Phase-specific changes in anthropometric and physical fitness outcomes among Chinese upper-secondary students before, during, and after the COVID-19 pandemic: the moderating role of educational track.

Background: The COVID-19 pandemic substantially disrupted school-based physical activity worldwide; however, how such disruptions differentially affect distinct domains of adolescent physical fitness across educational tracks remains insufficiently understood. Methods: Using large-scale school-based fitness surveillance data collected before, during, and after the COVID-19 pandemic, we examined phase-specific changes in anthropometric indicators and physical fitness outcomes among Chinese upper secondary school students aged 15-18 years. Students were stratified by educational track (general academic vs. vocational education), and outcomes spanning explosive power, endurance, flexibility, and anthropometric measures were analyzed to assess phase effects and phase-by-school-type interactions. Results: Statistically robust phase-related variations and phase-by-school-type interactions were observed across all fitness domains, with highly domain-specific responses. Anthropometric indicators followed overall upward trajectories consistent with long-term secular patterns, although students in vocational education exhibited greater temporal sensitivity to pandemic-related disruption. Explosive power showed relatively small phase effects but large and persistent between-school differences, whereas endurance and flexibility displayed pronounced phase-dependent changes, including delayed differentiation in endurance performance and a temporary narrowing of between-school disparities in flexibility during the pandemic. Conclusions: These findings demonstrate that large-scale societal disruptions, such as the COVID-19 pandemic, can alter adolescent physical fitness in domain- and context-specific ways, reshaping developmental patterns and inequality dynamics rather than uniformly depressing fitness levels. The educational track plays a critical role in moderating vulnerability and recovery across fitness domains, underscoring the need for fitness monitoring and intervention strategies that are sensitive to both domain-specific characteristics and the educational context.