Effects of Age, Long-term Endurance Training on VO2 Kinetics
Abstract and Introduction
Purpose This study examined the effects of age and training status on the pulmonary oxygen uptake (V̇O2p) kinetics of untrained and chronically trained young, middle-age, and older groups of men.
Methods Breath-by-breath V̇O2p and near-infrared spectroscopy-derived muscle deoxygenation ([HHb]) were monitored continuously in young (20–39 yr) trained (YT, n = 8) and untrained (YuT, n = 8), middle-age (40–59 yr) trained (MT, n = 9) and untrained (MuT, n = 9), and older (60–85 yr) trained (OT, n = 9) and untrained (OuT, n = 8) men. On-transient V̇O2p and [HHb] responses to cycling exercise at 80% of the estimated lactate threshold (three repeats) were modeled as monoexponential. Data were scaled to a relative percentage of the response (0%–100%), the signals time aligned, and the individual [HHb]-to-V̇O2p ratio was calculated as the average [HHb]/V̇O2 during the 20- to 120-s period after exercise onset.
Results The time constant for the adjustment of phase II pulmonary V̇O2 (τV̇O2p) was larger in OuT (42.0 ± 11.3 s) compared with that in YT (17.0 ± 7.5 s), MT (18.1 ± 5.3 s), OT (19.8 ± 5.4 s), YuT (25.7 ± 6.6 s), and MuT (24.4 ± 7.4 s) (P < 0.05). Similarly, the [HHb]/V̇O2 ratio was larger than 1.0 in OuT (1.30 ± 0.13, P < 0.05) and this value was larger than that observed in YT (1.01 ± 0.07), MT (1.04 ± 0.05), OT (1.04 ± 0.04), YuT (1.05 ± 0.03), and MuT (1.02 ± 0.09) (P < 0.05).
Conclusions This study showed that the slower V̇O2 kinetics typically observed in older individuals can be prevented by long-term endurance training interventions. Although the role of O2 delivery relative to peripheral use cannot be elucidated from the current measures, the absence of age-related slowing of V̇O2 kinetics seems to be partly related to a preservation of the matching of O2 delivery to O2 utilization in chronically trained older individuals, as suggested by the reduction in the [HHb]/V̇O2 ratio.
Studies of the physiological response to exercise in different age groups, particularly in the young and old, have demonstrated age-related differences in cardiorespiratory fitness, as determined by changes in maximal oxygen uptake (V̇O2max). In addition, comparisons of highly trained versus more sedentary groups of different ages have allowed assessment as to whether age-related changes in V̇O2max may be due partly to lack of physical activity or whether long-term physical activity prevents or reduces these losses. With respect to V̇O2 kinetics, although several studies have examined differences in the rate of adjustment of oxidative phosphorylation between older and young individuals as well as the effects of exercise training interventions, there is a dearth of information on the acute dynamic V̇O2 responses to submaximal exercise across age groups (young, middle-age, and older individuals) in chronically trained and untrained men.
It has been shown that V̇O2 kinetics during the transition to moderate-intensity exercise is typically slower in older compared with that in young healthy men; thus, older individuals rely more on nonoxidative sources for energy production during exercise on-transients, which could lead to intracellular disturbance of homeostasis and premature fatigue. Importantly, a relatively slow rate of adjustment in the V̇O2 response has also been observed in some young healthy men. Although the precise mechanisms determining the rate of adjustment of oxidative phosphorylation are yet to be fully elucidated, recent studies have proposed that a limitation in O2 delivery and/or distribution to the exercising muscles play a critical role in this sluggish response. Considering that endurance training exercise has been shown to speed V̇O2 kinetics in young, middle-age, and older men and that "late" middle-age adults of similar physical fitness as young adults exhibited similar V̇O2 kinetics responses, it remains unclear whether the age-related slowing of V̇O2 kinetics is due to aging per se or lack of physical activity. Understanding this issue could help further our knowledge on the mechanisms controlling the dynamic adjustment of oxidative phosphorylation.
Thus, the main goal of this study was to examine the age-related differences in V̇O2 kinetics of untrained young, middle-age, and older groups of men, compared with age-matched groups of chronically endurance-trained men, and to further explain the mechanisms controlling those responses. We hypothesized that there would be a continuous increase in the phase II V̇O2p time constant (τV̇O2p) from young to middle-age to older men, in both the trained and untrained groups; it was further hypothesized that in groups with slower V̇O2 kinetics, there would be an O2 delivery limitation, as indicated by a greater mismatch in the adjustment of V̇O2 and O2 extraction.