Fartlek (Swedish for “speed play”) is a form of interval training that involves changing from faster and slower paces.
Fartlek training is commonly applied to running. An example of Fartlek training is the runner deciding to sprint to an obstacle in the distance, then slowing down to another obstacle past that one, then running at a medium pace to yet another obstacle, running sideways to an obstacle, etc. in a playful manner rather than keeping track of metrics.
This type of training does not require timers or rep counting like other forms of interval training.