Muscle structural modifications in hypoxia.

The effects of prolonged severe hypoxia on human performance capacity and muscle structure and function have recently been studied during real and simulated ascents to Mt. Everest. The results of several independent research teams, using different techniques, are broadly compatible. It is found that body and muscle mass is significantly reduced after exposure to hypoxia. As a consequence, muscle fiber size is also reduced. The capillary density of muscle tissue is increased, not because of capillary neoformation, but because of the reduction in muscle fiber size. The activities of enzymes of the oxidative pathways are decreased in skeletal muscle tissue. A loss of mitochondria is the structural evidence of the diminished potential for muscle oxidative metabolism. In contrast to these results, recent experimentations with hypoxia in human exercise settings have demonstrated that if hypoxia is only present during a limited daily period of an endurance training session, hypoxia has a different effect on muscle tissue. It is found that muscle fiber size, capillarity, myoglobin concentration and muscle oxidative capacity are all enhanced with training in hypoxia. These controversial findings raise questions regarding the nature of the adaptational mechanisms triggered by the different hypoxic stimuli to which subjects had been subjected and thus offer important new venues for further studies on the control of protein metabolism in muscle tissue.