Understanding the course of fibre differentiation during development and ageing is important for analysis of normal and diseased muscles. Often, a certain fibre type of the developing or ageing muscle is more affected than another by altered conditions or diseases. However, postnatal differentiation of muscle fibre types and subtypes is not completely investigated to this day.

A fibre type can be identified by the myosin heavy chain (MHC) isoforms it contains. At first, two distinct fibre types are generated: slow fibres containing predominantly slow MHC and fast fibres containing predominantly fast MHC. This slow–fast differentiation begins early in histogenesis. Differentiation into subtypes occurs later during development, but the exact time point of subdifferentiation is unclear up to now. Indeed, the exact time point at which fibre subtypes become detectable may be important for investigations of developmental changes in muscle properties, adaption processes to altered conditions or muscle diseases.

Several studies have shown that the age at which different fibre types can be distinguished depends on the method used, and on species and muscles. For example, it was found that in man and rhesus monkeys the histochemical differentiation of leg muscles takes place before birth, whereas fibres of hindlimbs of newborn rats are histochemically undifferentiated. In this respect, one has to consider that muscle fibres can be differentiated on the basis of different classification systems). The criteria which underlie different classification systems are obviously not interchangeable. It means, for example, that fibre types IIA and IIB identified after histochemical ATPase activity staining are not metabolically differentiated and therefore do not strictly correlate with FOG and FG fibres as identified by cytophotometry. Some studies have reported developmental changes in MHC expression, in fibre population, and in fibre type-specific enzyme activities of several rat hindlimb muscles. However, a comprehensive study including developmental changes in fibre type population, fibre type-specific metabolic profile and the time of detectability of fibre types with the different classification systems during normal development and ageing of rat hindlimb muscles is missing to this day. Therefore, the aim of the present study was firstly to find out at which time of muscle development and ageing, the fibre types of different classification systems are detectable in soleus (SOL), extensor digitorum longus (EDL) and gastrocnemius (GAST) muscles of rats. To type the fibres according to different criteria and to compare different classification systems, fibre typing was performed by immunohistochemical, enzyme histochemical and cytophotometrical methods. Secondly, age-dependent changes in the fibre type population and metabolic profile of each fibre type were investigated.