Dose-Response Effects of a 4-Month FIT FIRST 10 School-Based Multisport Programme on Physical Fitness and Psychosocial Well-Being in Children Aged 7-11: A Three-Arm Cluster-Randomized Controlled Trial

Low physical activity (PA) and fitness in childhood are linked to adverse health outcomes, yet many Danish children do not meet national guidelines. The FIT FIRST 10 (FF10) multisport PE programme was developed to increase PA in primary schools. To evaluate dose-response effects of a 4-month FF10 intervention on physical fitness and psychosocial well-being. Three-arm, cluster-randomized trial (1:1:1). 1357 pupils (7-11 years) from 68 classes in 27 schools were allocated to control (CON, n = 529), 1.5 FF10 sessions/week (1.5INT, n = 495), or 3 FF10 sessions/week (3INT, n = 333); 1175 provided consent. The primary outcome was cardiorespiratory fitness via the Yo-Yo Intermittent Recovery Level 1 Children's test (YYIR1C). Secondary outcomes included cardiometabolic health, muscular fitness, and psychosocial well-being. No between-group effects were observed for the primary outcome (1.5INT vs. CON: 17 m [-19, 52]; 3INT vs. CON: -8 m [-47, 31]). Among secondary outcomes, 1.5INT showed small favorable effects on body composition (body fat percentage -0.4%; fat mass -0.2 kg; fat mass index -0.1 kg/m 2; BMI -0.1 kg/m 2) and handgrip strength (+0.4 kg) (all p < 0.05). No intervention effects were observed for cardiovascular or psychosocial domains. Small differences favoring CON over 3INT emerged in standing long jump (-5 cm), postural balance (-0.5 s) and perceived flexibility (-0.3 AU) (all p < 0.05). In conclusion, a moderate, feasible dose of approximately 1.5 FF10 sessions per week appeared more effective than a higher-frequency implementation, highlighting the importance of feasibility and implementation fidelity in real-world school settings. ClinicalTrials.gov (NCT06180772).