BACKGROUND: Maximum oxygen consumption (VO2max) is regarded as the gold standard measurement of cardiorespiratory fitness and is classically assessed by a graded exercise test (GXT) until volitional fatigue. The limitations of primary (VO2 plateau) and secondary criteria [max heart rate (HR) and respiratory exchange ratio (RER)] to confirm true VO2max achievement during the GXT led to the adoption of a verification stage­. This study aimed to determine the utility of including a verification stage when assessing VO2max in normobaric hypoxia (i.e., simulated high altitude), compared to sea level. Additionally, we sought to assess the efficacy of primary and secondary criteria traditionally used to confirm the attainment of VO2max during the GXT, in both normoxia and hypoxia. METHODS: Twenty male (n=15) and female (n=5) subjects completed two separate cycling VO2max trials consisting of a GXT and verification stage, separated by 10 min of passive rest. Tests in normoxia and hypoxia were separated by ≥ 48 h and completed in a randomly counterbalanced order. The highest VO2 aggregate over 30 s was used as the VO2max value from each stage and condition. Data were analyzed using a series of repeated-measures ANOVAs. RESULTS: VO2max was higher in normoxia compared to hypoxia (p < 0.05), with no significant differences between stages (GXT vs. verification) within each condition. When applying a threshold of 2% to classify whether VO2 attained in verification vs. GXT was similar or not, 12 of 20 subjects exhibited higher values during the verification stage in normoxia, whereas only 3 of 20 subjects had higher VO2max values during the verification stage in hypoxia. Based on sensitivity and specificity calculations, primary (plateau < 150 mL/min increase in VO2 with an increase in workload) and secondary criteria (peak HR within ± 10 bpm of age-predicted HR max and RER ≥ 1.10) were unable to verify VO2max attainment in the GXT due to low sensitivity/specificity. CONCLUSIONS: A stage x condition interaction was not observed; however, there appeared to be a meaningful difference in the proportion of subjects that achieved > 2% higher values during verification vs. GXT in normoxia when compared to subjects in hypoxia. Primary and secondary criteria were inadequate at confirming a true VO2max during the GXT. Including a verification stage when VO2max testing appears to be more useful when testing in normoxia vs. hypoxia.

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