Adaptation of skeletal muscle fibers to endurance training, confinement or selection for body weight and physical endurance of laboratory mice

By the help of histological methods the changes in skeletal muscle of laboratory mice, which had different levels of exercise during postweaning period (training, confinement) or had been selected for body weight and endurance fitness (Du-6+LB), were investigated. The animal groups with the better endurance fitness (Du-6+LB, trained) had a higher total number of muscle fibres. An increased aerobic capacity of metabolism for the trained and selected animals, a decreased one for the confined animals resulted, indicated by the composition of muscle fibre types of M. rectus femoris. The appearance of stress-induced pathological changes of muscle fibres was observed, with the highest extent in the confined group with decreased endurance fitness. Similar correlations between structure of muscle and fitness in domestic animals are to be expected.