Slow-Twitch vs. Fast Twitch Muscle Fibers & Endurance Performance

EndurElite Chief Endurance Officer Matt Mosman discusses the differences between slow and fast twitch muscle fibers, and which type is best suited for endurance activities.

Video Transcript:

What's the difference between slow and fast twitch?

Good morning endurance friends, Matt Mosman, the endurance guru over at EndurElite coming at you with another Endurance Fast Fact.

Today we're gonna be talking about fiber.

Now, not the type that makes you wanna go number two during the most inopportune times during running, cycling and other endurance exercise, we're gonna be talking about muscle fibers.

Now, most of you know muscles are composed of two types of fibers

  • Slow-twitch fibers
  • Fast-twitch fibers.

Now, the amounts that are found in your body are the percentages between slow-twitch and fast-twitch are really gonna depend on a lot of things:

  • Largely it's dependent on genetics
  • It also depends on blood levels of hormones in your body
  • And also your exercise habits.

So, if you're doing more purely endurance exercise or if you're doing more sprinting explosive-type exercises. So, muscle fiber type largely determined by that, and so how do you really find out what percentages do you have in your body?

How to tell if you have fast twitch muscles vs slow twitch?

The best way to do this is to have a muscle biopsy done, and besides having a needle stuck in my armpit once, having a muscle biopsy done is probably one of the most painful experiences that you can go through.

The researcher basically has this big needle, they plunge it into a muscle, like the quadricep, and they take a pinch out of it, like an actual chunk of your muscle and pull it out and then they can test it from there.

But even that has its drawbacks since different muscles in the body are composed of different ratios of fiber types.

So, your lower body will generally have more slow-twitch fibers and your upper body will have more fast-twitch fibers.

What Makes A Slow Twitch And Fast Twitch Muscle Fiber?

So, let's delve into this a little bit more.

What makes a slow-twitch fiber slow-twitch and what makes a fast-twitch fiber well, fast?

It's really dependent on three things:

  • One, is dependent on the oxidative capabilities of the fiber, meaning how many mitochondria are found in the fiber
  • How many capillaries around the fiber
  • How much myoglobin is found in the fiber as well.


Now, myoglobin is something that carries oxygen into the mitochondria. So, that's the first variable that determines what a fiber is.


The second is dependent on a compound is known as ATPase, and ATPase basically breaks down ATP and different fibers will have different amounts that can either rapidly break down ATP or ones that are slower.

Contractile proteins - actin and myosin

And then the third variable that determines what type of a muscle fiber it is is the number of contractile proteins within the fiber, specifically actin and myosin, where you have more of these, basically, the muscle can contract and relax faster and produce more power.

So, those are the three things that will determine what a muscle fiber is.

Type I, IIa, and IIx Muscle Fiber Types

So, let's go into this a little bit deeper.

Type I muscle fibers (slow twitch oxidative)

For your slow-twitch fiber, there's one type and this is type I.

So, type I fibers are very oxidative.

  • So, they have a lot of mitochondria, a lot of capillaries around it and a high-density of myoglobin.
  • Type I fibers are extremely fatigue resistance but they can't produce a lot of force. They contract slowly and relax slowly and that's why they're called a slow-twitch fiber.
  • Now, type I fibers will be found in a lot of long-distance endurance athletes more so than like a sprinter. So, that's the slow-twitch fiber, type I.

Now, fast-twitch fibers can be broken into two different types.

Type IIa muscle fibers (fast twitch oxidative)

You have type IIa, which is fast-twitch and then you have type IIx which we can think of as a super fast-twitch.

  • Now, the type IIa does have some aerobic or oxidative capabilities
  • It also has the ability to twitch a little bit faster or contract a little bit, and relax a little bit faster
  • Produce more power, but they're a little less fatigue resistant than your type I or slow-twitch fibers.

Type IIx muscle fibers (fast twitch glycolytic)

So, that's type IIa, and then type IIx is the super fastest muscle fiber out there.

  • These can produce a lot of force, they can contract very rapidly, relax very rapidly and just keep on going, so the muscle can keep on firing away.
  • But type IIx fiber's extremely, extremely not resistant to fatigue like the type I.

So, in type IIx you're gonna find in like the top tier sprinters in the world. So, that's kinda the breakdown of the slow versus the fast-twitch.

Now, in terms of athletes, the way I like to look at this is, let's look at track and field for example and see how these muscle fiber types would apply.

Examples of how muscle fibers are used during sports

So, for the type IIx, you're gonna find that in a lot of the short or a higher percentage of type IIx fibers in people that sprint the 100 and 200-meter dash.

Anything that lasts from like zero to maybe 20 seconds, where that kind of activity requires rapid contraction and relaxation of the muscle so they can just be going with speed.

So, you're thinking that's the 100 to 200-meter dashes. Those athletes probably have a high percentage of type IIx fibers.

Now, for the type IIa fibers, I would say runners like those going from 800 meters up to the mile would have a high percentage of type IIa fibers, and this is because they need to have that oxidative capability to run the distance but also need to produce a lot of power.

So, that would be the type IIa and then type I, you're looking at runners who run the 5k to the 10k that would have a really high percentage of type I fibers.

"All these athletes will have each fiber type but the amount will really determine their success in the event and a lot of it, especially with the type IIx fibers is genetics."

You could also look at it like this, like with cycling, Chris Froome probably has a really high percentage of type I fibers and a good percentage of type IIa and probably low type IIx, where Mark Cavendish would probably have a lot of type IIa and type IIx and fewer type I or a lower percentage of type I.

How to change the composition of slow vs fast twitch muscles?

So, this kinda begs the question, can muscle fiber types change with different types of training?

So, the research thinks that you really can't change type I fibers to type IIa or type IIx fibers, but the research does suggest that IIas can be converted to IIx and then IIx fibers can be converted to IIa depending on what you're doing, more specifically, strength training.

Moderate strength training like hypertrophy type strength training will probably lead to a conversion of type IIx to type IIa

and then really, really heavy strength training, you probably could see IIas being converted to IIx.

Which might be beneficial for some endurance athletes who already have a good percentage of type I fibers, but if you look at an obstacle course racer, where you're running but you're also doing a lot of explosive activities, it may be beneficial for these type of athletes to do pure strength training, develop those type IIx fibers out a little bit more.

So, when they're sprinting or climbing up a rope or doing explosive activities, those fibers are more developed and can produce a massive amount of power in a short amount of time so you can really just explode through that activity.

So, that's the breakdown of slow-twitch and fast-twitch fibers.