Changes in oxygen uptake kinetics after exercise caused by differences in loading pattern and exercise intensity

The kinetics of recovery-period oxygen uptake (VO₂ ) are affected by the O₂ deficit generated during exercise. However, studies using ramp tests (RTs) and constant work rate tests (CT) have differently characterized VO₂ responses to increased exercise intensity differently. We used these two types of loading patterns to investigate the effects of low-intensity, medium-intensity, and high-intensity exercises on the half time (T_1/2 ) of recovery-period VO₂ and the mechanism.

Ten healthy men aged 21.2 ± 0.9 years underwent symptom-limited cardiopulmonary exercise tests with the ramp protocol to determine their anaerobic threshold. All subjects subsequently underwent three submaximal RT and CT at low, moderate, and high intensities. In all RTs, subjects began exercise by warming up (20 W). In CT, T_1/2 was significantly lengthened as exercise intensity increased (CT-low: 34.0 ± 3.9 s, CT-moderate: 39.5 ± 3.5 s, CT-high:44.6 ± 4.2 s; P < 0.01, ANOVA), whereas no significant change was observed in RT, which began with the same work rate (RT-low: 46.0 ± 5.7 s, RT-moderate: 45.7 ± 4.8 s, RT-high: 44.6 ± 3.5 s, RT-max: 44.8 ± 3.2 s; P = 0.868, ANOVA). Only high-intensity exercise resulted in two components (the fast and slow components) of VO₂ decay, reflecting the increased O₂ deficit by anaerobic metabolism.

The exercise intensity at the beginning of an exercise affects early recovery-period VO₂ , which is a fast component. The T_1/2 of recovery-period VO₂ occurs during the fast component, and an increase in O₂ deficit affects both the fast and slow components, lengthening the T_1/2 . The T_1/2 of recovery-period VO₂ in CT at moderate or high intensities, even if not symptom limited, can be used to evaluate exercise intolerance and early occurrence of anaerobic metabolism. Submaximal exercise tests may be considered as convenient methods for evaluating exercise tolerance in patients with cardiac failure.