Changes in Fitness, physical activity, fatness, and screen time: A longitudinal study in children and adolescents

Abstract *Objective:* To analyze whether changes in Physical Activity Index (PAI), sedentary time (ST; TV and PC use), and Body Mass Index (BMI) made a significant contribution to longitudinal changes in Physical Fitness (PF) of children and adolescents. Additionally, we analyzed interaction between baseline fitness level and changes in fitness. *Methods:* This is a three years longitudinal study of 345 high school students aged 11-19 years. Students were invited to perform tests from Fitnessgram battery for strength (curl-ups, push-up), and Cardiorespiratory fitness (20m-shuttle run). PAI and ST were evaluated using a standard questionnaire. Standardized scores of physical fitness tests were summed (ZPF). Changes over time, were calculated Δ1 (TP1 minus TP0), Δ2 (TP2 minus TP1), and Δ~total~ (TP2 minus TP0). *Results:* Changes in PAI were positively and independently associated with changes in ZPF in Δ1, Δ2, and Δ~total~. No significant associations of ΔST and ΔZPF were found. ΔBMI was negative associated with ΔZPF in Δtotal. Participants with higher fitness levels at baseline were those who showed positive changes in PAI over Δ~total~, decreased screen time and had the lowest increase in BMI over three years compared with those who were low-fit at baseline. Conclusions: Changes in PAI were the best predictor for changes in Fitness in children and adolescents in each year and over the three years of evaluation. BMI changes were associated with longitudinal changes in PF.