Cardiorespiratory Fitness and Physical Activity among Children and Adolescents: 3-Year Longitudinal Study in Brazil

The aim of this study was to investigate the effect of physical activity (PA) levels over 36 months on the cardiorespiratory fitness (CRF) of children and adolescents. This was a longitudinal study carried out from 2016 to 2019 with 127 children and adolescents (82 males and 45 females) aged 13.5 ± 1.2 years at baseline. The 20 m shuttle run test (20 mSRT) was used to predict CRF. The questionnaires PAQ-C and PAQ-A were used to investigate PA. The 2 × 4 repeated measures ANCOVA was used, and the significance level was p ≤ 0.05. There was a linear trend in the interaction between level of PA and CRF throughout the 36-month study period in both sexes (males—distance covered: F1.78 = 4.567; p = 0.04; VO(2max): F1.78 = 5.323; p = 0.02; females—distance covered: F1.41 = 6.989; p = 0.01; VO(2max): F1.41 = 6.585; p = 0.01). Physically active males showed a gradual increase in CRF throughout the analyzed period. For females, CRF remained constant in the first 24 months, showing a slight increase only after 36 months in physically active ones. The findings of this study reinforce the importance of PA throughout childhood and adolescence to improve CRF.