Effects of ageing and physical training on rat skeletal muscle. An experimental study on the properties of collagen, laminin, and fibre types in muscles serving different functions.

The purpose of the present investigation was to study the effects of both advancing age and life-long endurance training on the connective tissue and fibre composition of two types of rat skeletal muscle. In particular, additional evidence was sought on age- and training-induced transformations of muscle fibres, and on the significance of intramuscular collagen in muscle functioning. For this purpose a combined study of exercise and ageing throughout the life-span of the experimental animals was constructed. To gain a broad view of muscle responsiveness the muscle fibre types, the estimates of different biochemical and histological properties of collagen as well as of the passive mechanical properties of both a slow (m. soleus = MS) and a fast (m. rectus femoris = MRF) skeletal muscle were determined. The results can be summarized as follows: 1) The percentage of type I fibres in MS increased from about 60% in one-month-old rats up to about 90% in the untrained and almost 100% in the trained rats at ages between 4 and 10 months. In adult animals the proportion of type I fibres was significantly higher in the MS of the trained than untrained animals. In the slow area of MRF, the percentage of type IIA fibres increased from a mean value of below 40% in one-month-old rats to above 50% in the untrained and near to 70% in the trained rats at the age of 24 months. Correspondingly, the proportion of type IIB fibres decreased with both age and training. In both types of muscles, there was a tendency towards smaller cross-sectional areas for the predominant fibre type in the trained rats when compared to the untrained rats. Consequently, the long-term endurance training used did not reverse the age-related shift in muscle fibre composition but clearly accelerated the fibre transformation towards more fatigue-resistant muscle fibres with slower contractile speeds. Nevertheless, the slowing due to endurance training is not necessarily deleterious, as different mechanisms may be involved in these age- and training-related alterations. The former appears to involve degenerative changes in the neuromuscular system whereas endurance-type activity could assist in the maintenance of the low-threshold neural activity important for the expression of slow contractile characteristics. 2) The slow postural soleus contained more collagen compared with the fast locomotor rectus femoris muscle. The concentration of total collagen as well as the area-fractions of both endomysium and perimysium were larger in MS than in MRF.(ABSTRACT TRUNCATED AT 400 WORDS)