Human muscle adaptations to chronic hypoxia.

Maximal aerobic power (V02max) decreases with increasing altitude, the drop being about the same in acute and chronic hypoxia. To elucidate this well established but still rather inexplicable finding, experiments were carried out on a group of participants before and immediately after return from a mountaineering expedition to Mount Lhotse Shar (8398 m). The results of a typical subject exposed over a period of 5 weeks to an altitude of at least 5200 m indicate that: A) In the vastus lateralis muscle the number of capillaries to the number of fibers ratio was unchanged, the mean fiber diameter was reduced from 77.3 to 67.9 mu while the mitochondrial to fiber volume ratio increased from 6.7 to 7.9%; in addition, muscle protein concentration diminished by 37% and succinate dehydrogenase activity (SDH) was 48% less than in control conditions; B) Cardiac output and muscular (vastus lateralis) blood flow at submaximal work loads were reduced by 21% and 53%, respectively; C) The V02 on-response kinetics at the mouth upon rectangular submaximal work loads was delayed (t 1/2 = 37.5 vs. 27.0 s). From the above results it is concluded that the apparent lack of beneficial effects of acclimatization on maximal aerobic power is likely the consequence of the interaction of a number of positive (increased hematocrit, rightward shift of the 02 dissociation curve) and negative (reduced thoracic and peripheral blood flow, decreased muscle mass and oxidative enzymes activity) changes induced by hypoxia.