How did the Swedish word "fartlek" make it to the English language running vocabulary?

It doesn't appear to be a commonly known Swedish word, but rather a term coined for a specific purpose: Swedish coach Gösta Holmér developed the fartlek training method in 1937.

The method was introduced to the US in the 1940s, after two Swedes made it famous:

The system originated in the 1930s and was made famous in the 1940s by Gunder Hagg and Arne Anderson, who took turns lowering the world record for the mile [in 1942, 1942, 1943 and 1945] as they tried to break the barier of the magic 4-minute mile.

The earliest citation in the OED is from a 1952 Scholastic Coach:

The answer that finally emerged was ‘fartlek’—the Swedish system of training.

An earlier snippet dated 1950 from the American Athletic Journal (by the National Association of Basketball Coaches of the United States and American Football Coaches Association) has a note from the editor saying this training method is spreading globally:

"... For example, it has been proven that it is much better to run at a fast pace for shorter distances, with walking and resting intervals and continued repetition, than it is to jog for too long a time at one pace, since excessive jogging will develop only a jogger and not a runner." (This was written ten years ago, but where may one find a better description and justification of the new training method, Swedish "Fartlek" or speed-play, which is now receiving such worldwide attention -Ed.)

And a book called Fartlek: The Swedish Distance Training Technique, from Track & Field News, March-Sept., 1949 was published in Los Altos, California. Contents:

Swedes alter distance training rules, by C. Nelson.--Fartlek, by G. Holmer.--More about Fartlek, by F. Wilt.--More about Fartlek; why Fartlek is successful, by C. Nelson.--More on distance training, by C. Nelson.--Swedish distance training schedules, by C. Nelson.

The technique was further described in American and British running and physical education publication in the fifties.

I'm Swedish and I've never used or even seen the word fartlek. It's not listed in the most common of Swedish dictionairies - although quite easy to understand in Swedish. "Fart" means speed, "lek" means play or game. So, a car race might be a "fartlek" if it's running for fun. Does that make sense?

I'm afraid it didn't take a great deal to find what it means: http://oxforddictionaries.com/definition/fartlek?q=fartlek

A system of training for distance runners in which the terrain and pace are continually varied

1940s, from Swedish fart "speed" + lek "play"

Since Swedish has an, um, interesting single word for that method of training and English doesn't really, it's not surprising that the Swedish word found its way in.

From wikipedia

Gösta Holmér
Fartlek training was developed in 1937 by Swedish coach Gösta Holmér (1891–1983) and has been adopted by many physiologists since. It was designed for the downtrodden Swedish cross country running teams that had been thrashed 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.

Linking to the article Finding Fartlek
The history and how-to of speed play
By Joe Schatzle, Jr.
As featured in the November 2002 issue of Running Times Magazine

AFAIK the transfer is attributable to trainer Jack Daniels and his popular book 1988 Daniel's Running Formula, in which he describes, how he learned about the training method from a Swedish trainer.

From his blurb:

"In the years between [the 1956 and 1960] Olympics, Daniels studied exercise science at the Royal Gymnastics Central Institute in Stockholm, Sweden, under Per Olof Astrand, one of the world's best sports scientists."