Influence of exercise intensity on pulmonary oxygen uptake kinetics in young and late middle-aged adults.

It is unclear whether pulmonary oxygen uptake (Vo2) kinetics demonstrate linear, first-order behavior during supra gas exchange threshold exercise. Resolution of this issue is pertinent to the elucidation of the factors regulating oxygen uptake (Vo2) kinetics, with oxygen availability and utilization proposed as putative mediators. To reexamine this issue with the advantage of a relatively large sample size, 50 young (24 ± 4 yr) and 15 late middle-aged (54 ± 3 yr) participants completed repeated bouts of moderate and heavy exercise. Pulmonary gas exchange, heart rate (HR), and cardiac output (Q) variables were measured throughout. The phase II τ was slower during heavy exercise in both young (moderate: 22 ± 9; heavy: 29 ± 9 s; P ≤ 0.001) and middle-aged (moderate: 22 ± 9; heavy: 30 ± 8 s; P ≤ 0.001) individuals. The HR τ was slower during heavy exercise in young (moderate: 33 ± 10; heavy: 44 ± 15 s; P ≤ 0.05) and middle-aged (moderate: 30 ± 12; heavy: 50 ± 20 s; P ≤ 0.05) participants, and the Q τ showed a similar trend (young moderate: 21 ± 13; heavy: 28 ± 16 s; middle-aged moderate: 32 ± 13; heavy: 40 ± 15 s; P ≥ 0.05). There were no differences in primary component Vo2 kinetics between age groups, but the middle-aged group had a significantly reduced Vo2 slow component amplitude in both absolute (young: 0.25 ± 0.09; middle-aged: 0.11 ± 0.06 l/min; P ≤ 0.05) and relative terms (young: 15 ± 10; middle-aged: 9 ± 4%; P ≤ 0.05). Thus Vo2 kinetics do not demonstrate dynamic linearity during heavy intensity exercise. Speculatively, the slower phase II τ during heavy exercise might be attributable to reduced oxygen availability. Finally, the primary and slow components of Vo2 kinetics appear to be differentially influenced by middle age.