Vol. 1, #16
November 20, 2004

Q: What are fast- and slow- twitch muscles, what are they good for, and can the percentages be changed? - Layperson

A: On the basis of certain structural and functional characteristics, muscle tissue is classified into three types and two subtypes (conditionally involuntary-normally not under conscious control and voluntary-normally under conscious control) : cardiac (conditionally involuntary) , smooth (conditionally involuntary) and skeletal (voluntary).

In regards to fast- and slow- twitch muscles, only skeletal muscles are of concern here.

Skeletal muscle tissue is named for its location - attached to 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).

Skeletal muscle tissue is composed of two general types of muscle fibers - fast-twitch and slow-twitch.

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. Fast-twitch muscle fibers are selectively recruited when heavy work is demanded of the muscles, and strength and power are needed. They contract quickly, providing short bursts of energy, and are therefore used for high-intensity, low-endurance activities, such as sprinting, weightlifting, shot-putting, and swinging a golf club. However, fast-twitch muscle fibers become exhausted quickly. Pain and cramps rapidly develop from the buildup of lactic acid, which is a byproduct of the metabolism of this kind of muscle fiber.

Slow -twitch muscle fibers produce a steady, low-intensity, repetitive contraction. They do not tire easily and are recruited when endurance is needed. Therefore, slow-twitch muscle fibers are used for low-intensity, high-endurance activities, such as long distance running.

Normally, people have roughly a 50%-50% division of fast- and slow-twitch muscle fibers, however, the duration and intensity of your activity will influence the physiology of your muscle tissue and the development of your muscle fibers. Endurance athletes tend to develop a greater percentage of slow-twitch muscle fibers, while power athletes tend to develop a greater percentage of fast-twitch muscle fibers. One reason for this is that in power athletes, the fast-twitch muscle fibers increase in size to store more ATP and CP (creatine phosphokinase) needed for explosive energy. Another reason is that power athletes need more muscle glycogen to fuel their muscles, while endurance athletes need both muscle glycogen and fatty acids.

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.

fiber Type Type I fibers Type II A fibers Type II B 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 High High
Major storage fuel Triglycerides CP, Glycogen CP, Glycogen

Body muscle make up

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 are 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 various motor units is determined in the brain and spinal cord. There is weak, but growing, evidence that the the number of the different skeletal muscle fibers and the characteristics of those present can be altered.

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.

On a side note here, the darker meat of animals, especially of fowl contain more Type I and Type II A fibers (due to higher myoglobin content) then Type II B (low myoglobin content) fibers.

November 13, 2004 Newsletter

DISCLAIMER:  The information in this column, is NOT intended to diagnose and/or treat any health related issues and is provided solely for informational purposes only. Consult the appropriate healthcare professional before making any changes to your healthcare regime. Even what may seem like simple changes in the diet for example, can interact with, and alter, the efficiency of medications and/or the body's response to the medications. Many herbs and supplements exert powerful medicinal effects. Neither the author, nor the website designers, assume any responsibility for the reader's use or misuse of this information.

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