Human skeletal muscle fiber type adaptability to various workloads.

TLDR

Muscle biopsies from controls, weight lifters, and distance runners were collected and each fiber was identified histochemically, then analyzed morphometrically by cryostatic retrieval and electron microscopy. Distance runners exhibited a higher proportion of type I and IIC fibers, greater mitochondrial and lipid content, and a stronger correlation between ATPase activity and oxidative potential (I > IIA > IIB), whereas weight lifters had more mitochondria than controls, suggesting inactivity rather than lifting reduces mitochondrial volume and increases type IIB fibers.

Abstract

Muscle biopsy specimens were removed from the vastus lateralis muscles of three groups of human subjects: controls, weight lifters, and distance runners. The runners proved to be a unique group with respect to the variables measured (low body weight and percentage body fat, and high VO2 max). Additionally, a histochemical analysis of the biopsy specimens revealed that the runners had a significantly higher percentage of fiber types I and IIC than either the controls or the weight lifters. Using a cryostatic retrieval method, each of the fibers identified histochemically was then analyzed morphometrically using electron microscopy. The results of volume-percent mitochondria demonstrated a strong relationship between the ATPase activity and oxidative potential of the fiber types for all three groups such that the oxidative activity would be ranked I greater than IIA greater than IIB. Irrespective of fiber type, there were significant differences between the groups with regard to muscle-fiber mitochondrial (runners greater than lifters greater than controls) and lipid content (runners greater than controls greater than lifters). The lifters had a significantly greater content of mitochondria than the controls, which may suggest that inactivity rather than the lifting exercise contributes to a low volume-percent mitochondria and a high percentage of type IIB fibers.