Section A: Applied Anatomy and Physiology 7. Muscle fibre types

Syllabus Structure and function of slow oxidative, fast oxidative glycolytic and fast glycolytic muscle fibre types

Types of Muscular Tissue Skeletal – Striated – Voluntary – Consciously controlled – Most are subconsciously controlled (stabilising) Cardiac – Only the heart – Also striated – Involuntary Smooth – Located in wall of hollow internal structures – NONstriated – Involuntary

The following slide shows a table of muscle fibre types. These muscle fibre types are found in skeletal muscle tissue.

Slow-oxidativeFast-oxidativeFast-glycolytic mitochondriamany few capillariesmany few myoglobin content high low myosin ATPase activity lowhigh contraction velocity slowfast rate of fatigueslowintermediatefast muscle fiber diameter smallintermediatelarge innervating neuron size smallintermediatelarge motor unit sizesmallintermediatelarge

Type 1 (slow oxidative) They are red in colour due to the presence of large volumes of myoglobin and so oxygen and high numbers of mitochondria Due to this fact they are very resistant to fatigue and are capable of producing repeated low-level contractions by producing large amounts of ATP through an aerobic metabolic cycle. For this reason the muscles containing mainly type I fibres are often postural muscles such as those in the neck and spine due to their endurance capabilities Also, athletes such as marathon runners have a high number of this type of fibre, partly through genetics, partly through training

Type 2a (fast oxidative) Are a hybrid of type I and II fibres These fibres contain a large number of mitochondria and myoglobin, hence their red colour They manufacture and split ATP at a fast rate by utilising both aerobic and anaerobic metabolism and so produce fast, strong muscle contractions, although they are more prone to fatigue than type I fibres Resistance training can turn type IIb fibres into type IIa due to an increase in the ability to utilise the oxidative cycle.

Type 2b (fast glycolytic) They are white in colour due to a low level of myoglobin and also contain few mitochondria They produce ATP at a slow rate by anaerobic metabolism and break it down very quicky This results in short, fast bursts of power and rapid fatigue As mentioned above, this type of fibre can be turned into type IIa fibres by resistance training This is a positive change due to the increased fatigue resistance of type IIa fibres These fibres are found in large quantities in the muscles of the arms

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, 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 is 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.

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

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, 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. You can develop your fast-twitch muscle fibre by conducting plyometric or complex training (combination of plyometrics and weights) to build the fast muscle (IIa) and performing sprinting types of training to build the super-fast (IIb) to the point where you can release exercise-induced growth hormone. The body itself produces the best form of growth hormone. If you want to accelerate muscle building then use large muscle group targeted weight training in combination with anaerobic sprinting-types of exercise to increase your body's natural muscle building steroids.