Effects of age on enzyme-histochemical fibre spectra and contractile properties of fast- and slow-twitch skeletal muscles in the rat.

Enzyme-histochemical fibre spectra and contractile properties were studied in fast-twitch (extensor digitorum longus (EDL) or tibialis anterior (TA)) and slow-twitch (soleus (S)) muscles of young adult (6 months) and old (20-24 months) male albino rats. It was found that ageing affected fibre size, fibre type proportions, and contractile properties of muscle tissue in both qualitative and quantitative terms and that these age-related alterations differed between fast- and slow-twitch muscles. In the fast-twitch TA and EDL, no differences were observed in either the total number of fibres, the cross-sectional area or the absolute and relative numbers of different muscle fibre types and subtypes between young adult and old animals. In the slow-twitch S, on the other hand, both the total number of muscle fibres and the average cross-sectional fibre area were smaller in the old animals. The fibre loss and fibre atrophy were most pronounced in type II fibres, especially type IIA. In TA, twitch force was higher and tetanus force was unaltered in the old as compared with the young adult animals, resulting in an increased twitch:tetanus ratio in old age. In S, on the other hand, both these forces were lower in the old animals and the twitch:tetanus ratio was accordingly unchanged with age. When the tetanus force was related to age-related differences in total muscle fibre cross-sectional area, no differences were found in the maximum force-generating capacity of maintained contractile material in either fast- or slow-twitch muscles between the two age groups. Probable mechanisms underlying the above alterations are discussed.