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Slow Twitch Vs Fast Twitch Muscle Fibers: A Complete Explanation

Understanding muscle anatomy and how the neuromuscular system works can be confusing, but many of the basic principles of muscle anatomy and physiology can be useful in helping guide your workouts to achieve your specific fitness goals better.

One of the most important concepts to understand in anatomy and exercise physiology is the differences between slow twitch vs fast twitch muscle fibers, also referred to as type 1 muscle fibers vs type 2 muscle fibers.

So, what are slow twitch muscle fibers? What are fast twitch muscle fibers? Most importantly, what are the primary differences between slow twitch vs fast twitch muscle fibers?

In this article, we will discuss what is meant by fast twitch fibers and slow twitch muscle fibers, and then we will compare fast twitch vs slow twitch muscle fibers to help illuminate how your workouts can be tailored to target fast twitch vs slow twitch muscle fibers and vice versa.

We will cover the following: 

  • What Are Switch Twitch Muscle Fibers?
  • What Are Fast Twitch Muscle Fibers?
  • Differences Between Slow Twitch vs Fast Twitch Muscle Fibers
  • Can You Change Your Muscle Fiber Type?
  • Training Slow Twitch Muscle Fibers vs Fast Twitch Muscle Fibers

Let’s dive in! 

A person flexing their calf muscles which include slow twitch and fast twitch muscles.

What Are Slow Twitch Muscle Fibers?

Before we delve more specifically into the differences between slow twitch vs fast twitch fibers, it is important to cover the properties and characteristics of fast twitch muscle fibers and slow twitch muscle fibers individually.

Slow twitch muscle fibers are also called type I muscle fibers.

They are smaller than fast twitch muscle fibers, contain many more mitochondria, and generate energy aerobically (in the presence of oxygen). 

The slow twitch fibers are considered endurance muscle fibers because they are relatively fatigue resistant. However, the peak force output and the speed at which slow twitch fibers can contract are lower and slower than type II muscle fibers.

This makes type I muscle fibers great for endurance activities like long-distance running.

A person running.

What Are Fast Twitch Muscle Fibers?

Fast twitch muscle fibers are also known as type II muscle fibers (or type 2 muscle fibers).

Unlike slow-twitch fibers (type 1 muscle fibers), there are actually two different types of fast twitch fibers or type II muscle fibers.

As the name describes, fast twitch muscles fibers contract quickly, enabling you to perform high-intensity exercises such as sprinting, jumping, explosive weight lifting, plyometric exercises, and explosive sports activities like whacking a baseball when you swing the bat or throwing a football as far as possible.

Because fast twitch fibers have to contract quickly, they rely on anaerobic energy systems, which means that they can produce cellular energy, ATP, in the absence of oxygen.

The oxidative pathway, or aerobic respiration, requires many more steps and takes more time to yield the output of ATP.

The net energy production from oxidizing a molecule of glucose or glycogen through aerobic metabolic pathways (the Krebs cycle and electron transport chain) is significantly higher than when generating ATP via anaerobic metabolism (anaerobic glycolysis and the ATP-PC or phosphocreatine system).

For example, the complete oxidation of a single molecule of glucose through aerobic metabolism yields a net total of 32 molecules of ATP.

In contrast, anaerobic glycolysis only produces a net of two ATP molecules.

A sprinter.

However, the complete oxidative phosphorylation and electron transport chain via aerobic metabolism takes significantly longer than one complete cycle of anaerobic glycolysis.

Therefore, when you need to produce ATP quickly in order to allow for rapid muscle contractions and force generation, muscle fibers rely on anaerobic energy production.

Because the fast twitch muscle fibers are responsible for producing rapid contractions for maximal force generation, these type II muscle fibers are considered anaerobic muscle fibers, and they have very few mitochondria.

Mitochondria are the cellular organelles, or small specialized structures within the cell, that perform aerobic respiration to produce ATP.

In contrast, anaerobic metabolism, such as anaerobic glycolysis and the ATP–PC energy system, takes place in the cytosol, or fluid portion of the cell, rather than in a specialized organelle.

The two subcategories of fast twitch muscle fibers are type IIA and type IIB muscle fibers. Note that type IIB fast twitch fibers are also called type IIX muscle fibers.

Type IIB muscle fibers are sort of an intermediary between type I and type IIa fast twitch muscle fibers in terms of their size, properties, energy generation, force output, and endurance.

The words fast and slow painted on the street.

