In any given muscle different fibre types can be found interspersed among one another

In any given muscle different fibre types can be found interspersed among one another.

Successive cross-sections of the same muscle stained for different properties. (A) for slow myosin-ATPase; (B) for succinic dehydrogenase (oxidative phosphorylation) activity; and (C) for glycolytic capacity.

Fast fibres (Type II) express a relatively potent myosin-ATPase variant. These fibres are specialised to contract rapidly and powerfully. They need to obtain a rich and ready supply of ATP. ATP from glycolysis is produced in the cytosol right in the vicinity of the myofibrils. Fast fibres are therefore designed to preferentially use glycolysis.

Since glycogen provides 3 ATP per glucose molecule, and can be stored right in the cytoplasm, they also 'prefer' this source over glucose from the blood (which will provide only 2 ATP per molecule). They do not rely on ATP produced by oxidative phosphorylation (OP).

	Type I - slow oxidative (red)	*Type IIA^- fast oxidative (red)**	Type IIB – fast glycolytic (white)
Myosin isozyme (ATPase)	Slow	Fast	Fast
SR calcium-ATPase	Moderate	High	High
Fibre diameter	Moderate	Small	Large
Oxidative capacity (mito- chondria, myoglobin etc.)	High	Very high	Low
Glycolytic capacity	Moderate	High	High
*Rare in humans & other primates

Oxidative Phosphorylation produces ATP efficiently and in abundance for each molecule of glucose or FFA burnt, but the ATP is made in the mitochondria and the process is dependent upon inward diffusion of oxygen. All this is too slow for fast fibres, which abandon OP. They are therefore supplied by relatively few capillaries, have few mitochondria and store relatively little myoglobin (but lots of glycogen and glycolytic enzymes). They are therefore pale in colour (white meat). The fibres are quite large in diameter, since diffusion is not a problem - but they do need to generate powerful contractions. The SR is usually abundant (to supply large quantities or Ca²⁺) and has a potent Ca²⁺-ATPase to rapidly resequester the released Ca²⁺. Sprinters have proportionately large numbers of fast fibres in their muscles.

Slow fibres express a relatively weak myosin-ATPase variant. Since they are not in so much of a hurry, they rely on the more efficient oxidative processes to produce ATP. The fibres are relatively thin, rich in mitochondria and myoglobin, richly supplied with capillaries, poor in glycogen, and have relatively sparse SR, with a relatively weak Ca²⁺-ATPase. These fibres are dark in colour (red meat). Long distance runners have abundant slow fibres in their muscles.

Fast fibres sacrifice durability for power. They produce pyruvate + NADH + H⁺ as end products which react to produce lactate and NAD⁺ in order to preserve the redox balance of the cells. The lactate does not leave the fibre easily. The intracellular pH therefore falls, impairing functioning of the Ca²⁺-ATPase in the SR and of the actomyosin ATPase. The fast fibres thus fatigue readily.

Fast & slow fibres do not readily interconvert, but training can increase capillary density, buffering capacity and ability to extract oxygen from the blood. An intermediate type of fibre (IIA) may convert more to fast type with strength training and more to slow type with endurance training.