Almost every post I read on Fartlek running starts with “Fartlek is Swedish for Speed Play”, so that’s where we’ll start.
Fartlek is, normally, a set of variable length self-paced intervals with variable length self-paced rest, i.e., depending on how you feel you run quick for a bit, then slow for a bit and repeat until you run out of time or enthusiasm.
The literature on Fartlek  is somewhat unsurprising – compared to either continuous running or being inactive – doing variable-length intervals improves your fitness. I haven’t dug deep enough to see if anyone has done a more useful comparison of ‘defined length and effort’ intervals vs. Fartlek, but either way Fartlek is a good way of getting some additional fitness in an engaging manner.
There are a number of variations, but my preferred variation for a group is for the group to decide an order, running in single file if necessary to remember the order, the first runner then chooses when to start the interval, how fast to run the interval and where to end the interval – they do no need to tell the rest of the group their decision, the rest of the group will simply attempt to keep up as best they can. Once the first runner reaches the end point they have selected for their interval they turn around and run to the back of the group.
Using this pattern the group will get stretched out when a faster running is selecting the interval and compress again when a slower run chooses the interval – which means faster runners will have further to run to get to the back of the group than the slower runners.
- Ramya, R. and Dr. Rajalakshmi, D. 2019. “Effect of fartlek training on selected physical fitness and skill performanceintercollegiate level hockey players.”, International Journal of Current Research, 11, (09), 7404-7407.
- Connolly, Luke J et al. “Effects of self-paced interval and continuous training on health markers in women.” European journal of applied physiology vol. 117,11 (2017): 2281-2293. doi:10.1007/s00421-017-3715-9