Until the 1950s, Swedish cross-country skiers relied on natural training. But in the wake of failure at the 1952 Winter Olympics, the Swedish Ski Federation initiated a scientification of training. They sought aid from physiologists to “rationalize” the training of elite skiers. But the advocates of natural training resisted this new, scientific model. A battle was fought between the two sides, and a number of different power strategies were used by the federation and the physiologists to promote scientific training. In this article, the battle between different training regimes is analyzed using theories of power and sportification. The article concludes that the shift towards rational training during the period 1948-1972 was part of a larger rationalization of Swedish society. And the relative slowness of implementation was due to a lack of professionalization (of coaches). Rationalization should therefore be seen as one of the later stages of sportification. QC 20150114Rationell träning: Vetenskapliggörandet av träning för längdskidåknin