Fartlek, which means "speed play" in Swedish, is a training method that blends continuous training with interval training. The variable intensity and continuous nature of the exercise places stress on both the aerobic and anaerobic systems. It differs from traditional interval training in that it is unstructured; intensity and/or speed varies, as the athlete wishes. Most fartlek sessions last a minimum of 45 minutes and can vary from aerobic walking to anaerobic sprinting. Fartlek training is generally associated with running, but can include almost any kind of exercise.
Gösta Holmér 
Swedish coach Gösta Holmér developed fartlek in 1937, and, since then, many physiologists have adopted it. It was designed for the downtrodden Swedish cross country running teams that had been beaten throughout the 1920s by Paavo Nurmi and the Finns. Holmér's plan used a faster-than-race pace and concentrated on both speed and endurance training.
Fartlek sessions 
This is the first session that was designed by Gösta Holmér for a cross country runner. This is also an example of what a fartlek session might look like, but fartlek sessions should be designed for an athlete's own event or sport, as well as catering to their individual needs. Sessions should be at an intensity that causes the athlete to work at 60% to 80% of his or her maximum heart rate. This should mean that the body will not experience too much discomfort while exercising. An athlete should also include a good warm up at the beginning of the session, and a cool down at the end of the session, to improve performance, minimize post-workout muscle soreness, to decrease the chances of injury and for other reasons.
- Warm up: easy running for 5 to 10 minutes.
- Steady, hard speed for 1.5–2.5 kilometres (0.93–1.6 mi); like a long repetition.
- Recovery: rapid walking for about 5 minutes.
- Start of speed work: easy running interspersed with sprints of about 50–60 metres (160–200 ft), repeated until a little tired.
- Easy running with three or four "quick steps" now and then (simulating suddenly speeding up to avoid being overtaken by another runner).
- Full speed uphill for 175–200 metres (574–660 ft).
- Fast pace for 1 minute.
- The whole routine is then repeated until the total time prescribed on the training schedule has elapsed.
Fartlek in American culture 
The fartlek method of training was introduced to the United States in the 1940s. Fartlek Hill in Quantico, Virginia, on the grounds of United States Marine Corps Officer Candidates School, is named after fartlek training. The hill is the central part of the fartlek-type physical training regularly conducted throughout the training cycle. The fartlek training done at Officer Candidate School differs from traditional fartlek training, however, by incorporating a number of calisthenic exercises at various intervals.
See also 
- McArdle, William D.; Katch, Frank I.; Katch, Victor L. (2009) . "Training for Anaerobic and Aerobic Power". Exercise Physiology: Nutrition, Energy, and Human Performance (7th ed.). Lippincott Williams & Wilkins. p. 483. ISBN 978-0-7817-9781-8. Retrieved May 7, 2012.
- Barker, Jill (29 March 2011). "Making the leap from lazy jogging to real racing". Vancouver Sun. Archived from the original on 4 April 2011.
- McDonald, Lyle (1998). The Ketogenic Diet: A Complete Guide for the Dieter and Practitioner. p. 247. ISBN 978-0-9671456-0-0.
- Schatzle, Jr., Joe (November 2002). "Finding Fartlek: The history and how-to of speed play". Running Times Magazine.
- Fick, Nathaniel (2005). One Bullet Away: The Making of a Marine Officer. Houghton-Mifflin, Mariner. ISBN 0-618-55613-3.