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.
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 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 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 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.
Muscle Fiber Diagram
The diagram above shows the structure of the muscles. The inset picture (brown colored) shows you the fast and slow twitch fibers. Lighter = fast twitch / darker = slow twitch.
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|
|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.
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.