Differences Between Slow Twitch vs Fast Twitch Muscle Fibers

Nearly all of the skeletal muscles in the body contain some of each type of muscle fiber, but the relative percentage of the fiber types within the muscle varies based on factors such as your genetics, the muscle’s function, and the type of training you do. 

For example, the soleus, which is a small, flat muscle under the gastrocnemius in the calf, is an endurance muscle, so it is composed of nearly all Type I muscle fibers, whereas the gastrocnemius (the larger calf muscle) is composed of a mix of both of fiber types. 

With that said, there are various differences between slow twitch and fast twitch muscle fibers, both in terms of structure, function, energy systems, and performance.

Anatomically, fast twitch muscle fibers tend to be thicker than slow twitch fibers. 

Slow-twitch fibers have more capillaries and mitochondria to help produce energy through aerobic means.

A person jumping over a hurdle.

There is also more myoglobin, which is the protein that carries oxygen.

For this reason, the oxidative capacity of slow twitch vs fast twitch muscle fibers is quite different, with slow twitch fibers being nearly entirely aerobic.

Type IIA fast twitch fibers are almost entirely anaerobic, whereas type IIB fibers fall somewhere in the middle, capable of producing energy aerobically and anaerobically.

In terms of performance, fast twitch muscle fibers provide faster and more powerful and forceful contractions, but they fatigue quickly, whereas slow twitch muscle fibers produce less rapid and powerful contractions but have better endurance.

A similar pattern is seen for energy production. Fast twitch muscles produce small amounts of energy rapidly, whereas slow twitch muscles can produce more energy, but energy production takes longer.

The following chart summarizes some of the key differences in properties of slow twitch vs fast twitch muscle fibers:

Two people jogging.
Muscle Fiber PropertiesSlow-Twitch Muscle Fibers (Type I)Fast/Intermediate Twitch Muscle Fibers (Type II-B or Type II-X)Fast-Twitch Muscle Fibers (Type II-A)
Diameter (size)SmallMediumLarge
Force of ContractionLowHighHigh
Speed of ContractionSlowFastFast
ATP ProductionAerobicAnaerobicAnaerobic
Mitochondrial DensityHighMediumLow
Capillary DensityHighMediumLow
EnduranceHighMediumLow

Can You Change Your Muscle Fiber Type?

Your genetics influences your natural relative percentage of slow twitch vs fast twitch muscle fibers.

However, generally speaking, untrained individuals have about a 50/50 split between fast-twitch fibers vs slow twitch fibers.

Consistent exercise training can then modify this ratio to some degree, depending on the type of training that you do, your sex, your age, and your genetic predispositions.

Chronic endurance training, such as long-distance running, cycling, swimming, and hiking, will increase the proportion of slow twitch muscle fibers vs fast-twitch whereas power athletes, competitive weightlifters, sprinters, etc., can achieve a higher ratio of slow twitch vs fast twitch fibers.

For example, a marathon runner might have about 70 to 80% type I vs type II muscle fibers, while a trained sprinter might have 70-75% type II vs type I muscle fibers.

The percentage of fast twitch muscle fibers declines with age, which is part of the underlying causes of age-related sarcopenia and loss of functional strength since type II fibers are larger and faster, likely able to react more quickly to prevent falls and other issues associated with elderly individuals.

Sprinters sprinting off the blocks.

Training Slow Twitch Muscle Fibers vs Fast Twitch Muscle Fibers

In general, aerobic endurance exercises such as long-distance running, walking, cycling, and swimming strengthen slow-twitch muscle fibers, whereas type II fast-twitch muscle fibers are targeted with high-intensity exercises like sprinting, plyometrics, jumping, HIIT training, and heavy resistance training.

Using the low weight high reps approach with weightlifting activates the slow twitch muscles, while a high weight low reps approach targets fast twitch fibers.

However, even endurance training can increase and strengthen fast twitch muscle fibers.

One study looked at the changes in muscle fibers in recreational runners training for a marathon.

Runners underwent 13 weeks of increasing mileage and then did a three-week taper.

Results showed that the functional strength of both type I and type IIa fibers improved during the build phase, and the type IIa fast-twitch fibers continued to improve during the marathon taper.

Type II muscle fibers are larger and more responsive to hypertrophy with heavy resistance training

Therefore, the higher the percentage of Type II muscle fibers you have, the greater your potential for visible muscle growth.

To learn more about muscle physiology and how it can apply to your strength training or other workouts, check out our guide to muscle maturity here.

A muscular man sitting.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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