Final answer:
Marathon training predominantly recruits slow-twitch muscle fibers (SO fibers), which are suited for prolonged endurance activities. Marathon runners ideally have about 80% slow-twitch fibers for the long-distance stamina required. Training can increase endurance features in these fibers.
Step-by-step explanation:
Marathon training primarily recruits slow-twitch muscle fibers (SO fibers). These fibers are highly efficient at using oxygen to generate more fuel (known as ATP) for continuous, extended muscle contractions over a long time. They are different from fast-twitch fibers (FG fibers), which are used for short, powerful bursts of speed but fatigue quickly.
Marathon runners generally have a higher proportion of slow-twitch fibers compared to fast-twitch fibers. The ideal composition for a marathon runner's muscles would be around 80 percent slow-twitch muscle fibers and 20 percent fast-twitch muscle fibers. This composition supports the prolonged endurance required for long-distance running.
Endurance athletes benefit from muscles with a higher proportion of SO fibers, which may be enhanced through continuous endurance training. However, certain genetic factors also determine the ratio of muscle fibers in individuals. Extended endurance training can lead to an increase in the number of mitochondria and the amount of myoglobin within these slow-twitch muscle fibers, making them even more resistant to fatigue.