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

Abstract

The time course and mechanisms of adjustment of pulmonary oxygen uptake (V(O(2))) kinetics (time constant tauV(O(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. V(O(2p)) 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(O(2p)) and Delta[HHb] were modeled with a monoexponential model. Training was performed on a cycle ergometer three times per week for 45 min at approximately 70% of peak V(O(2)). Pretraining tauV(O(2p)) was greater (P < 0.05) in O (43 +/- 10 s) than Y (34 +/- 8 s). tauV(O(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 Delta[HHb] was faster than tauV(O(2p)) in both O and Y, resulting in Delta[HHb]/V(O(2p)) displaying an "overshoot" during the transient relative to the subsequent steady-state level. After 3 wk of training the Delta[HHb]/V(O(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 V(O(2p)) kinetics in O and Y at 3 wk of training was associated with an improved matching of local O(2) delivery to muscle V(O(2)) (as represented by a lower Delta[HHb]/V(O(2p))). The continued overshoot in Delta[HHb]/V(O(2p)) in O may reflect a reduced vasodilatory responsiveness that may limit muscle blood flow distribution during the on-transient of exercise.

Figures

Fig. 1.

Changes in the phase II pulmonary oxygen uptake (V̇o2) time constant (τV̇o2p) over the course of the endurance training program in older and young adults. Pre, pretraining; W3, W6, W9, weeks 3, 6, and 9 of training; Post, posttraining. *P < 0.05 compared with pretraining; NS, not significant.

Fig. 2.

1: Group mean profiles for the adjustment of change in deoxygenated hemoglobin concentration (Δ[HHb]) and V̇o2p (left shifted such that data from phase I V̇o2p were not included) during the initial 180 s of a step transition in work rate in older adults pretraining (A), at week 3 (B), week 6 (C), and week 9 (D) of training, and posttraining (E). ●, Time points at which the relative increase of Δ[HHb] is greater than the relative increase of V̇o2p (P < 0.05). 2: Group mean profiles for the adjustment of Δ[HHb]/ΔV̇o2p during the initial 180 s of a step transition in work rate in older adults pretraining (A), at week 3 (B), week 6 (C), and week 9 (D) of training, and posttraining (E). a.u., Arbitrary units. *Δ[HHb]/ΔV̇o2p significantly different from 1.0 (P < 0.05).

Fig. 3.

1: Group mean profiles for the adjustment of Δ[HHb] and V̇o2p (left shifted such that data from phase I V̇o2p were not included) during the initial 180 s of a step transition in work rate in young adults pretraining (A), at week 3 (B), week 6 (C), and week 9 (D) of training, and posttraining (E). ●, Time points at which the relative increase of Δ[HHb] is greater than the relative increase of V̇o2p (P < 0.05). 2: Group mean profiles for the adjustment of Δ[HHb]/ΔV̇o2p during the initial 180 s of a step transition in work rate in young adults pretraining (A), at week 3 (B), week 6 (C), and week 9 (D) of training, and posttraining (E). *Δ[HHb]/ΔV̇o2p significantly different from 1.0 (P < 0.05).

Fig. 4.

A: correlation between changes in Δ[HHb]/ΔV̇o2p and τV̇o2p in response to training for older men (O) and young men (Y). B: time course of changes in Δ[HHb]/ΔV̇o2p and τV̇o2p in Y. C: time course of changes in Δ[HHb]/ΔV̇o2p and τV̇o2p in O. *P < 0.05 compared with pretraining.