Fartlek is a word that derives from Swedish and means "Speed game" and in fact it was born in Sweden. Moments of high intensity running alternate with moments of recovery run.
The fartlek differs from the classic repeats in that it has no distances or speeds decided a priori. Therefore we will not speak of a predetermined external load but we will speak of internal load so we run using our own perception of fatigue.
What we can vary is the perception of fatigue in the intense running part: medium intense, at 70% of one's fcm, intense, at 80% of one's fcm, very intense, at 90% of one's fcm, and then return to a smooth running pace.
So in the fartlek there can be fixed time intervals or the rhythm variations can be managed by sensation, that is, following one's own state of form or adapting to the type of path (uphill, downhill, flat, dirt road, etc.)
This type of training mainly develops the ability to manage energy and a good sensitivity to rhythm. From a physiological point of view, it certainly acts on aerobic power and helps to increase speed in competition.
