training can simply be described as alternating periods of faster and slower exercise (i.e., running), intermixed. In this adaptation of these other well-characterized training methods, the interplay between the effort (exercise) and recovery (relief) are not systematically manipulated; instead, the athlete and coach determine the interplay "based on 'how it feels'" during the training. To some extent, in distinction to the earliest forms of fartlek, it's evolution has taken it further in directions away from the track, toward natural outdoor terrain, including both "level and hilly terrain". From the perspective of exercise physiology,[w]hen properly applied, [a fartlek training approach] overloads one or all of the energy systems... [and so] provides ideal general conditioning and off-season training strategies... [adding] freedom and variety to workouts.In relation to continuous and interval training methods, these same authors note that there is "[i]nsufficient evidence... proclaiming superiority of any specific training method to improve aerobic capacity and associated physiologic variables... Each form of training produces success." They go on to argue that, "[o]ne can probably use the various training methods interchangeably, particularly to modify training and achieve a more psychologically pleasing exercise or training regimen." History Gösta Holmér Swedish coach Gösta Holmér developed fartlek in the 1930s; since then, many runners and running coaches have adopted it. It was designed for the Swedish cross country running teams, which had been beaten throughout the 1920s by Paavo Nurmi and the Finnish team. Holmér's new training plan applied a faster-than-race training pace, and concentrated on a mix of speed and endurance training. Early uses of fartlek In the late 1930s, the decade following Finnish runners' supremacy under Paavo Nurmi, Gosta Holmer was the national coach for Sweden (see Gösta Holmér article), and he devised an approach that has been called as "innovative as any idea in athletics’ history", introducing "faster-than-race-pace, simultaneous speed/endurance training" which he termed "Fartlek" (with a capital "f"), meaning "speed play". In Holmér's hands, the sustainable speed of his runners was not achieved through Fartlek workouts alone; his training regimen included "[r]epeat track workouts, tempo runs and time trials... alternated or combined with Fartlek" through each training week. For his Swedish runners, which were world-class, the original Fartlek workout has been described as "a total of 12 kilometres running[,] with up to 5,000 metres... being at faster than race pace." Described by another, a typical workout might be "seven total miles of running with 4,000