Speeding of VO₂ kinetics in response to endurance-training in older and young women

The goal of this study was to examine the time-course of changes in oxygen uptake kinetics (τVO_2p) during step-transitions from 20 W to moderate-intensity cycling in response to endurance-training in older (O) and young (Y) women. Six O (69 ± 7 years) and 8 Y (25 ± 5 years) were tested pre-training, and at 3, 6, 9, and 12 weeks of training. VO_2p was measured breath-by-breath using a mass spectrometer. Changes in deoxygenated-hemoglobin concentration of the vastus lateralis (∆[HHb]) were measured by near-infrared spectroscopy in Y (but this was not possible in O). VO_2p and ∆[HHb] were modeled with a mono-exponential. Training was performed on a cycle-ergometer three times per week for 45 min at ~70% of VO_2peak. Pre-training τVO_2p was greater (p < 0.05) in O (55 ± 16 s) than Y (31 ± 8 s). After 3 weeks training, τVO_2p decreased (p < 0.05) in both O (35 ± 12 s) and Y (22 ± 4 s). A pre-training “overshoot” in the normalized ∆[HHb]/VO_2p ratio relative to the subsequent steady-state level (interpreted as a mismatch of local O₂ delivery to muscle VO₂) was observed in Y. Three weeks of training resulted in that “overshoot” being abolished. Thus there was a training-induced speeding of VO₂ kinetics in O and Y. In the Y this appeared to be the result of improved matching of local O₂ delivery to muscle VO₂. In O, inadequate systemic O₂ distribution (as indirectly expressed by the arterial-venous O₂ difference/VO_2p ratio) seemed to play a role for the initial slower rate of adjustment in VO_2p.