This study was undertaken to test the hypothesis that endurance training in hypoxia
is superior to training of the same intensity in normoxia. To avoid adaptation to
hypoxia, the subjects lived under normoxic conditions when not training. A secondary
objective of this study was to compare the effect of high- vs. moderate-intensity
training on aerobic performance variables. Thirty-three men without prior endurance
training underwent a cycle ergometer training of 6 weeks, 5 d/week, 30 minutes/d.
The subjects were assigned to 4 groups, N-high, N-low, H-high and H-low based on the
training criteria normoxia (N; corresponding to a training altitude of 600 m), vs.
hypoxia (H; training altitude 3850 m) and intensity (high; corresponding to 80 % and
low: corresponding to 67 % of V˙O2max). V˙O2max measured in normoxia increased between 8.5 to 11.1 %, independent of training
altitude or intensity. V˙O2max measured in hypoxia increased between 2.9 and 7.2 %. Hypoxia training resulted
in significantly larger increases than normoxia training. Maximal power that subjects
could maintain over a thirty-minute period (measured in normoxia or hypoxia) increased
from 12.3 - 26.8 % independent of training altitude. However, subjects training at
high intensity increased performance more than subjects training at a low intensity.
Muscle volume of the knee-extensors as measured by magnetic resonance imaging increased
significantly in the H-high group only (+ 5.0 %). Mitochondrial volume density measured
by EM-morphometry in biopsy samples of m. vastus lat. increased significantly in all
groups with the highest increase seen in the H-high group (+ 59 %). Capillary length
density increased significantly in the H-high group only (+ 17.2 %). The main finding
of this study is that in previously untrained people, training in hypoxia while living
at low altitude increases performance in normoxia to the same extent as training in
normoxia, but leads to larger increases of aerobic performance variables when measured
under hypoxic conditions. Training intensity had no effect on the gain of V˙O2max. On the level of skeletal muscle tissue, the combination of hypoxia with high training
intensity constitutes the most effective stimulus for increasing muscle oxidative
capacity.
Hypoxia, training intensity, endurance, V˙O2max, biopsy, human, mitochondrial density, capillary length density.