Oxygen uptake does not increase linearly at high power outputs during incremental exercise test in humans

A group of 12 healthy non-smoking men [mean age 22.3 (SD 1.1) years], performed an incremental exercise test. The test started at 30 W, followed by increases in power output (P) of 30 W every 3 min, until exhaustion. Blood samples were taken from an antecubital vein for determination of plasma concentration lactate [La-]pl and acid-base balance variables. Below the lactate threshold (LT) defined in this study as the highest P above which a sustained increase in [La-]pl was observed (at least 0.5 mmol x l[-1] within 3 min), the pulmonary oxygen uptake (VO2) measured breath-by-breath, showed a linear relationship with P. However, at P above LT [in this study 135 (SD 30) W] there was an additional accumulating increase in VO2 above that expected from the increase in P alone. The magnitude of this effect was illustrated by the difference in the final P observed at maximal oxygen uptake (VO2max) during the incremental exercise test (Pmax,obs at VO2max) and the expected power output at VO2max(Pmax,exp at VO2max) predicted from the linear VO2-P relationship derived from the data collected below LT. The Pmax,obs at VO2max amounting to 270 (SD 19) W was 65.1 (SD 35) W (19%) lower (P < 0.01) than the Pmax,exp at VO2max. The mean value of VO2max reached at Pmax,obs amounted to 3555 (SD 226) ml x min(-1) which was 572 (SD 269) ml x min(-1) higher (P < 0.01) than the VO2 expected at this P, calculated from the linear relationship between VO2 and P derived from the data collected below LT. This fall in locomotory efficiency expressed by the additional increase in VO2, amounting to 572 (SD 269) ml O2 x min(-1), was accompanied by a significant increase in [La-]pl amounting to 7.04 (SD 2.2) mmol x l(-1), a significant increase in blood hydrogen ion concentration ([H+]b) to 7.4 (SD 3) nmol x l(-1) and a significant fall in blood bicarbonate concentration to 5.78 (SD 1.7) mmol x l(-1), in relation to the values measured at the P of the LT. We also correlated the individual values of the additional VO2 with the increases (delta) in variables [La-]pl and delta[H+]b. The delta values for [La-]pl and delta[H+]b were expressed as the differences between values reached at the Pmax,obs at VO2max and the values at LT. No significant correlations between the additional VO2 and delta[La-]pl on [H+]b were found. In conclusion, when performing an incremental exercise test, exceeding P corresponding to LT was accompanied by a significant additional increase in VO2 above that expected from the linear relationship between VO2 and P occurring at lower P. However, the magnitude of the additional increase in VO2 did not correlate with the magnitude of the increases in [La-]pl and [H+]b reached in the final stages of the incremental test.