So, while the basic composition of muscle fibers is predetermined, training can have some effect.
As much as I’ve gotten faster doing various fitness programs, I always seem to lag a little behind the folks on the DVD.
Later we will discuss how fast twitch fibers can be further broken down into Type IIa and Type IIb.
But, in general, a successful sprinter will have a much higher than average percent of fast twitch muscle (say, 80%), while a successful marathon runner will have a much higher than average percent of slow twitch muscle (again, around 80%). Also, it’s easier for fast twitch muscles to become slow twitch muscles, rather than the other way around.
If you just train longer and harder, you are basically building endurance – in other words, slow twitch muscle. Slow twitch muscles are your endurance muscles. They are the most efficient at using oxygen to generate more fuel (ATP) for continuous, muscle contractions. Some folks have designed very specific training regimes designed specifically to increase fast twitch muscle, but that is beyond the scope of this article. They fire more slowly than fast twitch fibers and can keep going for a long time before they fatigue.
Although everyone can add muscle mass, and that muscle mass will be composed of both fast and slow twitch muscle, the percentage of one to the other holds fairly constant for each individual. Fast twitch fibers use anaerobic respiration (not using oxygen) to create fuel, making them much better at generating short bursts of activity than slow muscles. Fast twitch muscles typically produce the same amount of force per contraction as the slow twitch type – the main difference is how rapidly they are able to fire.
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