Adaptation of human skeletal muscle to endurance training of long duration

The aim of this study was to examine the adaptive response of skeletal muscle to dynamic training of long duration and low intensity. The response of m. triceps brachii, (TB) and m. quadriceps femoris, vastus lateralis (VL) to training of long duration at an intensity corresponding to approximately 45% of VO2 max, has been studied in six subjects. Muscle specimens were taken during training (18.5 miles (30 km)), which involved skiing with a backpack, (6 days/week, 8 weeks), as well as during a subsequent period of detraining (33 weeks). Increases in oxidative enzyme activities and capillarization during training were limited to the TB. There were parallel increases in citrate synthase (CS) and malate dehydrogenase (MDH) during the first 3 weeks of training (30%). CS then continued to rise (to 90% above pretraining level), while the increase in MDH levelled off. The activity of 3-hydroxyacyl-CoA-dehydrogenase (HAD) and the number of capillaries per fibre increased by 50% and 40% respectively. A selective decrease (12%) in area of fibre type IIA was noted during training after a 2-week period of energy deficiency. During the first 6 weeks of detraining, the decrease in CS and HAD (30%) was greater than the decrease in capillaries per fibre (14%). It is concluded that prolonged duration of training is not, in itself, sufficient to elicit an adaptive response in the leg muscles, while a modest training effect was seen for the arm muscles. The finding emphasizes the importance of adequate intensity in physical training programmes.