Association between body mass index and physical fitness among multiethnic children aged 9-12 years in Zhejiang Province, China: a cross-sectional quantile regression analysis.

Background: Evidence on the association between body mass index (BMI) and physical fitness among children of different ethnicities remains limited. Based on a multiethnic sample of students aged 9-12 years in Zhejiang Province, this study examined the association between BMI and physical fitness across different performance levels using quantile regression. Methods: From September to October 2024, students in grades 3-6 were recruited from primary schools in Zhejiang Province using school-based cluster sampling. Schools were the primary sampling units, and eligible students aged 9-12 years from the four predefined ethnic groups were invited. Physical fitness indicators included lung capacity, 50-meter sprint, sit-and-reach, 1-min rope skipping, 1-min sit-ups, and 50 × 8-meter shuttle run. Quantile regression models assessed BMI-fitness associations across quantiles, adjusting for gender, grade, and ethnicity. Because the analyses were conducted on the final analytic sample without full design-based variance adjustment, the findings should be interpreted as sample-based associations. Results: Of the 12,000 students approached, 11,560 students with complete data were included in the final analytic sample. The associations between BMI and physical fitness outcomes showed clear quantile-specific patterns. For lung capacity, BMI was positively associated with the outcome across all prespecified quantiles from 0.10 to 0.90. For 50-meter sprint time and 50 × 8-meter shuttle run time, BMI was positively associated with completion time across all prespecified quantiles, indicating that students with higher BMI tended to have poorer speed and shuttle-run performance. The association between BMI and 50-meter sprint time was strongest around the middle quantiles. For sit-and-reach performance, significant positive associations were observed at selected quantiles, including higher quantiles. For 1-min sit-ups and 1-min rope skipping, the direction and strength of the associations varied across quantiles, demonstrating heterogeneous BMI-fitness associations across the distribution of physical performance. Conclusion: BMI showed quantile-specific associations with multiple physical fitness indicators. Higher BMI was consistently associated with greater lung capacity and longer completion times in the 50-meter sprint and 50 × 8-meter shuttle run, indicating poorer speed-related performance. Associations with sit-ups and rope skipping varied in direction across quantiles, while the positive association with sit-and-reach performance was more evident at selected higher quantiles.