Permanent and intermittent hypoxia as response modifiers of skeletal muscle tissue with exercise training

Resumen

It is still a widely held belief that the skeletal muscles of animals and humans exposed to chronic hypoxia have an enhanced oxidative capacity and increased capillarity. However, analysis of biopsies from subjects both prior to and after real or simulated ascents to the Himalayas have consistently shown a decrease in muscle oxidative capacity and an unchanged capillarity. Furthermore, in vastus lateralis biopsies derived from highland populations indigenous to the Himalayas and Andes, the mitochondrial content was markedly reduced in comparison with that of lowlanders matched for age and training status. Combined, these studies indicate that permanent exposure to severe hypoxia (incurred by living at altitudes above 3500m) elicits a decrease in muscle oxidative capacity as well as in the aerobic work performance. If severe hypoxia is incurred only during the constrained limits of endurance training sessions, then the resulting improvements are comparable to those elicited by training for a similar period under normoxic conditions. Additionally, it is observed that skeletal musele volume and myoglobin concentration increase with training in hypoxia but not in normoxia. Bearing these findings in mind, endurance athletes should limit their hypoxia exposure to the minimum period commensurate with induction of an erythropoietin response. If a gain in oxidative muscle mass and myoglobin concentration are desired, then they could be achieved by training sessions conducted under severe hypoxic conditons. The cellular mechanisms responsible for the response of skeletal muscle tissue to hypoxia are currently not known.