File:Muscle fiber types.jpg

Muscle fiber types

Muscle fiber types identified by ATPase staining.

Type I fibres (red fibres)

Red muscles contain predominantly (but not exclusively) red muscle cells. Red muscle fibres are comparatively thin and contain large amounts of myoglobin and mitochondria. Red fibres contain an isoform of myosin with low ATPase activity, i.e. the speed with which myosin is able to use up ATP. Contraction is therefore slow. Red muscles are used when sustained production of force is necessary, e.g. in the control of posture.

Type II fibres

White muscle cells, which are predominantly found in white muscles, are thicker and contain less myoglobin. ATPase activity of the myosin isoform in white fibres is high, and contraction is fast. Type IIA fibres (red) contain many mitochondria and are available for both sustained activity and short-lasting, intense contractions. Type IIB/IIX fibres (white) contain only few mitochondria. They are recruited in the case of rapid accelerations and short lasting maximal contraction. Type IIB/IIX fibres rely on anaerobic glycolysis to generate the ATP needed for contraction

Image source: UWA Blue Histology

http://www.lab.anhb.uwa.edu.au/mb140/CorePages/Muscle/Muscle.htm#SKELETAL

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