Third, I personally am not convinced that a high percentage of ST fibers is essential for success in prolonged endurance activities like marathon running. I suggest that the different muscle fiber compositions found between middle- and long-distance runners are artifactual and that within the next 20 to 50 years, the muscle fiber compositions of elite marathon runners will be found to approach those currendy found in elite middle-distance runners (i.e, approximately 50% FT fibers). My reason for believing this is simply that those athletes who currently excel at the marathon distance probably do so because they are not quite fast enough (possibly because their muscles have too few FT fibers) for success in middle-distance running, in particular the 1-mile race. A good example is the former world marathon record holder Derek Clayton, who chose to specialize in the marathon when he realized he could not win middle-distance races or succeed in the mile (see post 8).
Thus, I suggest that selection pressures force these “slow” milers with low percentages of FT fibers to compete at the longer distances at which faster milers, who have faster muscles and higher percentages of FT fiber content, do not choose or indeed need to compete.
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However, as the longer distance races, especially the standard marathon, become more lucrative and therefore more attractive than track running, these elite middle-distance runners with higher percentage FT fibers will begin to dominate these races. Thus, a repeat of the original study of David Costill and his colleagues on the elite American marathon runners of the 1970s (Fink et al, 1977) done in 10 or 20 years may show different results and could indicate that athletes with even mixes of ST and FT fibers are probably better able to succeed in marathon and ultramarathon races than are those with high percentages of ST fibers. Data on world-class black distance runners show this to be the case; these runners have 40 to 60% FT fibers (Noakes et al, 1989d).
Understanding the different muscle fiber types helps us to appreciate that during exercise the fibers in the active muscles have specific activation patterns. This is a function both of the type of exercise and of its intensity and duration. Thus, during low-intensity running, ST fibers are initially active, but as the exercise intensity increases, a greater number of FT fibers become active in the sequence of FTa FTb (Saltin, 1981). Similarly, during prolonged exercise, a recruitment pattern occurs with the ST fibers being activated first. As the ST fibers become progressively energy depleted, the FTa fibers become active, followed finally by the FTb fibers. Thus the noncompetitive jogger who exercises at a low intensity for a short duration will train predominantly ST fibers, whereas the middle-distance runner who includes high-intensity training will train both ST and FT fibers.
Logically, we would think that optimal training should be at all running intensities so that all muscle fiber types are equally trained. Indeed, the success of the “peaking” training technique developed by Forbes Carlile (1963) and Arthur Lydiard (Lydiard & Gilmour, 1978) and described by Daws (1977), Osier (1978), and others (see post 5) may lie in the fact that the athletes achieve optimum training of FT fibers.
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