How unequivocal is the muscle fibre type concept?

Different histochemical identification methods for muscle fibre types have been introduced over the years. Most of them have been based on myosin ATPase activity after different kinds of preincubations, alone or in combination with oxidative enzymes. Comparative studies have shown, however, that the different methods result in nonidentical subgroups of type II fibres. Optical density values of individual fibres after incubation of serial sections for alkali- or copper-preincubated ATPase, NADH-TR, and fibre diameter, combined in two-dimensional plots, have for a long time been used in our laboratory to separate three subgroups of type II fibres. A cluster analysis, based on the data mentioned above, results in three subgroups of type II fibres in rat plantaris muscle. In comparison, earlier studies comparing different histochemical methods and reporting lack of correspondence between them have been based on two subgroups of type II fibres only. It is suggested that part of the lack of correspondence is due to unequal and incomplete separation by the methods used in the comparative studies, and that the three subgroups of type II fibres identified in the cluster analysis are type IIA, IIX and IIB, respectively. The need for a consensus on a common basis for histochemical identification of muscle fibre types is emphasized.