The oxygen uptake threshold during incremental exercise test.

The linear relationship between oxygen consumption (VO2) and exercise intensity is a well established phenomenon observed during incremental exercise. Recently, a non-linear increase in VO2 has been reported by Zoladz et al., who used a relatively complicated method to describe the phenomenon. In this study, we tried to ascertain whether the same phenomenon, which we named the oxygen uptake threshold (OUT), could be described by a simple method, using the two best fitting lines adopted for the less and more steep parts of the VO2 increase. Our hypothesis was that the non-linear VO2 increase was the result of a continuous VO2 increase (oxygen drift) occurring during the more intense steps only. Therefore, we analysed the VO2 time course during each step. Six cyclists performed an incremental exercise test on a cyclo-ergometer. The lactate threshold (LT) was calculated by using the intersection point of the two best fitting lines in the diagram of log LA (lactate concentration) dependence on log P (Power). The time course of VO2 during each step was analysed by an exponential rise to the maximum model. The results showed that OUT could be determined in five of the six subjects, whereas LT could be determined in all six subjects. The power output determined by OUT (168 +/- 13 W) was similar to that determined by LT (180 +/- 25 W). The VO2 time course during each step showed steady values during low intensity exercise. At intensities above LT and OUT, however, VO2 increased continuously, showing oxygen drift. It may be concluded that OUT is a realistic phenomenon, which is based on oxygen drift.