Muscle Fiber Types: What They Are And How to Train Them

Doug Lawrenson
Written By: Doug Lawrenson
November 17th, 2006
Updated: June 4th, 2021
197.4K Reads
Muscular man shirtless doing bicep curls in the gym
Design your workouts to suit different types of muscle requirements. The more you know about your muscles, the bigger you can build them.

We train them. But how much do we know about them. And how do we know what type of muscle fibers we have in the muscle we are training; by knowing what muscle fibers a specific muscle has, can take the guess work out of what type of training that specific muscle requires.

Muscle Types

Our muscle tissue consists of fibers (cells) that are highly specialized for the active generation of force for our muscle contraction. Muscle tissue provides motion, maintenance of our posture, and heat production. On the basis of certain structural and functional characteristics, the muscle tissue that our body has is classified into three types: cardiac, smooth and skeletal.

Cardiac Muscle

Cardiac muscle tissue forms the bulk of the wall of the heart. Like skeletal muscle tissue, it is striated. Unlike skeletal muscle tissue its contraction is usually not under conscious control and is classed as involuntary.

Smooth Muscle

Smooth muscle tissue is located in the walls of hollow structures such as blood vessels, the stomach, intestines, and the bladder. Smooth muscle fibers are usually involuntary, and they are non-striated (smooth). Smooth muscle tissue, like skeletal and cardiac muscle tissue, can undergo hypertrophy (growth).

Skeletal Muscle

Skeletal muscle tissue is attached to our bones. It is striated; that is, the fibers (cells) contain alternating light and dark bands (striations) that are perpendicular to the long axes of the fibers. Skeletal muscle tissue can be made to contract or relax by conscious control (voluntary).

All skeletal muscle fibers are not alike in structure or function. For example, skeletal muscle fibers vary in colour depending on their content of Myoglobin (Myoglobin is found in muscle tissue, where it binds oxygen, helping to provide extra oxygen to release energy to power muscular contractions.) Skeletal muscle fibers contract with different velocities, depending on their ability to split Adenosine Triphosphate (ATP). Faster contracting fibers have greater ability to split ATP. In addition, skeletal muscle fibers vary with respect to the metabolic processes they use to generate ATP. They also differ in terms of the onset of fatigue. On the basis of various structural and functional characteristics, skeletal muscle fibers are classified into three types: Type I fibers, Type II B fibers and type II A fibers.

Type I Fibers

These fibers, also called slow twitch or slow oxidative fibers, contain large amounts of Myoglobin, many mitochondria and many blood capillaries. Type I fibers are red, split ATP at a slow rate, have a slow contraction velocity, very resistant to fatigue and have a high capacity to generate ATP by oxidative metabolic processes. Such fibers are found in large numbers in the postural muscles of the neck.

Type II A Fibers

These fibers, also called fast twitch or fast oxidative fibers, contain very large amounts of Myoglobin, very many mitochondria and very many blood capillaries. Type II A fibers are red, have a very high capacity for generating ATP by oxidative metabolic processes, split ATP at a very rapid rate, have a fast contraction velocity and are resistant to fatigue. Such fibers are infrequently found in humans.

Type II B Fibers

These fibers, also called fast twitch or fast glycolytic fibers, contain a low content of Myoglobin, relatively few mitochondria, relatively few blood capillaries and large amounts glycogen. Type II B fibers are white, geared to generate ATP by anaerobic metabolic processes, not able to supply skeletal muscle fibers continuously with sufficient ATP, fatigue easily, split ATP at a fast rate and have a fast contraction velocity. Such fibers are found in large numbers in the muscles of the arms.

