The PO2 at which hemoglobin is half-saturated with O2 (P50) at 37\ub0C, PCO2 = 42 Torr, measured pH and 2,3-diphosphoglycerate-to-hemoglobin concentration ratio ([2,3-DPG]/[Hb]) values, Hill's coefficient (n) at rest, and maximal O2 consumption (V\u307O2(max)) were determined in 11 world-class professional bicycle racers off-season (control, C), after 3 mo of 3 h daily training (preseason, PrS), and after additional 6 mo of competitions (competitive season, CoS). The results indicate that the P50 observed in trained athletes was the same as that of a comparable group of sedentary subjects (Sed) under the same conditions of pH, PCO2, and [2,3-DPG]/[Hb] and was similar to that obtained after 'normalization' in respect to pH and the [2,3-DPG]/[Hb]; [2,3-DPG]/[Hb] increased as a function of training from 0.72 to 0.95 (P < 0.001); the slope of the central portion of the O2 equilibrium curve (OEC) was nearly unaffected by endurance training as indicated by the n value (n(CoS) = 2.70 \ub1 0.08; n(Sed) = 2.65 \ub1 0.08); and V\u307O2(max) increased in the course of training 7 and 9% (P < 0.001), respectively, when expressed in absolute units or per kilogram body weight. The V\u307O2(max) predicted on the basis of a computer simulation does not increase significantly as a consequence of the measured rise in [2,3-DPG]. Therefore, the observed increase of V\u307O2(max) cannot be explained with adaptive changes of the OEC. The present results differ from previous findings reported in other types of athletes
