Effects of plyometric-based high-intensity interval training on muscle-to-fat ratio in children: protocol for a randomized controlled trial in primary school settings

Background Childhood obesity is associated with cardiometabolic and musculoskeletal health risks. Because fat reduction may be accompanied by loss of skeletal muscle, this trial aims to evaluate whether a plyometric-based high-intensity interval training (Plyo-HIIT) program improves muscle-to-fat ratio (MFR) in children aged 9–12 years. Methods This pragmatic, assessor-blinded, three-arm randomized controlled trial will recruit 210 children aged 9–12 years from primary schools in Hong Kong, China. Participants will be randomly assigned to Plyo-HIIT, structured fitness (SF) program, or the health education (HE) control group in a 1:1:1 ratio. Each group will engage in two 45-minute weekly sessions for 16 weeks. The Plyo-HIIT program involved developmentally appropriate plyometric exercise (e.g., jumping, skipping). The SF program includes moderate- to high-intensity activities maintaining 60%–80% of maximal heart rate. The HE group will receive weekly health education sessions. Outcomes will be assessed at baseline, post-intervention (16 weeks), and at 6-month follow-up. The primary outcome will be MFR measured using magnetic resonance imaging. Secondary outcomes will include body composition, muscular fitness, cardiorespiratory fitness, flexibility, body mass index, physical activity, and cortisol levels. Results Participant recruitment was conducted between September 26, 2024, and March 14, 2025. The 16-week intervention has been completed. However, as one participating cohort delayed the start date of the intervention, the 6-month follow-up assessments are still ongoing, with the final follow-up assessment scheduled for August 2026. No trial outcome data are reported in this protocol article. Conclusions This study is expected to provide evidence for the effectiveness of Plyo-HIIT in improving body composition by increasing the muscle-to-fat ratio among school-age children. This will support the integration of Plyo-HIIT training into school-based physical activity programs, offering a practical and time-efficient strategy to prevent early-onset obesity. By comparing Plyo-HIIT with the SF and HE groups, this study may further demonstrate the value of plyometric training for primary school settings.