muscle fibers

The Different Muscle Fiber Types (and how to train them)

Hey Angels and Alphas,

Whether you realize it or not, your muscles literally help power your every move. From running, to lifting weights, to smiling. Your muscles are made up of an enormous number of cells known as muscle fibers. These hard-working little critters are split into two different types, which each have their own specific function and ultimately impact how your body responds to movement and training.

Today, we’re going to review everything you need to know about the types of fibers that make up the muscles in our bodies.

Let’s start with the basics.

There are two primary muscle fiber types.

  • Slow-Twitch Muscle Fibers (also known as Type I)

Type I muscle fibers are focused around endurance. This means they slowly relax and develop force, and they have the unique ability to work for extended periods of time without facing fatigue. Under a microscope, they look red because of their huge concentration of oxygen-carrying red blood cells.

  • Fast-Twitch Muscle Fibers (also known as Type II)

As opposed to slow-twitch muscle fibers, fast-twitch muscle fibers develop force and relax quickly, which makes them responsible for more explosive movements and rapid strength generation. However, they tire out much faster, so they’re tailored toward short-duration efforts. They look light red under a microscope due to their lower concentration of red blood cells.

Most of the muscles in your body contain a mixture of type I and type II muscle fibers, although some muscles generally have higher concentrations of one as opposed to the other.

For example, one of the two main muscles in your calf is the soleus, and it has the highest percentage of type I muscle fibers in the body. And that’s precisely because it plays a vital role in endurance-focused activities such as walking, jogging, even standing. Generally, muscles that are working against gravity at low levels all day are more fatigue-resistant and contain more type I fibers.

At the same time, muscles that are used for more power-generating tasks such as the glutes and deltoids tend to contain more type II fibers.

Not only that, but some people are also genetically predisposed to have higher concentrations of certain muscle fibers. These are exactly the differences responsible for the variations we see in different athlete’s body shapes. This means that athletes who possess a higher percentage of type I fibers in certain areas will inherently be better at endurance tasks, while people with type II will be better at short-duration power tasks. (If all other things are equal.)

So which muscle type should you favor when training?

It’s probably easy for you to guess which type of muscle fibers you have more of, based on the certain activities you’re better at. If you’re, let’s say, a slow sprinter, but you can run a 13-mile dash pretty quickly, then it’s pretty safe to say you’re more type I dominant.

You may also be able to tell which type of muscle fibers you’re abundant in by examining your body type. If it’s usually easier for you to gain muscle when you start working out, you probably have a high percentage of type II. On the opposite side, if you struggle to put on muscle mass despite all your efforts, you’re most likely rich in type I fibers.

Is it possible to change muscle fiber types?

Your proportions of type I and type II fibers are largely determined by genetics. You can’t really turn one type into the other.

That being said, within those primary muscle fiber types there are also subtypes including type I, Ic, IIc, IIax, IIac, and IIx. This allows you to change a muscle fiber’s subtype through training, but not its original type.

For example, the type IIx muscle fibers are known to produce the most force out of all fiber types, but they’re also the ones that tire out the fastest. But exercising a bit converts those type IIx fibers into IIa fibers, which have more endurance than IIx.

Training for slow vs. training for fast-twitch muscle fibers…

Even if you’re not naturally fast and explosive, or you’re not naturally equipped for endurance, you can still do your best to maximize the muscle fibers you have through exercise.

You should keep in mind that you won’t achieve the same result as someone who is genetically predisposed to better muscle fiber proportions.

If you want to focus your attention on developing your type I muscle fibers for endurance, here’s what you should do:

  • Isometric exercises such as the plank (variations of it, not the plain ‘ol plank), because isometric exercises keep your muscle fibers engaged for extended periods of time.
  • High-rep resistance training with lighter weights.
  • Circuit training, in which you can jump from one exercise to the next without any rest, training your endurance even further.

And if you’re someone who wants to make the most out of their type II muscle fibers and train for strength and explosiveness, you should:

  • Strength train with progressively heavier and heavier weights in the 2-6 rep range. Take long rest between sets (upwards of 5 minutes.)
  • Explosive moments and high-intensity interval training (HIIT) such as kettlebell swings, jump squats, barbell snatches, and more.

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