Muscle tissue adaptations of high-altitude natives to training in chronic hypoxia or acute normoxia

Twenty healthy high-altitude natives, residents of La Pat, Bolivia (3,600 m), participated in 6 wk of endurance exercise training on bicycle ergometers, 5 times/wk, 30 min/session, as previously described in normoxia-trained sea-level natives (K. Hoppeler, H. Howald, K. E. Conley, S. L. Lindstedt, H. Claassen, P. Vock, and E. R. Weibel. J. Appl. Physiol. 59: 320-327, 1985). A first group of 10 subjects was trained in chronic hypoxia (HT barometric pressure = 500 mmHg; inspired O-2 fraction = 0.209) a second group of 10 subjects was trained in acute normoxia (NT; barometric pressure = 500 mmHg; inspired O-2 fraction = 0.314). The workloads were adjusted to similar to 70% of peak O-2 consumption (Vo(2)peak) measured either in hypoxia for the HT group or in normoxia for the NT group. Vo(2peak) determination and biopsies of the vastus lateralis muscle were taken before and after the training program. Vo(2peak) in the HT group was increased (14%) in a way similar to that in NT sea-level natives with the same protocol. Moreover, Vo(2peak) in the NT group was not further increased by additional Oz delivery during the training session. HT or NT induced similar increases in muscle capillary-to-fiber ratio (26%) and capillary density (19%) as well as in the volume density of total mitochondria and citrate synthase activity (45%). It is concluded that high-altitude natives have a reduced capillarity and muscle tissue oxidative capacity; however, their training response is similar to that of sea-level residents, independent of whether training is carried out in hypobaric hypoxia or hypobaric normoxia.