The oxygen uptake efficiency slope (OUES) has been proposed as a useful, objective, non effort-dependent and submaximal index of cardiorespiratory functional reserve by Baba and colleagues (1996). The OUES corresponds to the rate of increase in oxygen uptake (VO₂) in response to the logarithmic transformation of ventilation (VE) during an incremental exercise test. The transformation of the x-axis showed a linear relation between VO₂ and log10VE which can be modelised to obtain the OUES:VO₂=OUES×log10VE+b. Therefore OUES could be obtained from the whole or only a part of the test duration allowing submaximal determination of cardiorespiratory fitness. Thus, OUES could be used in subjects unable to perform a really maximal incremental exercise.

The OUES has been already used and tested in a lot of populations such as healthy subjects, or patients with chronic heart failure or ischemia, depression, obesity or renal deficiency in whom the attainment of the maximal oxygen uptake (VO2max) is difficult or clinically impossible. The OUES, even if calculated from only part of the test, has been shown to be reproducible, reliable and highly correlated with VO2maxor with the first ventilatory anaerobic threshold. It can be considered as a good parameter to evaluate functional exercise capacity and predict mortality with a good specificity and sensitivity in several populations, and even when subjects where unable to reach maximal intensity. Nevertheless, its use to predict VO2peak seems compromised. The physiological significance of OUES is also discussed.