Associations Between Morning And Afternoon Moderate-to-vigorous Physical Activity With Physical Fitness In Children And Adolescents

PURPOSE: Physical activity (PA) mediates fitness attributes and body composition in youth. Yet, how the timing of PA relative to circadian rhythms further influences these outcomes remains to be resolved. Our purpose is to Investigate the associations of morning and afternoon moderate-to-vigorous PA (MVPA) with physical fitness tests and body mass index (BMI) in Portuguese children and adolescents. METHODS: We used children and adolescents’ data (n = 2641; boys: 14.0 yrs ± 2.2 yrs; girls: 14.0 yrs ± 2.2 yrs) from the 2018 Portuguese national surveillance of PA and sports. The children had accelerometry, handgrip strength, and a fitness battery assessed that included the curl-ups, push-ups, standing long jump, and vertical jump tests, and the 10 and 20-m shuttle run. To assess morning and afternoon MVPA, we used accelerometry data from 7 AM to 7 PM. Morning and afternoon MVPA were defined as 7 to 12 AM and 12 PM to 7 PM, respectively. We used the sum of the mean MVPA from each hour and adjusted it by the accelerometer wear time. Linear regression models were fit to assess to what degree the morning and afternoon MVPA could explain the variability in our response variables. The models were then adjusted for age, sex, and both morning and afternoon MVPA were included simultaneously in the models. RESULTS: Afternoon MVPA was associated with higher values on the 20-m shuttle run [β = 0.63; (IC 95%: 0.40; 0.86)], the curl-ups [β = 0.42; (IC 95%: 0.13; 0.71)], the push-ups [β = 0.16; (IC 95%: 0.03; 0.28)], and the standing long jump [β = 0.75; (IC 95%: 0.37; 1.13)] tests, while morning MVPA was negatively associated with the vertical jump test [β = -0.15; (IC 95%: -0.28; -0.03)]. No associations of PA timing were found for handgrip strength and BMI. CONCLUSIONS: Afternoon MVPA was related with higher physical fitness test scores in children and adolescents compared to the morning period, providing observational evidence that the time of the day in which PA is accumulated may yield differentiated benefits for physical fitness attributes. This work was supported by fellowships fromthe Portuguese Foundation for Science and Technology 2020.07856.BD, SFRH/BD/149394/2019, and UIDB/00447/2020.