Our muscle fibers can generally be classified as fast-twitch or slow-twitch:
Slow twitch fibers (Type I): These muscle fibers can carry more oxygen and sustain longer periods of aerobic activity (activities that require your muscles to use oxygen) than other types of fibers, using fats or carbs as fuel. They can contract for long periods of time, but are weak. Think: long distance running or hours of cardio.
Fast twitch fibers (Type IIb): These muscle fibers can carry less oxygen and only work with short periods of anaerobic activity (activities that require your muscles to burn glycogen) before becoming exhausted. They have the greatest potential for strength and for gaining size. Think: sprinting, powerlifting, strength training.
Fast twitch fibers (Type IIa): These are a mix of Type I and Type IIb fibers, and thus can be used for either aerobic or anaerobic activities.
We can actually change our muscle fibers based on our training. It might be a difficult task but Body type, metabolism, muscle fibers, can be altered by our sheer work and dedication.
We can change our size and the percentage of our muscle fibers with the right training, just like we can change our body composition with the right diet.
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