Impact of a school-based intervention on physical activity, muscular fitness, and sports participation in Chilean children: single-arm cluster study

Physical inactivity among children remains a major public health concern, particularly in low- and middle-income countries. School-based interventions are a promising strategy to promote physical activity and improve muscular fitness. This study evaluated the impact of a multicomponent school-based intervention on physical activity, muscular fitness and sports participation among Chilean children. A single-arm school-based intervention was conducted with 209 elementary school children from public schools in southern Santiago, Chile. The 24-month intervention included structured physical education, integrated health education, and environmental strategies such as active recess. Physical activity intensities were objectively measured using accelerometry and muscular fitness was assessed through standardized tests. Sports participation was assessed through a structured questionnaire. Linear mixed models were used to estimate changes over the follow-up period, adjusting for age, sex, and body mass index z-score. Over the follow-up, sedentary behavior decreased by 3.7%, whereas light-intensity physical activity increased by 6.1%. Significant increases were observed in moderate (4.9%), vigorous (3.7%), and moderate-to-vigorous physical activity (4.6%). Adjusted models confirmed reductions in sedentary behavior (mean difference = − 25.13 min/day; 95%CI: − 33.55 to − 16.71) and increases in light physical activity (mean difference = 22.38 min/day; 95%CI: 15.31 to 29.46), while changes in moderate-intensity physical activity were borderline significant (mean difference = 2.01 min/day; 95%CI: 0.00 to 4.02). Muscular fitness improved in absolute terms across lower-limb, upper-limb, and abdominal muscle strength outcomes; however, only upper-limb strength remained significantly improved when expressed relative to body mass. Exploratory analyses stratified by baseline body mass index category showed consistent reductions in sedentary time (− 3.2% to − 4.6%) and increases in light-intensity physical activity (6.1% to 6.9%) across categories. Moderate-intensity physical activity increased significantly only among children with normal weight (5.7%). Upper-limb strength increased across all body mass index categories (29.9% to 39.3%), whereas lower-limb strength (13.5%) and abdominal muscle strength (15.5%) improved only among children with normal weight. Participation in cycling, volleyball, dance, soccer, and running also increased. The intervention improved muscular fitness and lower-intensity physical activity but had limited impact on higher-intensity activity, particularly among girls and children with higher BMI, highlighting the need for targeted strategies. ClinicalTrials.gov Identifier: NCT07597460. Registered on May 18, 2026.