Muscle fibre type characteristics in endurance trained and untrained individuals.

The main fibre types of M. vastus lateralis of 10 trained or untrained male individuals (25-35 years) were quantitatively determined by morphological techniques; the fibre types being defined according to the M-band appearance. The volume density of mitochondria ( Vmit ) was higher in endurance-trained muscles. Vmit was higher in Type 1 than in Type 2 fibres, there being no difference between subtypes of Type 2 fibres. The volume density of lipid droplets ( Vli ) showed a wide range of values both with respect to degree of training and between fibre types. Z-band width was not influenced by endurance training, but was considerably larger in Type 1 than in Type 2 fibres. Discriminant analysis showed that 46% of the fibres, preclassified according to the M-band appearance, would have been correctly allocated on basis of the Vmit . The corresponding value for lipid droplets was 42% and for the Z-band width, 62%. It is concluded that Vmit is not a satisfactory criterion for discriminating between fibre types, especially between Type 2A and Type 2B in trained subjects. The study also shows that endurance training reduces the relative importance of individual-dependent factors in comparison with muscle fibre properties when concerning Vmit .