In the 1920's, the top distance runners were coming out of Finland. They did one thing differently in their training from everyone else: they ran on trails. This did not catch on elsewhere for a couple of reasons. First, logic dictates that if one races on tracks, one should train on tracks. Second, trails are irreproducible, but one mile on one track is essentially the same as one mile on any other. The Finns claimed that the advantage of trails was in decreased frequency of injury (actually, there's merely a switch from impact shock injuries to torsion injuries), but the advantage may have been more in recruitment of more muscle fibers to aid in balance and in better body awareness. That there were a few world class runners living near each other probably had a greater effect than their training methods.
Paavo Nurmi had two trail loops he used for training, one about 2K, one about 3K. These had hills of differing length and grade and terrain that included rocks, sand and mud. In winter, he ran on snow and ice. He would run multiple loops of these courses, keeping track of specific workouts. By racing frequently, he was able to compare workouts with races and thus know from his workouts what kind of racing shape he was in. By running the same workouts regularly, he was able to chart his progress and knew that when his times were not improving, that he was reaching a peak and should go for a record time in a race.
Those who prefer training on trails tend not to run the same trails all the time, but seek variety and thus lose the ability Nurmi had to track progress. Fortunately, technology has made a dramatic change in accountability possible. Global positioning systems allow runners to measure the distances they run on trails; though they are notoriously inaccurate in valleys, they do allow runners a good sense of how far they've run. Also, heart rate monitors have allowed runners to attempt to run at a constant effort by running at a constant heart rate. Runners can find what their heart rate tends to be during specific road and track workouts, then use that information to reproduce the same workout on more difficult terrain, getting the same benefits while running at a slower speed.
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