Structure of skeletal muscle tissue diagram

Characteristics of Different Muscle Fibers

Fibre Type Type I fibers Type II A fibers Type II B Type fibers
Contraction time Slow Fast Very Fast
Size of motor neuron Small Large Very Large
Resistance to fatigue High Intermediate Low
Activity Used for Aerobic Long term anaerobic Short term anaerobic
Force production Low High Very High
Mitochondrial density High High Low
Capillary density High Intermediate Low
Oxidative capacity High High Low
Glycolytic capacity Low Low High
Major storage fuel Triglycerides CP Glycogen CP Glycogen

Types of Muscle Fibers

Most skeletal muscles of the body are a mixture of all three types of skeletal muscle fibers, but their proportion varies depending on the usual action of the muscle. For example, postural muscles of the neck, back, and leg have a higher proportion of type I fibers. Muscles of the shoulders and arms are not constantly active but are used intermittently, usually for short periods of time, to produce large amounts of tension such as in lifting and throwing. These muscles have a higher proportion of type I and type II B fibers.

Even though most skeletal muscle is a mixture of all three types of skeletal, all the skeletal muscle fibers of any one motor unit are all the same. In addition, the different skeletal muscle fibers in a muscle may be used in various ways, depending on need. For example, if only a weak contraction is needed to perform a task, only type I fibers are activated by their motor units. If a stronger contraction is needed, the motor units of type II A fibers are activated. If a maximal contraction is required, motor units of type II B fibers are activated as well. Activation of the various motor units is determined in the brain and spinal cord. Although the number of the different skeletal muscle fibers does not change, the characteristics of those present can be altered.

The fast muscle (what the researchers call type IIa) moves 5 times faster than the slow muscle, and the super-fast (called type IIb) moves 10 times faster than the slow muscle fiber.

The average person has approximately 60% fast muscle fiber and 40% slow-twitch fiber (type I). There can be swings in fiber composition, but essentially, we all have three types of muscle fiber that need to trained.

Fiber Type Modification

Various types of exercises can bring about changes in the fibers in a skeletal muscle. Endurance type exercises, such as running or swimming, cause a gradual transformation of type II B fibers into type II A fibers. The transformed muscle fibers show a slight increase in diameter, mitochondria, blood capillaries, and strength. Endurance exercises result in cardiovascular and respiratory changes that cause skeletal muscles to receive better supplies of oxygen and carbohydrates but do not contribute to muscle mass. On the other hand, exercises that require great strength for short periods of time, such as weight lifting, produce an increase in the size and strength of type II B fibers. The increase in size is due to increased synthesis of thin and thick myofilaments. The overall result is that the person develops large muscles.

You can develop your fast-twitch muscle fiber by conducting plyometrics or complex training (combination of plyometrics and weights.) to build the fast muscle (IIa), and performing weight/strength training to build the super-fast (IIb) to the point where you can release exercise-induced growth hormone.

How to find your muscle fiber composition.


The objective of the muscle fiber test is to determine the fiber composition of the muscles being used for a particular exercise. Two test protocols are described: The Dr F. Hatield muscle fiber test and the Charles Poliquin muscle fiber test.

Required Resources:

To undertake this test you will require:

  • Weight training facilities
  • An assistant/spotter
  • Selection of exercises

How to conduct the Dr F. Hatfield muscle fiber test:

  • Determine your one repetition maximum (1RM) on an exercise
  • Rest for 15 minutes
  • Perform as many repetitions as possible with 80% of your 1RM


  • Less than 7 repetitions – Your fast twitch (FT) dominant
  • 7 or 8 repetitions – You have a mixed fiber type
  • More than 8 repetitions – You are slow twitch (ST) dominant

If you are FT dominant, then you should use heavier loads and lower repetitions predominantly in your training. ST dominant individuals, on the other hand, will respond better to lighter loads and higher repetitions

How to conduct the Charles Poliquin muscle fiber test:

  • Determine your one repetition maximum (1RM) on an exercise
  • Rest for 15 minutes
  • Perform as many repetitions as possible with 85% of your 1RM


  • Less than 5 repetitions – you are fast twitch (FT) dominant
  • 5 repetitions – you have mixed fiber type
  • More than 5 repetitions – you are slow twitch (ST) dominant

If you are FT dominant, then you should use heavier loads and lower repetitions predominantly in your training. ST dominant individuals, on the other hand, will respond better to lighter loads and higher repetitions.

