Stringer et al. (1997) developed a method to estimate cardiac output during incremental exercise testing based on Fick's principle. The authors proposed that the arterio mixed venous oxygen content difference increases linearly with percent of maximal O2 consumption. We hypothesized an S-shaped pattern in the published data and calculated the inflection point of this curve and of the standard resting oxygen dissociation curve. Using a partial F-test, we compared the linear model with a third-order polynomial model, which showed a better fit to the data (F(2,101) = 9.5, p < 0.001). This finding was reproduced in a dataset published by Astrand et al. in 1964 (F(2, 122) = 10.6, p < 0.001). The inflection point of the curve coincided with the lactate anaerobic threshold (first ventilatory threshold = VT1) as measured by Stringer et al. (VT1 at 50% and inflection point at 56% of maximal O2 consumption). The inflection point of the standard resting oxygen dissociation curve was calculated at a partial pressure of 21.5 mmHg and a saturation of 36%, matching the "critical capillary PO2" concept of Stringer et al. (21.2 mmHg). We conclude that the arterio mixed venous oxygen content difference increases in an S-shaped manner with percentage of maximal oxygen consumption, and that the inflection point of this curve may correspond to VT1 and that of the in vivo oxygen dissociation curve. Further research is needed to confirm these findings and improve the method.