The participants included in our cardiopulmonary exercise testing study were recruited from a population-based sample in which several individuals reported asthma, cardiovascular disease, cancer, or type 2 diabetes. Hence, it may be argued that the health status of the original study population included in our study and that of those in the HUNT Fitness study were different. However, only a few individuals who reported underlying disease were included in the exercise test in the cardiopulmonary exercise testing study, suggesting that this is not the main explanation for the differences observed in o2max between the studies. Further, the difference in o2max between the studies was consistent across age strata, implying that the health aspect may be less important. Nevertheless, the differences between the study populations may still explain the differences observed in o2max between the HUNT Fitness study and ours, for instance, with respect to differences in exercise habits and physical activity. When discussing our observations, we mentioned “level of exhaustion” as one of the possible reasons for the different results between studies. Even though maximal heart rate (HRmax) is not a variable that predicts maximal effort well, it may provide a picture of the degree of fatigue in larger groups. Unfortunately, to our knowledge, the information about HRmax from the HUNT Fitness study3 was not publicly available when we submitted our manuscript. In retrospect, the higher observed HRmax values in the HUNT Fitness study may well partly explain the difference in o2max between the study populations.