Abstract
Abstract The adjustment of pulmonary oxygen uptake (VO2p), heart rate (HR), and muscle deoxygenation [HHb] were examined during the transition to moderate-intensity cycling exercise, in eight older adults (70 ± 2 years) and eight young adults (29 ± 4 years) under 2 conditions: normoxia (FIO2=20.9%) and hypoxia (FIO2=15%). The subjects performed repeated step transitions from an active baseline (12 W) to a relative power output (80% VO2 of the first ventilatory threshold) in both conditions. Phase 2 VO2p, HR, and HHb data were fit with an exponential model. For both groups slower phase 2 VO2p kinetics, were found in hypoxia compared to normoxia (34±8 vs 29±6 sec for older adults and 25±4 vs 19±4 sec for young adults). Only the young adult group showed greater HR (33±6 vs 19±4 sec), [HHb] (18±3 vs 15±2 sec) and (24±2 vs 22±2 sec) in hypoxia compared to normoxia, whereas only the older adults group exhibited a greater [HHb]/VO2 (1.13±0.1 vs 1.09±0.1) ratio in hypoxia compared to normoxia. These findings support the idea that, for older adults the main constrain to the VO2p kinetics in hypoxia lies in the microvascular O2 delivery, whereas for young adults factors other than the microvascular O2 delivery (i.e slowed mitochondrial respiration) are responsible for the slowed VO2p kinetics in hypoxia.
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