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Since publication, an error has been identified in Figure S1C. In the initial production of this figure, we had mistakenly duplicated the column under the heading ''L15 + 3% FBS'' to the left of ''L15 + 10% FBS.'' We have removed the second L15 + 3% FBS column to resolve this image duplication. We also duplicated images at 5 hpc L15 +3% FBS and L15 + 10% FBS. We have checked the metadata and origin of this image and confirmed it originates from the L15 + 10% FBS condition. As this error occurred in figure production and not during data analysis, this error has not impacted the conclusions of this experiment. This error has now been corrected online. The authors apologize for this error and any confusion it may have caused.

TO THE EDITOR: The proposal by our well-respected colleagues (2) to introduce a new metric-incorporating the altitude elevation and the total exposure duration, termed "kilometer hours"-for better describing the "hypoxic dose" is decidedly a step forward. By only quantifying the "external" stress, this metric presents several limitations: It suggests a linear relationship between altitude elevation and saturation decrease [but the Fick curve is curvilinear (3)] or that it applies to all athletes irrespectively of their training background [but elite endurance athletes suffer the largest decrease in V O 2max (1)], altitude experience [but elite athletes who have had previous hypoxic exposure better adapt to hypoxic condition (4)], or type of hypoxia [but hypobaric vs. normobaric hypoxia induces larger desaturation (5)].The large intersubject variability in the physiological responses to a given "hypoxic dose" implies that the magnitude of the stimulus rather than the altitude elevation should instead be considered. We therefore propose a new metric based on the sustained duration at a given arterial saturation level. Hence, desaturation levels in normoxia (exercise-induced arterial hypoxemia) or in hypoxia (3) predict the decrement in V O 2max in hypoxia and therefore the˙amplitude of the "hypoxic stimulus." This metric termed "saturation hours" is defined as %·h ϭ (98/s -1) ϫ h ϫ 100, where s is the saturation value (in %) and h the time (in hours) sustained at any second level.Practically, with the development of new sport gears incorporating the oximeter inside the textile, this metric will readily be measured without any disturbances to individuals.TO THE EDITOR: are to be congratulated for their "kilometer hours" (km·h) approach predicting increasing response along with increasing hypoxic dose during altitude training. Previous literature has clearly shown that both endurance training and hypoxic exposure as such can increase hemoglobin mass (Hb mass ), and the responses to their doses are individual. Wehrlin et al. (5) with a 1,080 km·h (24 days 18 h/day, 2,500 m) showed an average 5.3% increase in Hb mass