By Elena Voropay

Increase in muscle size is called hypertrophy. How does a muscle’s size increase? There are two types of hypertrophy: transcient and chronic. The first one refers to the muscle pump you experience after completing a set of exercise. It is the result of fluid accumulation, or edema, that is lost from the blood plasma and lasts only for a short time.

Chronic hypertrophy is the consequence of long-term resistance training which changes the structure of the muscle. There is a lot of controversy about the theories that try to explain the exact mechanism of muscle growth. One theory suggests that hypertrophy takes place due to the increase in the number of muscle fibers (fiber hyperplasia), another presumption holds that the number of fibers does not change, but the size of existing individual fibers grows (fiber hypertrophy) in response to overloading the muscle with resistance such as weight training.

Resistance training will increase the muscle size (hypertrophy). Muscle growth depends on the type of the muscle fibers activated and the pattern of recruitment. Muscle growth is due to one or more of the following adaptions:

• Increased contractile proteins (actin & myosin)
• Increased number of and size of myofibrils per muscle fibre
• Increased amounts of connective, tendinous & ligamentous tissues
• Increased enzymes and stored nutrients

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.