Speeding of (V) over dotO2 kinetics with endurance training in old and young men is associated with improved matching of local O2 delivery to muscle O2 utilization

Murias JM, Kowalchuk JM, Paterson DH. Speeding of (V) over dot(O2) kinetics with endurance training in old and young men is associated with improved matching of local O-2 delivery to muscle O-2 utilization. J Appl Physiol 108: 913-922, 2010. First published February 11, 2010; doi:10.1152/japplphysiol.01355.2009.-The time course and mechanisms of adjustment of pulmonary oxygen uptake ((V) over dot(O2)) kinetics (time constant tau(V) over dot(O2p)) 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. (V) over dot(O2p) was measured breath by breath with a volume turbine and a mass spectrometer. Changes in deoxygenated hemoglobin concentration (Delta[HHb]) were measured by near-infrared spectroscopy. (V) over dot(O2p) and Delta[HHb] were modeled with a mono-exponential model. Training was performed on a cycle ergometer three times per week for 45 min at similar to 70% of peak (V) over dot(O2). Pre-training tau(V) over dot(O2p) was greater (P < 0.05) in O (43 +/- 10 s) than Y (34 +/- 8 s). tau(V) over dot(O2p) 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 Delta[HHb] was faster than tau(V) over dot(O2p) in both O and Y, resulting in Delta[HHb]/(V) over dot(O2p) displaying an overshoot during the transient relative to the subsequent steady-state level. After 3 wk of training the Delta[HHb]/(V) over dot(O2p) 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 (V) over dot(O2p) kinetics in O and Y at 3 wk of training was associated with an improved matching of local O-2 delivery to muscle (V) over dot(O2) (as represented by a lower Delta[HHb]/(V) over dot(O2p)). The continued overshoot in Delta[HHb]/(V) over dot(O2p) in O may reflect a reduced vasodilatory responsiveness that may limit muscle blood flow distribution during the on-transient of exercise.