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 (tau VO2p) 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. VO2p was measured breath-by-breath using a mass spectrometer. Changes in deoxygenated-hemoglobin concentration of the vastus lateralis (Delta[HHb]) were measured by near-infrared spectroscopy in Y (but this was not possible in O). VO2p and Delta[HHb] were modeled with a mono-exponential. Training was performed on a cycle-ergometer three times per week for 45 min at similar to 70% of VO2peak. Pre-training tau VO2p was greater (p < 0.05) in O (55 +/- 16 s) than Y (31 +/- 8 s). After 3 weeks training, tau VO2p decreased (p < 0.05) in both O (35 +/- 12 s) and Y (22 +/- 4 s). A pre-training overshoot in the normalized Delta[HHb]/VO2p ratio relative to the subsequent steady-state level (interpreted as a mismatch of local O-2 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 O-2 delivery to muscle VO2. In O, inadequate systemic O-2 distribution (as indirectly expressed by the arterial-venous O-2 difference/VO2p ratio) seemed to play a role for the initial slower rate of adjustment in VO2p.