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

Abstract

Seven young females were subjected to 24 wk of intensive endurance training. Adaptive changes in myofibrillary ATPase activity, capillary supply and mitochondrial content were investigated with light microscopy and EM in needle biopsies from the quadriceps femoris. The average value for the maximal O2 uptake increased from 45.7 to 57.2 (ml .cntdot. kg-1 min-1) (25.2%, P < 0.005). The average number of capillaries per muscle fiber increased from 1.39 to 1.79 (28.8%, P < 0.005). Since no significant change in fiber area was found, a considerable number of new capillaries may have been formed during the training period. An increased capillary supply of all fiber types was found, being greatest for type I and smallest for type IIB. The relative amount of type I fibers before and after the training period was 57.9 and 56.5%, respectively, for type IIA fibers 26.4 and 31.5% (P < 0.005), for type IIB fibers 9.2 and 3.4% (P < 0.005) and for type IIC fibers 0.4 and 2.2% (P < 0.005). In the type II group, significant changes in subtypes take place during endurance training. Type IIAB may represent a transitional state between type IIA and IIB. Correlation of capillary supply, myofibrillar ATPase activity and mitochondrial content (determined semiquantitatively) of individual muscle fibers indicates that the capillary supply to a given fiber is more closely related to its mitochondrial content than to the fiber type as determined on the basis of myofibrillar ATPase activity.