Inside the muscle fiber are movement molecules called myosin, which form cross-bridges with a protein filament called actin. Well, skeletal muscle in general is composed of bundles of individual muscle fibers called myocytes. You can’t take a completely red (pure endurance fiber) and turn it into a completely white (fast twitch) fiber but the intermediate fibers (IIa), which would be the various shades you see in the muscle are plastic (changeable) and  can be transformed  into more of a red (slow twitch) version or more of a white (fast twitch) version. A pure white fiber can also  become  a little redder, or  a pure red fiber can become a little whiter. So for these myocytes (muscle fibers), we can break it down into two main categories: slow twitch (Type I) muscle fibers and fast twitch (Type II) muscle fibers.
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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