Jill Tucker, RN says:

“American Mark Geiger has already earned plaudits for his work as a referee during the World Cup. Keeping up with the best soccer players in the world requires the kind of stamina that can only be attained by adhering to strict methods of exercise. Such as ‘fartlek’—not the punchline of a schoolyard joke but a form of Swedish interval training.”
Endurance Key for World Cup Refs

ESPN.go.com  June 10, 2014