Dr. Andy Galpin

It may have something in the neighborhood of 3,000 nuclei.

If you were to then count up all the muscle fibers in total, this would mean you would have in the ballpark of 750 million nuclei in the entire biceps muscle.

You extend this out to all 600 plus of your muscles, and that puts you somewhere in the neighborhood of around 500 billion nuclei throughout your entire body.

That's how responsive an adaptive skeletal muscle is to everything you're doing in your life.

We're going to talk a lot more about these myonuclei in future episodes, because as I mentioned, they are the primary place that are going to regulate how your muscle cells respond and change to external stressors.

But what regulates the contractile and metabolic properties?

That's actually something different, and it's what we call the muscle fiber type.

You've probably heard of this, referred to as a fast twitch or slow twitch muscle fibers, but it actually extends beyond that.

Now, on the surface, that's a fine, broad explanation, but we can go a little bit past that, and I think it's helpful to do so without being unnecessarily exhaustive.

Really, fibers can be classified as the following, slow twitch, or another way to say that is type I, and then your fast twitch fibers are broken down into two major categories, your type IIa, as well as your IIx fibers.

Many of the properties are the same between all these fiber types, the microanatomy, how they're designed, all the things we've talked about, how they're wrapped in connective tissue, their myonucleation status, et cetera, et cetera.

The properties that are really distinct are, again, the contractile and metabolic ones.

Slow-twitch fibers tend to be but are not always smaller.

In fact, in endurance-trained individuals, they can oftentimes be larger than fast-twitch fibers.

But their contraction is exactly that.

The twitch or the speed of contraction is slower.

It doesn't necessarily mean the strength or force behind the contraction is smaller or less.

It's just that it contracts a little bit slower.

The advantage, though, is they tend to be highly fatigue resistant.

They are better at utilizing fat as a fuel source.