Similar pattern of change in V˙O₂ kinetics, vascular function, and tissue oxygen provision following an endurance training stimulus in older and young adults

The purpose of this study was to examine the time course of changes in the oxygen uptake (V˙O₂) kinetics response subsequent to short-term exercise training (i.e., 24, 48, 72, and 120 h posttraining) and examine the relationship with the time course of changes in microvascular [deoxygenated hemoglobin concentration ([HHb])-to-V˙O₂ ratio ([HHb])/V˙O₂)] and macrovascular [flow-mediated dilation (FMD)] O₂ delivery to the active tissues/limbs. Seven healthy older [OA; 74 ± 6 (SD) yr] and young men (YA; 25 ± 3 yr) completed three endurance cycling exercise training sessions at 70%V ˙ O2peak. Moderate-intensity exercise on-transient V˙O₂ (measured breath by breath) and [HHb] (measured by near-infrared spectroscopy) were modeled with a monoexponential and normalized (0–100% of response), and the [HHb])/ V˙O₂ was calculated. Ultrasound-derived FMD of the popliteal artery was assessed after 5 min of cuff occlusion. %FMD was calculated as the greatest percent change in diameter from baseline. Time constant of V˙O₂ (τV˙O₂) was significantly reduced in both OA (~18%) and YA (~23%) at 24 h (P < 0.001) posttraining and remained decreased at 48 h before returning toward pretraining (PRE) values. Both groups showed a significant decrease in the [HHb])/V˙O₂ at 24, 48, and 72 h (P = 0.001, 0.01, and 0.03, respectively) posttraining before returning toward PRE values at 120 h. %FMD followed a similar time course to that of changes in the [HHb])/V˙O₂, being significantly greater in both OA (by ~64%) and YA (by ~26%) at 24 h (P < 0.001), remaining increased at 48 and 72 h (P = 0.02 and 0.03, respectively), and returning toward PRE values at 120 h. These data suggest the rate of adjustment of V˙O₂ may be constrained by O₂ availability in the active tissues.