Intensity and Duration of Physical Activity and Cardiorespiratory Fitness

bjective: There is no clear guidance on the intensity and duration of physical activity (PA) that adolescents need to achieve maximal cardiorespiratory fitness (CRF). We aimed to establish the extent to which each PA intensity is associated with CRF, independently of other intensities, and the duration of PA at each intensity associated with maximum CRF. Methods: PA and CRF were assessed in 339 adolescents aged 13-14 years by wrist-worn accelerometers and 20-meter shuttle runs, respectively. Partial regression modelling was used to construct residualised PA variables (rPA) at each PA intensity that were uncorrelated with each other. Moving average models were optimally fitted to determine relationships between rPA variables and CRF. Threshold regression models determined the duration of PA above which CRF improvement was minimal. Results: Greater VPA was associated with better CRF until about 20 minutes of daily VPA, when the relationship plateaued. Moderate and light PA, and sedentary time were not associated with CRF in partial models. Adolescents performing 14 (range 12-17) minutes of daily VPA had median CRF. Participants in the upper quartile of VPA had 1.03 z-scores higher CRF than those in the lowest quartile (95% CI: 0.75, 1.30). Conclusion: Our data suggest that 20 minutes of daily VPA on average may be best for achieving maximal CRF in adolescence. As moderate-to-vigorous PA guidelines can be satisfied by only undertaking moderate PA, we suggest that future guidelines based on VPA may be better. Our findings should simplify public messaging, aiding the development of PA guidelines for adolescents.