fartlek

Fartlek is a middle and long-distance runner's training approach developed in the late 1930s by Swedish Olympian Gösta Holmér. It has been described as a relatively unscientific blending of continuous training (e.g., long slow distance training), with its steady pace of moderate-high intensity aerobic intensity, and interval training, with its “spacing of more intense exercise and rest intervals.” Simply stated, in its widely adapted contemporary forms, Fartlek training is alternating periods of faster and slower running, often over natural terrain, including both “level and hilly terrain.”

While Fartlek training is generally associated with running, it can be incorporated into almost any kind of exercise. The variable intensities and the continuous nature of the exercise stresses both the aerobic and anaerobic parts of the runner's physiology. It differs from traditional interval training by being less structured.