Dr. Andy Galpin

They have more mitochondria.

And they're overall good at what we call anti-gravity or postural.

So when you think about slow twitch fibers, they tend to be the ones that are on kind of all day, keep you upright and erect, keep you moving, and keep you at a low level of contraction, but continuously.

Fast twitch fibers, particularly the 2A fibers, are really the opposite.

So they're going to be hedged more towards fast speed of contraction.

But really, they're not particularly effective at fatigue resistance.

They are going to prefer using things like phosphocreatine or carbohydrates as a fuel source and are not as effective at things like fat or long-term sustained contractions.

The third and final type, 2X, are the fastest of the bunch.

While your 2A and fast-twitch fibers are somewhere in the neighborhood of five to six times more powerful than a slower type 1 fiber, a 2X fiber is in the neighborhood of 20 times more powerful.

The downside is we don't really have any good evidence of normal humans having many, if any, pure 2X fibers.

Oddly enough, we see it in extreme muscle disuse situations, things like spinal cord injury or coming back from extended space flight or situations like that.

And so there's a lot of mystery still behind these 2X fibers.

We could get into that again in maybe more detail a little bit later.

So on its surface, while we really have these three distinct unique fiber types, type 1, type 2A, and type 2X, it is really appropriate to discuss basically the type 1 and the type 2As from now on.

Now, within any individual muscle you have on your body, the proportion of fast twitch to slow twitch, in other words, the amount of fast or slow twitch fibers in that muscle, varies widely from muscle to muscle, as well as from human to human.

Now, that actually determines a lot about the function of that muscle.

For example, if you take, say, the soleus muscle, which is one of the small muscles kind of behind the back of your calf there that goes into the bottom of your heel.

That is heavily based towards slow twitch muscle fibers.

Depending on the person, it would be something in the neighborhood of 70% to all the way up to 90 plus percent slow twitch fibers.

And that's because the primary purpose of the soleus is to keep you standing and moving all day.