The 2 main categories of muscle fibres become 3 when we split the white muscle fibres into 2 sections.
So we expand further:
Type I
Slow oxidative (also called slow twitch or fatigue resistant fibres), contain large amounts of myoglobin, mitochondria, many blood capillaries, generate ATP by the aerobic system, hence the term oxidative fibres, split ATP at a slow rate, slow contraction velocity, resistant to fatigue, found in large numbers in postural muscles, needed for aerobic activities like long distance running.
Type IIa
These are fast oxidative fibres (also called fast twitch A or fatigue resistant fibres). These fibres contain large amounts of myoglobin, many mitochondria, many blood capillaries, high capacity for generating ATP by oxidation, split ATP at a very rapid rate and, hence, high contraction velocity, resistant to fatigue but not as much as slow oxidative fibres, needed for sports such as middle distance running and swimming.
Type IIb
Type IIb fibres are fast glycolytic fibres (also called fast twitch B or fatigable fibres).These contain, low myoglobin content, few mitochondria, few blood capillaries, large amount of glycogen, split ATP very quickly, fatigue easily, and are also needed for sports like sprinting.
Individual muscles are a mixture of 3 types of muscle fibres (type 1 and type 2a and b), but their proportions vary depending on the action of that muscle.
It must be remembered that skeletal muscles, although a mixture, can only have one type of muscle fibre within a motor unit. For example in contractions if a weak contraction is needed only the type 1 motor units will be activated. These fibres are used mainly for endurance activities.
If a stronger contraction is required the type 2a fibres will be activated or used to assist the type 1 fibres.
Maximal contractions facilitate the use of type 2b fibres which are always activated last. These fibres are used during ballistic activities but tire easily. This is explained in the sliding filament theory.
Sliding filament theory
During muscle contraction the myofilaments myosin and actin slide toward each other and overlap. This shortens the sarcomeres and the entire muscle. Muscle cells are "shocked" by nerve impulses from motor neurons. The point of attachment of the nerve to the muscle is called a neuromuscular junction. A motor neuron and its muscle cells are referred to as a motor unit. The nerve impulse is carried from the neuron across the gap to the membrane (sarcolemma) of the muscle cell by a chemical called acetylcholine. After the impulse is passed an enzyme called cholinesterase "de-activates" acetylcholine, readying the muscle for the next nerve impulse. Stimulation of the muscle cell causes Ca++ions to be released into the cell. This binds with the actin filaments causing them to expose active sites to the myosin cross bridges. The cross bridges bind to the active sites, forming a new molecular structure which causes the cross bridge to bend toward the center, pulling the actin filament with it. Energy from ATP is used to break the bond, straighten the cross bridge, and allow the cross bridge to form a new bond with another active site further down the actin filament. This cycle continues until the muscle contraction is complete. Then ATP is used to cause active transport to move the calcium ions out of the muscle fiber causing relaxation of the muscle.
It is believed there are no sex or age differences in fibre distribution; however, relative fibre types vary considerably from muscle to muscle and person to person.
Inactive men and women (as well as young children) have 45% type 2 and 55% type 1 fibre. People at the higher end of any sport tend to demonstrate patterns of fibre distribution e.g. endurance athletes show a higher level of type 1 fibres. Sprint athletes, on the other hand, require large numbers of type 2 b fibres. Middle distance event athletes show approximately equal distribution of the 2 types. This is also often the case for power athletes such as throwers and jumpers. It has been suggested that various types of exercise can induce changes in the fibres of a skeletal muscle. It is thought that if you perform endurance type events for a sustained period of time, some of the type 2b fibres transform into type 2a fibres. However, this is argued about. It may well be that the type 2b fibres show enhancements of the oxidative capacity after high intensity endurance training which brings them to a level at which they are able to perform oxidative metabolism as effectively as slow twitch fibres of untrained subjects. This would be brought about by an increase in mitochondrial size and number and the associated related changes not a change in fibre type.