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!


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    28 thoughts on “Fast Twitch vs. Slow Twitch: What Kind of Muscles Do You Want?

    1. Hard to change though.
      Prof Boreham. “If you do a lot of training you can gradually change
      the fast fibres to the slow type. This may take several years of chronic
      training six or seven days a week, hours each day. But you can’t
      do it the other way around, you can’t change slow to fast – so for
      sprinters or middle distance runners who do a lot of training, as the
      years go by they tend to go up a distance”

    2. I can deadlift 500, bench press 315, overhead press 225, squat 365 at a bodyweight of 183 aged 18 with a lean defined body and can run 2 hours straight, at a steady, relatively fast pace.

      What does that say about my muscle composition? Type 3 hybrid? Best of both worlds!

    3. Depends upon how fast you can run I suppose. It probably says you’re not going to be elite at either sprint or distance running because like the average person you have a 50/50, or near to it, split of muscle fiber types.

    4. i am trying the new freeletics gym workouts lately and have noticed much more soreness in my muscles after finishing these workouts than I would doing a workout with a 1-5 or even 10-12 rep range. Both types of exercises I use enough weight to go to failure on each set with good form. The freeletics workouts work in pretty high rep ranges. Is the soreness I get any indication of the type of muscle fibers I have or am I reading too much into this?

    5. Sadly, this is bullshit, I can never build enough fast twitching muscles as some guy who has genetics for such body, Ive been weight lifting my whole life, and I never got those lean quick twitch muscles, its just not gona happen, not in a million years. Do what you can be best at, not what “makes you happy”, you’ll get disappointed, because it does matter what card you’ve been dealt, thats just a reality.

    6. Yeah, you’re right. I know this from experience. I’m a white guy who tried to be a sprinter when I was a kid. I just ended up sad and disappointed. Now I’m gonna start trying things that my body is more suited for: cycling, swimming, lifting heavy objects, and maybe other things. White people have a good amount of options in sports, but we have very little chance in really any running event at the top level, from the 100 m dash to the marathon. West Africans or people of West African descent (which includes black Americans) dominate the 100 m to 400 m. East Africans (Kenyans, Ethiopians, etc.) or people of East African descent dominate all distances longer than that. Check out Jon Entine’s book Taboo. It’s an oldie, but a goodie.

    7. Now, I don’t want a lecture on this question I’m about to ask–which muscle type is more prone to bulking? I would guess the fast-twitch fibres, right? So, if you’re prone to growing really big muscles, would you genetically have more fast-twitch fibres? If so, how do you get them to “calm down”–do you do more endurance based activities to help balance them out with the slow twitch? For women, who say want their calves to shrink down so they can wear awesome knee boots, this is an important consideration in fitness. . .thank you.

    8. My trainer just told me about the big an small fibers so of course I had to come home an Google it….while your article is condense it certainly answered alot of my questions, thanks I’m going to post you to my FB page an follow you….

    9. enjoy it while your young…it changes trust me I know…Ihave been wking out since the age of 21 now at 68 I can’t do what I did but I keep striving to do better all the time….

    10. I have to disagree with you on that, My son has become one of the top runners here in Jax Fl….He had never gone to the gym or ran until his late 30’s…overweight an a couch potato until then…He now has the body of a 20 y/o an is placing 3rd in most marathon’s ….he has trained, chg his way of eating an is just simply conditioned his body to be a long distance runner an is amazing…He’s been invited to the Boston an NYC marathon…you can chg your body ,you only have to work at it all the time…lindap

    11. How are you going to introduce oxygen to your muscle to give it the required amount to change from type 2 to type 1 or vice versa no amount of training or will power can do that its down to genetic hardwireing and like those born with downs there sadly isnt anything that can be done yet maybe try injecting air into your leg before running a 5k marathon

    12. I think Etine’s book has been disproven. First off, the tribe in East Africa he points to run EVERY DAY to and from their schools. It is a part of their culture. Sometimes, they run back and forth for lunch. We drive our kids to school, and send them with overly processed junk food for lunch.

      I don’t believe it is totally genetic. And not because you’re white. You just might not have been blessed with enough height, and/or athletic gifts to be a sprinter. There are many white people who are faster than 11 seconds in the 100m. Breaking the 10s barrier is what people crow about. We’re already starting to see faster white wide receivers and running backs enter into the NFL. As black people ascend into more wealth, it will be more telling if they are still dominating in most sports, or if it is then PROVEN to be a nurture, more than a nature, argument.

      I, for one, don’t believe the whole genetic thing. Any more than I believe this article’s main point. I think “certain” humans with robust athletic gifts have the abilities described. Not all Africans, even from those particular tribes, have the traits you’re talking about.

      Only a small fraction of the human population will have the necessary build for their training to maximize their ascent to the highest echelons of sports.

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