Improved V'O2 uptake kinetics and shift in muscle fiber type in high altitude trekkers.

The study investigated the effect of prolonged hypoxia on central (i.e. cardiovascular O2 delivery, Q'aO2) and peripheral (i.e. O2 utilization) determinants of oxidative metabolism response during exercise in humans. To this aim, 7 male mountaineers were examined before and immediately after the Himalayan Expedition INTERAMNIA 8000 - MANASLU 2008 lasting 43 days, among which 23 above 5000 m. The subjects showed a decrease in body weight (p < 0.05) and of power output during a Wingate test (p < 0.05) and an increase of thigh cross sectional area (p < 0.05). Absolute maximal uptake (V'O2max) did not change. The mean response time (MRT) of V'O2 kinetics at the onset of step sub-maximal cycling exercise was significantly reduced from 53.8 s ± 10.9 to 39.8 s ± 10.9 (p < 0.05), whereas that of Q'aO2 was not. Analysis of single fibers dissected from vastus lateralis biopsies revealed that the expression of slow isoforms of both heavy and light myosin subunits increased, whereas that of fast isoforms decreased. Unloaded shortening velocity of fibers was significantly decreased. In summary, independent findings converge in indicating that adaptation to chronic hypoxia brings about a fast-to slow transition of muscle fibers resulting in a faster activation of the mitochondrial oxidative metabolism. These results indicate that a prolonged and active sojourn in hypoxia may induce muscular ultra structural and functional changes similar to those observed after aerobic training.