Correlation of individual skeletal muscle fibres from "semithin" sections stained with p-phenylene-diamine and histochemical sections incubated for myofibrillar ATP-ase.

Muscle fibres in transverse "semithin" sections of osmium-fixed and plastic embedded muscle tissue stained with p-phenylene-diamine has been classified into three types (M, INT and H) and correlated to the same fibres in neighboring sequential preincubated myofibrillar ATP-ase sections. A close correlation, but no equality was found between the fibre types classified according to the ATP-ase and the p-phenylene-diamine methods. On the average, the content of mitochondria and fat droplets was highest in the ATP-ase classified type I fibres (dominated by M fibres), and higher in the endurance trained than in the untrained subjects.