RESUMEN
Background: The increasing prevalence of obesity among children and adolescents is a major public health challenge worldwide. This study examined the relationship between physical fitness and BMI spanning the range from underweight to obese among Chinese mainland children and adolescents. Methods: Participants were 22,681 children and adolescents (11,300 boys and 11,381 girls) aged 10-18 years from the Chinese mainland. Weight status was classified as underweight, normal weight, overweight, and obese using WHO 2007 standards. Physical fitness parameters such as cardiorespiratory fitness (VO2max), lower body explosive strength (standing broad jump), upper body explosive strength (handgrip strength), abdominal muscular endurance (sit-ups in 30 s), flexibility (sit-and-reach), and agility (repeat bestride (20 s)) were assessed. Results: There was a significant association between weight status categories and physical fitness in all age groups and sex (plinear < 0.001, pquadratic < 0.001). Underweight adolescents performed better in lower limb strength, flexibility, agility, and cardiorespiratory fitness than their obese peers, but worse in upper limb strength. Underweight boys aged 10-11 and 12-13 years and girls aged 10-11 years showed significantly (p < 0.05) high odds of meeting a low physical fitness index. Obese adolescents have high odds of meeting a low physical fitness index with age. Conclusion: The present study showed a nonlinear relationship between weight status and physical fitness. Children and adolescents who were classified as underweight or obese had poorer physical fitness than their normal-weight peers.