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Abstract Background/Objectives The aim of this study was to examine the effect of physical activity (PA) and sedentary behaviour (SB) on body mass index (BMI) and fat mass index (FMI) in children over the course of five years and identify potential bi-directional associations. Subjects/Methods Data were drawn from the EU Childhood Obesity Project (CHOP). PA and SB were measured with the SenseWear Armband 2 at the ages of 6 (T1), 8 (T2) and 11 (T3) years. Height and weight were measured and BMI was calculated at each time point, resulting in 1254 complete observations from 600 children. Bio impedance analysis was used to measure body fat mass and eventually calculate FMI. To examine the longitudinal association between PA/SB and BMI/FMI as well as to account for repeated measure on these children, mixed model analysis was employed. Results Higher levels of total PA and moderate-to-vigorous PA (MVPA) were associated with lower BMI and FMI and higher SB with higher BMI and FMI over the five year period. When looking at the age dependent effects, negative associations of MVPA (βMVPA x age: − 0.05, 95% confidence interval (CI): − 0.09 – -0.01, p = 0.007) and positive associations of SB (βSB x age: 0.04, 95% CI: 0.02–0.06, p

Abstract Background Physical activity (PA) and its health benefits are a continuous point of discussion. Recommendations for children’s daily PA vary between guidelines. To better define the amount of PA necessary to prevent overweight and obesity in children, further research is needed. The present study investigates children’s compliance to physical activity guidelines (PAGs) and the association between objectively measured PA and body mass index (BMI). Methods Participating children were 11 years old (n = 419) and part of the European CHOP trial, which was conducted in Germany, Belgium, Poland, Spain, Italy. At least 2 days of PA measurements were collected from each child using a SenseWear™ armband. BMI was calculated from children’s height and weight. Thresholds of min·day−1 in PA needed to differentiate between normal and excess weight (overweight/obesity) were determined with Receiver Operator Characteristics (ROC) analysis. Additionally, adjusted linear and logistic regressions models were calculated for group differences and effects of a 5, 15 and 60 min·day−1 increases in PA on BMI. Results Median time spent in total PA was 462 min·day−1 (25th percentile; 75th percentile: 389; 534) and 75 min·day−1 (41; 115) in moderate to vigorous PA (MVPA). Girls spent 36 min·day−1 less in MVPA than boys and overweight/obese children 24 min·day−1 less than normal weight children (linear regression, p