Type 1 vs. Type 2 Muscle Fibers

Type 1 and type 2 muscle fibers. Oh, you didn’t know you have different muscle types? In fact, as researchers learn more about our body, they discover a wide spectrum of muscle fibers. There are at least three distinct muscle fiber types that are recognized with consensus, while evidence indicates many more types exist. But what is a muscle type? And how does it affect your training? That’s what we’ll get into in this article.

What Are Muscle Fibers? 

Before we discuss Type 1 and Type 2 muscle fibers, we need to understand what muscle fibers are! To fully understand this, we need to know the basic anatomy of the muscle and then go a little deeper and see what’s happening in the actual muscle fiber.

Anatomy Of A Muscle And Muscle Fibers 

We often talk about muscle as if it were a solid block. However, a better way to think about it is like a piece of rope. A piece of rope is made of hundreds or thousands of smaller threads that are intertwined together. When wrapped together, they create a thick rope with increased strength.

Now, keep that thought in mind and transfer it to your muscles! Muscles are composed of thousands of smaller muscle fibers that are about 1 to 40 microns long and 10 to 100 microns in diameter¹ (Remember this, as it’s going to be crucial). For comparison, a strand of hair can be anywhere from 50 to 120 microns in diameter, and a piece of paper is 70 microns thick.

These fibers are surrounded by connective tissue known as endomysium and then grouped together in bundles known as fascicles. These fascicles are then surrounded by connective tissue known as perimysium, which are grouped together to create the entire muscle. Not that it matters, but the muscle is then surrounded by another tissue known as epimysium.

Regardless, this is what your muscle is: a bunch of muscle fibers wrapped into bundles that are then wrapped with other bundles. 

Anatomy Of The Muscle Fiber

We now need to examine the actual muscle fibers.

This is crucial as it’s the individual muscle fibers that contract to create force. The force produced can vary from extremely light, such as your eyes twitching to read these words, to maximal efforts during your deadlift PR attempt. We’ll get into this down below.

Either way, each muscle fiber works on its own. In fact, within each muscle fiber is an even smaller fiber called the myofibrils. These myofibrils are composed of multiple contractile units placed end-to-end called sarcomeres. It’s within each sarcomere, we find where the magic happens.

Each sarcomere contains contractile units known as myosin and actin. Each muscle fiber has a motor neuron on its head, and when this motor neuron fires, the muscle contracts with myosin and actin, pulling on each other, which contracts the muscle.

A quick rundown of how this looks is as follows;

Muscle → Muscle fascicles
Muscle fascicles → Muscle fibers
Muscle fibers → Sarcomeres
Sarcomeres → Myosin & Actin
Myosin & Actin → Muscle contraction

The Differences Between Type 1 And Type 2 Muscle Fibers

Now we know that a muscle’s individual muscle fibers fire to generate force and movement. However, not all muscle fibers are the same size, and we’re primarily concerned with the diameter of the muscle fiber.

At the beginning of this article, we mentioned that the width of a muscle fiber would be important, and here we are! As we mentioned, there is a wide range of widths and different muscle fibers, which dictates their type.

For the purposes of this article, there’s generally a consensus on the existence of 3 types of muscle fibers.² Originally, there was just type 1 and type 2. However, recent research has led to the division of type 2 muscle fibers. This is how the muscle fibers are divided and their width.

Type I: Small diameter 

Type IIa: Large diameter

Type IIb: Largest diameter

Now, the diameter of the muscle fiber is just the beginning of what differentiates these muscle fibers. Each muscle fiber contains several other characteristics that help determine the muscle fiber’s function. Some of the main ones include;

Motor neuron: Helps to fire each contraction of a muscle

Myoglobin: Helps transport oxygen in the muscle

Capillaries: Helps distribute blood 

Mitochondria: Where glucose is converted into ATP during the oxidative process

Here’s a breakdown of all the characteristics of the muscle fiber types:

Characteristic 

Type I

Type IIa

Type IIb 

Contraction speed

Slow 

Fast 

V. Fast

Force produced

Low

High

V. High

Fiber size (diameter)

Smallest

Large

Largest

Fatigue Resistance

High

Medium

Low

Motor neuron size

Small

Large

V. Large

Oxidative Capacity

High

Medium

Low

Mitochondrial density

High 

Medium

Low

Capillary density

High

Medium

Low

Myoglobin

High

Medium

Low

Metabolic Type

Primarily aerobic/oxidative 

Fast oxidative/Hybrid of aerobic and anaerobic

Primarily anaerobic/glycolytic

Fast Twitch Vs. Slow Twitch

You’ve probably heard the phrases “fast twitch” and “slow twitch” to describe muscle fiber types. This refers to the speed at which a muscle contracts. Basically, slow twitch muscle fibers contract slowly, resulting in low force production. On the other hand, fast twitch muscle fibers contract quickly, being able to produce more force.

Type 1 Muscle Fibers

Type 1 muscle fibers are slow-twitch muscle fibers and possess the thinnest diameter of all muscle fibers.

Properties

These thin muscle fibers also have the smallest motor neuron. This small motor neuron means that Type 1 muscle fibers cannot produce high levels of force.

