Fast Twitch vs. Slow Twitch: What Kind of Muscles Do You Want?

Alright guys, this article is going to jump into the more scientific side of fitness, but it’s still fascinating and something you should know.

Ask yourself this question: Do you want to be able to run ridiculously fast for a short period of time, or do you want to be able to run for hours and hours without wearing down?  Believe it or not, your muscles are composed of certain fibers that are already destined to be one or the other.  However, with enough proper training you can actually control how much of each type of fiber you have in that body of yours.  What are these fibers?

Type I Fibers

Type I fibers are “slow twitch” fibers.  They use oxygen to fire, and they take longer to get going, but they can go for a longer period of time without getting tired.  The force per contraction on these muscle fibers is spread out over time.  People who have trained for marathons and enjoy running long distances (not me) will have a higher percentage of Type I fibers (that are more efficient) than a sprinter or a couch potato.

Type IIb Fibers

Type IIb fibers are “fast twitch” fibers.  These fire anaerobically (without oxygen), they fire extremely quickly, but they get tired easily.

Sprinters and most fast animals in nature (cheetahs, lions, deer, etc.) will have more “fast twitch” fibers: longer periods of rest, followed by ridiculous amounts of quick speed and energy.  Guys loaded with Type IIb fibers will tire more easily on long distance runs, but they can definitely beat your ass off the line in a quick race.  Because it’s such a great amount of force in such a short amount of time, these fibers are also used in weight training.

Type IIa Fibers

Fence riders…halfway between type I and Type IIb.  These are equal parts aerobic and anaerobic.  Not great at long distances, not great at sprinting, but pretty good for either.  The “jack of all trades” muscle fiber, if you will.

Those are your three different types of muscles fibers. You’re born with these fibers in certain proportions, and they will affect how successful you are at either developing as a long distance guy, or a sprinter guy.  Most bodies have 50% of Type 1 and 50% of Type 2 (A and B), but many elite athletes (world class marathon runners, Olympic sprinters) can have up to 80% of one or the other.  Obviously a sprinter with 80% fast twitch fibers will have a better chance of being fast than somebody with only 30% fast twitch fibers.

Your muscle fiber composition is already built into your genetics.  It’s like picking a random race whenever you start playing an RPG: initially, you already have certain strengths and weaknesses (stamina and strength vs. intelligence and wisdom, etc.), but by the end of the game you’ve done enough leveling that you can erase those differences and mold your guy into exactly what you want.

The real world is no different.  This study suggests that it’s possible through training to adjust the levels of each of your types of muscle fibers.  If yo genetics say you should be a sprinter, but you really want to run marathons…enough training will make your muscles conform and function better with increased amounts of Type II fibers.  Of course, had you been born with 80% slow twitch muscles your path to better marathons might be easier, but you can still get there…you just need to bust your ass and work harder!

Moral of the story: your genetics might have you already set up to be better at one thing or the other, but hat doesn’t mean you don’t have a choice.  Do what makes you happy, and with enough persistence and solid training you can control how your muscles function.  Personally, the thought of running really fast excites me more than running for a really long time, so that’s how my training is directed.  Plus, looking like a sprinter isn’t a bad side effect.

Thanks Mr. Wizard!

-Steve

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