Effects of endurance training on muscle fibre ATP‐ase activity, capillary supply and mitochondrial content in man.

Seven young females were subjected to 24 weeks of intensive endurance training. Adaptive changes in myofibrillary ATP-ase activity, capillary supply and mitochondrial content were investigated with light- and electron microscopy in needle biopsies from the quadriceps femoris. 1. The average value for the maximal oxygen uptake increased from 45.7 to 57.2 (ml . kg-1 min-1) (25.2%, P less than 0.005). 2. The average number of capillaries per muscle fibre increased from 1.39 to 1.79 (28.8%, P less than 0.005). Since no significant change in fibre area was found, this suggests that a considerable number of new capillaries have been formed during the training period. 3. An increased capillary supply of all fibre types was found, being greatest for type I and smallest for type IIB. 4. The relative amount of type I fibres before and after the training period was 57.9 and 56.5% respectively (n.s.), for type IIA fibres 26.4 and 31.5% (P less than 0.005), for type IIB fibres 9.2 and 3.4% (P less than 0.005) and for type IIC fibres 0.4 and 2.2% (P less than 0.005). Thus, in the type II group, significant changes in subtypes take place during the endurance training. The data suggest that type IIAB may represent a transitional state between type IIA and IIB. 5. Correlation of capillary supply, myofibrillar ATP-ase activity and mitochondrial content (determined semiquantitatively of individual muscle fibres indicators that the capillary supply to a given fibre is more closely related to its mitochondrial content than to the fibre type as determined on the basis of myofibrillar ATP-ase activity.