The biochemical basis of fibre types in bovine muscle.

Recent studies using isolated muscle fibres have established a link between the histochemical fibre types, I, IIA and IIB, which can be defined by myofibrillar ATPase activity, and three forms of myosin heavy chain. This work is reviewed, as is work on metabolic variability within these fibre types. Results are them presented which show that the activity of myofibrillar ATPase in sections, although principally determined by the myosin heavy chain, is modified by other myofibrillar components, as yet unidentified. Evidence is also presented for hybrids of myosin heavy chain in muscle fibres. Finally, metabolic variability within the fibre types is examined for bovine muscle. It is concluded that, in spite of wide metabolic variability within types, a classification based on myofibrillar ATPase activity is still useful.