BackgroundPromoting health-enhancing and sustainable physical activity levels across childhood and adolescence contribute to positive health outcomes as an adult. This study will aim to: a) examine the immediate (pre- to post-intervention) and sustained (1-year post-intervention follow-up) effects of the Children’s Health Activity Motor Program-Afterschool Program (CHAMP-ASP) on physical activity, motor competence, and perceived motor competence relative to the comparison ASP, b) examine the immediate and sustained effects of CHAMP-ASP on secondary health outcomes, specifically health-related physical fitness (i.e., cardiorespiratory fitness, muscular strength, percent body fat) and weight status compared to children in the comparison ASP, and c) determine if perceived motor competence mediates the effect of CHAMP-ASP on moderate-to-vigorous physical activity.MethodsThis multicenter cluster randomized trial will be implemented by ASP staff and will be conducted in ASPs located in two city-based cohorts: East Lansing/Lansing and Ann Arbor/Ypsilanti, Michigan. Children (N = 264) who are K-2 graders will participate 35 min/day X 3 days/week for 19 weeks (1995 min) in their afterschool movement program (i.e., CHAMP-ASP vs. comparison). The research team will train ASP staff to implement the program, which will be delivered within the existing ASP offering. Measures of physical activity (accelerometer), motor competence (process and product measures of fundamental motor skills), health-related fitness, perceived motor competence, and anthropometry will be collected pre-, immediately post-, and one-year post-intervention. Random-effects models will be used to assess the clustered longitudinal effect of the intervention on outcome measures.DiscussionThe long-term goal is to provide a sustainable, ecologically-relevant, and evidence-based program during the early elementary years that can be delivered by ASP staff, is health-enhancing, and increases physical activity in children. Findings hold the potential to help shape public health and educational policies and interventions that support healthy development and active living during the early years.Trial registrationClinicalTrials.gov Identifier NCT05342701.Ethics and disseminationEthical approval was obtained through the Health Sciences and Behavioral Sciences IRB, University of Michigan (HUM00208311). The CHAMP-ASP study is funded by the National Institutes of Health. Findings will be disseminated via print, online media, dissemination events, and practitioner and/or research journals.