There are three types of muscle: skeletal, smooth, and cardiac. Skeletal muscle is what we train when we lift weights. Scientists and exercise physiologists divide it into two types, slow twitch muscle fiber (Type I), and fast twitch muscle fiber (Type II).
To contract and do work, muscle fiber needs energy. The main distinction between Type I and Type II muscle fiber is the metabolic pathway that each uses for this energy. Type I – the slow type – uses an aerobic metabolic pathway; it can contract over and over at a relatively slow rate, as long as oxygen and sugar is available. Type II – fast-twitch fiber – is anerobic; it is capable of quick contraction but it rapidly depletes its internal energy stores.
Type II fiber is grouped further into two (or more) subtypes, Type IIa and Type IIb. Again, the distinction is between the metabolic pathways that the muscle fibers use for energy.
We all have a baseline ratio of Type I to Type II muscle fiber. This ratio is genetically determined. In most people, the ratio of slow- to fast-twitch muscle fiber is around 50/50. In elite endurance athletes, however, it can be skewed as much as 80/20 either way: world-class Olympic sprinters will have 80% fast-twitch fiber in their legs, while the muscles of ultra-long distance runners are 80% slow-twitch fiber. If you want to know what your fast/slow fiber ratio is, you will have to undergo a needle biopsy.
Can you change your ratio of fast twitch to slow twitch muscle fiber through training?
They jury is still out on this one. While there is some scientific evidence to suggest that you can train to alter the overall ratio of fast-twitch to slow-twitch fiber, it is probable that you shouldn’t worry too much about trying to fight your natural body type. Instead, try to maximize what you have and don’t worry too much about it one way or the other. If you are built like a marathoner, you probably won’t become a champion powerlifter no matter how hard you train.
However, science has shown that it is possible to change your Type II muscle fibers from Type IIb to Type IIa and vice versa. What this means is that you can train specifically for strength endurance or maximum strength. Powerlifters train for max strength, while rock climbers try to maximize their strength endurance. You can tailor your training methodologies to maximize your potential in either of these areas of athletic performance.
Skinny guys on a bulking program shouldn’t worry much about muscle fiber type
As you begin to bulk up, don’t pull out your hair worrying about the fact that some people have more fast-twitch fiber than others. Unless you are a body-snatcher, there isn’t much you can do about it; you have to play the hand you’re dealt.
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