Speeding of Vo₂ kinetics with endurance training in old and young men is associated with improved matching of local O₂ delivery to muscle O₂ utilization

The time course and mechanisms of adjustment ot pulmonary oxygen uptake (Vo₂) kinetics (time constant VO_2P) were examined during step transitions from. 20 W to moderate-intensity cycling in eight older men (O; 68 ± 7 yr) and eight young men (Y; 23 ± 5 yr) before training and at 3, 6, 9, and 12 wk of endurance training. Vo_2p was measured breath by breath with a volume turbine and a mass spectrometer. Changes in deoxygenated hemoglobin concentration (△[HHb]) were measured by nearinfrared spectroscopy. Vo_2p and △[HHb] were modeled with a monoexponential model. Training was performed on a cycle ergometer three times per week for 45 min at ∼70% of peak Vo₂. Pretraining VO _2P was greater (P < 0.05) in. O (43 ± 10 s) than Y (34 ± 8 s). VO_2P decreased (P < 0.05) by 3 wk of training in both O (35 ± 9 s) and Y (22 ± 8 s), with no further changes thereafter. The pretraining overall adjustment of △[HHb] was faster than. VO_2P in both O and Y, resulting in △[HHb]/Vo_2p displaying an "overshoot" during the transient relative to the subsequent steady-state level. After 3 wk of training the △[HHb]/Vo _2p overshoot was attenuated in both O and Y. With further training, this overshoot persisted in O but was eliminated after 6 wk in Y. The training-induced speeding of Vo_2p kinetics in O and Y at 3 wk of training was associated with an improved matching of local O₂ delivery to muscle Vo₂ (as represented by a lower △[HHb]/Vo _2p). The continued overshoot in △[HHb]/Vo_2p in O may reflect a reduced vasodilatory responsiveness that may limit muscle blood flow distribution during the on-transient of exercise.