely run at a constant pace. You climb, crest, accelerate on runnable sections, slow down to negotiate a descent, push again when the trail allows, and stop to drink or dodge someone in a bottleneck. Fartlek reproduces that pattern of variable demand, and the body learns what it practices.

At a physiological level, this game is no game. When you alternate hard segments with easy ones without stopping, you train the ability to produce energy at high intensity, as well as to clear and reuse metabolites. You improve your tolerance for hard effort and your ability to recover while running. For me, being able to apply that active recovery is gold in ultras and alpine marathons. I’m not going to achieve my challenge thanks to my best minute, but I can certainly lose it because of my worst twenty.

I see a second reason why fartlek fits us like a glove: the terrain forces you to vary mechanics, no matter what. The flat asks me for running economy; the climb demands power and efficiency; the descent requires control, elasticity, and eccentric musculature capable of braking without disintegrating.

A well-designed fartlek session in the mountains should include micro-doses of all that without turning the workout into a catalog of torture. The bet is that over time, that variety builds specific resilience: the body doesn’t panic when the script changes, because it already lives in change on every run it does.

Having fun also counts

Performance in the mountains is a mix of engine and willpower, but willpower is a muscle that also gets fatigued. Fartlek trains decision-making under stress: “I push until that bend,” “I hold to the col,” “I recover here without switching off,” “On this descent I run fast without losing technique.” That constant dialogue between ambition and prudence is the key to racing well.

Not to forget, as Mikel pointed out, that recreational trail runners are a bit like birds: anarchic by nature, and fartlek adapts brilliantly to our real day-to-day training. With heavy legs, you can make the changes shorter or less aggressive; if you’re fresh, you extend a hard block. If you’re running through mud, the session becomes technical. If it’s windy, you play with exposure. You can adjust the load without giving up the objective, which allows you to stack good weeks, the kind that truly build a more complete mountain runner.

Mikel and Mayayo, team bronze at Ultrafiord

FARTLEK TRAIL RUNNING: THREE EXAMPLES.
TIME, TECHNIQUE, OR ELEVATION

TIME-BASED FARTLEK: The watch as a traffic light, not a chain

You head out for a 75 to 90-minute run over mixed terrain, preferably with short climbs and some runnable sections. Warm up for 20 minutes easy, then enter the main block: alternate 2 minutes “lively” with 2 minutes “comfortable,” repeating the pattern for 30 to 40 minutes. “Lively” here is not a sprint: it’s a controlled effort, that point where you’re breathing hard but could still say a short sentence without sounding like an asthmatic wild boar. Recovery is active: you keep running, but clearly lowering the heart rate.

What’s it for? To fine-tune threshold and the ability to change pace without losing economy. It’s a “civilized” fartlek, excellent during building periods or when you want quality without the mental stress of measured intervals. And if you do it on trails, the bonus is learning to sustain a high pace with variable footing, which is where you lose those “free” seconds in races.

TECHNICAL FARTLEK: Playing on climbs and descents

This one separates the trail runner from the “runner who also goes downhill.” You look for a climb of 3 to 6 minutes and a technical descent of 2 to 4 minutes, repeatable and safe (no gambling with your teeth). Warm up well, then do a block of 6 to 10 repetitions where you climb at moderate intensity (not all-out) and descend at high intensity but with absolute priority on technique: gaze 2–3 meters ahead, stable torso, short steps, high cadence, “light” feet, no heel-striking as if you wanted to dig trenches.

For veterans, the trick is dosing: if you notice technique degrading, you stop the block. Don’t “push” when you’re already clumsy; poorly managed fatigue on technical descents is a workshop for injuries. Managed well, on the other hand, you gain confidence and real speed, the kind that translates into minutes in races.

ELEVATION FARTLEK: The mountain “accordion”

This is the most trail-like fartlek of all because it uses the profile as a score. You choose an undulating route of 60 to 120 minutes, with a succession of hills or ridges. The rule of the game is simple: on every climb you run “strong but controlled” until the top; on every flat or false flat you run “steady” (without switching off); and on every descent you run “easy technical” or “lively technical” depending on difficulty, but always without breaking technique.

The goal is for intensity to be dictated by the terrain, not the ego. You climb with intent, crest without stalling, and use runnable sections to consolidate pace. Personally, sudden changes in gradient choke me, even bringing on cramps. That’s why this session teaches a crucial trail skill: changing gears.

Some runners climb well but stall at the crest; others descend like meteors but crawl on false flats. The “accordion” trains you not to give away those transition moments where far more is decided than it seems.

FARTLEK AND TRAIL RUNNING: CONCLUSION

Whether with Mikel’s approach from Irún or Alejandro’s from Cercedilla, the truth is that the speed play created by the Swedes adapts especially well to veteran recreational runners who want to maximize the method without paying absurd tolls.

In the end, the way I see it, mountain fartlek is just that: learning to play with effort… without letting effort play with you. If you’ve already used it, leave Mikel and yours truly a comment telling us how your experience went.