Muscle Types

Cardiac

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 (involuntary).

Smooth

Smooth muscle tissue is located in the walls of hollow internal structures such as blood vessels, the stomach, intestines, and urinary bladder. Smooth muscle fibers are usually involuntary, and they are nonstriated (smooth).

Skeletal

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).

All skeletal muscle fibres are not alike in structure or function. For example, skeletal muscle fibres vary in colour depending on their content of myoglobin (myoglobin stores oxygen until needed by mitochondria). Skeletal muscle fibres contract with different velocities, depending on their ability to split Adenosine Triphosphate (ATP). Faster contracting fibres have greater ability to split ATP. In addition, skeletal muscle fibres 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 fibres are classified into three types: Type I fibres, Type II B fibres and type II A fibres.

Type I Fibres

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

Type II A Fibres

These fibres, also called fast twitch or fast oxidative fibres, contain very large amounts of myoglobin, very many mitochondria and very many blood capillaries. Type II A fibres 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 fibres are infrequently found in humans.

Type II B Fibres

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

Characteristics of Muscle Types

Body muscle make up

Most skeletal muscles of the body are a mixture of all three types of skeletal muscle fibres, 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 fibres. 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 fibres.

Even though most skeletal muscle are a mixture of all three types of skeletal, all the skeletal muscle fibres of any one motor unit are all the same. In addition, the different skeletal muscle fibres 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 fibres are activated by their motor units. If a stronger contraction is needed, the motor units of type II A fibres are activated. If a maximal contraction is required, motor units of type II B fibres are activated as well. Activation of various motor units is determined in the brain and spinal cord. Although the number of the different skeletal muscle fibres does not change, the characteristics of those present can be altered.

Fibre type modification

Various types of exercises can bring about changes in the fibres in a skeletal muscle. Endurance type exercises, such as running or swimming, cause a gradual transformation of type II B fibres into type II A fibres. The transformed muscle fibres 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 fibres. The increase in size is due to increased synthesis of thin and thick myofilaments. The overall result is that the person develops large muscles.

Plyometric exercise and weight training can be combined in complex training sessions to maximise the use of Type II B fibres and develop explosive power.