Glen Spencer
Posted on: Thu, 05/18/2023 - 20:30

I am still not sure if I did this correctly I don't see my comment how many sets and reps and what percentage of my one rep max should I be using for myofibriller hypertrophy and sarcoplasm hypertrophy and maybe a blend of both and endurance everybody at the gym and all the instructors have their own guidelines none of them are the same and I'm confused

M&S Team Badge
Posted on: Wed, 05/31/2023 - 08:36

Glen, we got you. The comments have to be approved before they get answered. So if you ask anything again in the future, please know we see you.

If you're new to training, then my advice would be to keep it as simple as possible while you take the time to learn more away from the gym by doing research. For the time being, the only thing you should be focusing on is proper form, feeling challenged, and knowing you did your best so you can leave and recover. As you progress, then you could start worrying more about one rep maxes and percentages for hypertrophy, etc. This shouldn't be that complicated for you at this stage of the game. Go in, train hard, recover, and improve. Hope this helps, and thanks for reading M&S. When you are ready to start implementing more, we're here to support you.

Glen Spencer
Posted on: Thu, 05/18/2023 - 20:21

Hello Doug my name is Glen, I assist with ministry to people out on the street I Pell a bicycle couple of miles a day give or take and I carry a 30 lb backpack I'm 66 years old and I started going to the gym and I hear people talk about hypertrophy for gaining muscle and strength

Laila Garcia
Posted on: Wed, 09/01/2021 - 10:17

Very nice

Ricardo C Beck
Posted on: Sat, 09/05/2015 - 14:05

This is very descriptive about the various muscle types. However it 's not mentioned that we are born with a certain numbers and percentages of the various muscle fibers. No matter what we do we cannot add on more muscle fibers than from our birth amounts. We CAN make them very strong for their purpose, but not "growing" broad shoulders from having a narrow and slim body. Adapt to the sport where your body style is to the advantage.

Posted on: Tue, 03/19/2013 - 08:40

I work out err day.

Posted on: Sat, 06/23/2012 - 23:14

If you want to develope massive strength then lift heavy weights for low reps, if you want to develope bigger muscles then lift as heavy as you can for 8 to 10 reps. Always try to progress if you want to get stronger and do not do too many sets or reps if you are trying to get big and strong, but you have to lift heavy as possible. Try H.I.T. training, look it up.

Posted on: Wed, 01/30/2013 - 22:06

Aaron, did you write for Flex magazine back in '88? Those recs are simple at best and inaccurate at worst. You didn't address frequency, aggregate load, the benefit of sub-maximal training, deloads, etc. "Lifting heavy as possible" is terrible advice unless you're IMing some serious androgens.

Daniel Trout
Posted on: Thu, 12/01/2011 - 16:31

what excercise would I use like bench press or leg press for 1rep max? and what if a person is mixed fiber type do they do heavier weight with low reps or lighter weight with high reps?

Posted on: Tue, 02/07/2012 - 03:13

bench press as heavy of weight as you can one time and that is your one rep max for bench. squat would be a better exercise in my opinion to determine a one rep max for leg muscles. and if you are in the middle, then meet in the middle with your workout, instead of training at 5 reps per set or 15 reps per set, train at 8-12.

Posted on: Fri, 04/15/2011 - 21:54

These 2 tests are useless - IMO. Why ? It is impossible to measure 1-RM exactly;
there are so many variables to it.
There is a site which has a Range of 80% Reps broken down into:
<7 7-12 >12.
That 'looks' realistic ( if it has a solid science behind it - there was no reference to any publication. )

Posted on: Fri, 02/12/2010 - 03:40

Nice work Charles and Dr Hatield, true greats in the sport and who I look up too!contius hard work in the field and wlays learning new ways to help athletes.