Effects of ageing on enzyme-histochemical, morphometrical and contractile properties of the soleus muscle in the rat.

Enzyme-histochemical, morphometrical and physiological properties were studied in the slow-twitch soleus muscle of male Wistar rats 2-25 months old. There was a clear age-related shift in the enzyme-histochemical profile of the muscle, resulting from transformation of fast-twitch muscle fibres to slow-twitch ones. This process occurred in younger animals up to the age of 7-8 months. At older ages there was a slight reduction in the total number of fibres and an increasing incidence of atrophic and angular fibres, indicating a denervation process. The tetanus force was reduced with age, but when it was related to the total muscle fibre cross-sectional area, no age-related difference was found. The contraction time was significantly altered only in 20-25-month-old animals, where it was prolonged, whereas the half-relaxation time showed a tendency towards prolongation throughout the entire age-span. Thus, the force-generating capacity of residual contractile material was not significantly affected by age and the age-associated decrease in the number of fast-twitch muscle fibres was not an important contributing factor to the decreased speed of contraction.