Muscle oxygen extraction is higher during constant work exercise than during incremental ramp exercise.

A cardiopulmonary exercise test (CPX) can provide objective measures of exercise capacity. Specifically, incremental ramp exercise (IRE) and constant work-rate exercise (CWE) protocols are frequently used in clinical practice and for research. The CWE endurance time has shown larger increases than other indexes assessed by IRE after rehabilitation intervention. Muscle oxygen extraction is one of the important physiological factors of exercise capacity; however, the differences in muscle oxygen kinetics between IRE and CWE remain unclear. The purpose of this study was to compare the muscle oxygen kinetics during IRE and CWE. Each of the 15 participants performed IRE and CWE to exhaustion on a cycle ergometer. Ventilatory and muscle deoxygenation responses were measured during the tests; muscle deoxygenation was determined using near-infrared spectroscopy. No differences in oxygen uptake and heart rate were observed between the two tests. A comparison of the muscle deoxygenation kinetics between the two tests indicated significantly greater deoxygenation during the CWE than during the IRE at all time points (p < 0.05). The muscle deoxygenation kinetics, as percentages of maximal oxygen uptake (VO2max), were higher during CWE than during IRE, except at 80% of VO2max (p < 0.05). These results suggest that skeletal muscle during CWE extracts oxygen at a rate comparable to that during IRE, and that exercise capacity assessed using CWE might be linked to a higher overall O2 extraction. The fact that endurance time during CWE is more sensitive to rehabilitation intervention may be due to improvements in muscle oxygen extraction.