Therefore, creating strength and power is not what Type 1 muscle fibers are for. Rather, it’s endurance. This occurs through several other internal key characteristics. This includes;

Higher density of mitochondria
Higher amounts of myoglobin
Higher capillary density

Function

Together, these characteristics may not allow Type 1 muscle fibers to produce a lot of force, but they can work for a long time. This means they are important for low-intensity activity and exercise, such as

Running a marathon
Long distance cycling
Hiking

Metabolic System

Due to this, Type 1 muscle fibers rely on the body’s oxidative system to produce energy. This is why they have more capillaries, myoglobin, and mitochondria. These allow more blood to be distributed and ATP to be produced in the mitochondria.

Type IIa Muscle Fibers

Type IIa muscle fibers are fast-twitch muscle fibers and lay in the middle of the spectrum. They’re larger than Type I but not as they’re Type IIb. This also means their force production lies in between as well.

Properties

The motor heads of Type II muscle fibers are large, so they can create more force for their strength than Type I. However, they fatigue much faster due to their structure. This includes;

Medium density of mitochondria
Medium amounts of myoglobin
Medium capillary density

Function

Type IIa muscle fibers can create moderate amounts of force for moderate durations. To visualize their role, think of sports such as;

Soccer
Basketball
Baseball

Metabolic System

Your Type IIa muscle fibers are unique as they’re a hybrid and can use both our aerobic and anaerobic metabolic systems to supply energy. This allows it to generate higher levels of force while fending off fatigue.

Type IIb Muscle Fibers

On the other end of the spectrum, we have Type IIb muscle fibers, which are also fast-twitch muscle fibers. These are the largest muscle fibers, so their properties reflect the opposite of Type I.

Properties

The motor heads of Type IIb are the largest, meaning they can produce very high levels of force. However, they also fatigue very easily. This is reflected in their other properties. 

The low density of mitochondria
Low amounts of myoglobin
Low capillary density

Function 

Type IIb muscle fibers can produce large levels of force, so various strength and power sports benefit greatly. This includes things like;

Weight lifting
Olympic lifting
Sprinting

Metabolic

Type IIb muscle fibers don’t need to worry about myoglobin and capillaries so much as they utilize the anaerobic metabolic systems, which means they don’t require oxygen.

Hybrid Muscle Fibers

Now that we have gone over the three primary muscle types, we want to briefly mention other hybrid muscle fiber types. The most commonly recognized is;

Type IIx: Hybrid of Type IIa and Type IIb

In addition, there are others that have been recognized. These are simply expressed by their combination of muscle fibers, including;

I/IIa fibers
IIa/IIx fibers 
I/IIa/IIx fibers

Now, a lot is being discovered lately so there will most likely be other muscle types added in the future.

Can Muscle Fibers Transition Types?

In the past, muscle fibers were believed to not change; you were born with what you have. However, we now know that this isn’t true. In fact, multiple studies have seen numerous instances of different types transitioning into the others. This includes;

Hybrid muscle fiber type → Pure muscle type
Pure muscle fiber type → New muscle fiber type
Type I → Type II
Type II → Type I

Below is a list of instances that show this to happen.

A study showed that bodybuilders and non-trained individuals shared a similar proportion of type IIa muscle fibers (45%). However, bodybuilders had much higher Type IIb muscle fibers (15% vs. 5%) and significantly fewer Type I (35% vs. 48%).³

A study found that 8 weeks of sprint training in male sprinters increased the proportion of type IIa fibers in their vastus lateralis (from 35% to 52%) with a corresponding reduction in the percentage of type I fibers (from 52% to 41%).⁴

A study looked at the effect of 13 weeks of marathon training and a 3-week taper on fiber-type shifts. They found that the vastus lateralis saw an increase in type I (42.6% to 48.6%), an increase in type I/IIa (5.1% to 8.2%), a decrease of IIa (40.1% to 35.8%), a decrease of IIa/IIx (11.9% to 6.4%), and increase of IIx (0% to 1%).⁵

In addition to these, we generally lose Type II muscle fibers as we age.⁶

Should You Train A Muscle By Fiber Type?

The human body’s muscles are composed of different proportions of fiber types, or at least to some extent. This means that even though different people are naturally born with different muscle types, the triceps tend to have higher proportions of Type II muscle types.

Due to this, many people have claimed that you should train a muscle based on their type. For example, the triceps generally have a higher percentage of Type II muscle fibers, so many suggest you should primarily use heavy weights. However, our muscles don’t operate on such a strict mechanism. For example;

This study found that Type 1 muscle fiber growth saw similar growth with heavy and low loads.⁷

This study saw no growth in Type 1 muscle fiber growth with low-load training.⁸

This study demonstrates Type II muscle responds well to low-load training.⁹

Further, people’s proportion can vary. While everyone has different variations in their muscle type, most people are actually closer to a 50/50 split. 

Applying Muscle Fiber Types To Your Training

So, what does all this mean for your training? Well, nothing really. While this stuff is very cool to understand, there aren’t really any key takeaways that we can give you to apply to your training.

Your body is smart and will do what it needs to do to optimize performance. If you’re an endurance runner, increase your endurance training and adaptations will occur to support this activity. If you’re a weight lifter, train like a weightlifter and your body will adapt and the necessary muscle fibers will transition.

The main point is your body will adapt to your needs. Trying to alter your training to force these adaptations isn’t really necessary, as your body already knows what’s best.

References

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Marshall-McKenna, Rebecca, et al. “Resistance Exercise Training at Different Loads in Frail and Healthy Older Adults: A Randomised Feasibility Trial.” Experimental Gerontology, vol. 153, 1 Oct. 2021, p. 111496, www.sciencedirect.com/science/article/pii/S0531556521002783, https://doi.org/10.1016/j.exger.2021.111496.