Stop giggling — “fartlek” is a word borrowed from Swedish, in which “fart” means “speed” and “lek” means “play.” Fartlek is a type of training for runners that uses quick changes in speed to improve endurance and address psychological aspects of the sport. Fartlek training has become the standard for distance runners hoping to improve their paces before running marathons and ultramarathons. Switching between varying speeds and terrains (including steep uphill and downhill stretches, as well as slow grades) helps runners prepare for the variety of challenges they may face on race day.