Ha ha, you said fart! Actually, this is a Swedish word and it has nothing to do with arse. It means ‘speed play’ and is usually used in running to improve your speed and endurance.
In a fartleking session, a runner mixes up speeds as they go. For example, you may sprint as fast as you can from one lamppost to the next, then take it down to a gentle jog, before cranking it up to a medium pace.
Fartlek training is good for improving overall running speed and helps break up the monotony of a regular run. It can be done on all types of terrains, and is less structured than interval training.