An inverse association between physical activity or fitness status and health outcomes has been reported by several cohort studies. When fitness categories are established in quartiles or quintiles based on the peak exercise capacity achieved, the association is graded. Although significant health benefits of increased cardiorespiratory fitness (CRF) have been uniformly reported, the degree of protection has varied substantially between studies. This variability is likely due to varying methods used to define CRF categories, and not considering age, despite its strong effect on CRF. To ameliorate these methodological discrepancies, we propose standardized guidelines by which age-specific CRF categories should be defined.
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