[Association between body mass index and physical fitness index among children and adolescents].

Objective: To explore the relationship between body mass index (BMI) and physical fitness index (PFI) among children and adolescents.

Methods: The study sample included 8941 students aged 7 to 18 years in Anhui who attended the Physical Fitness and Health Surveillance program of Chinese School Students, in 2010. Within each sex- and age-specific group, students were classified into five BMI categories which were: very low, BMI<5th percentile; low, BMI≥5th but <15th percentiles; normal, BMI≥15th but <85th percentiles; high, BMI≥85th but <95th percentiles; and very high, BMI≥95th percentiles. Z-scores based on urban-rural, sex- and age-specific means and standard deviations were calculated, and the sum of Z-scores for the fitness tests was used as a PFI. Differences in PFI between BMI categories were compared with ANOVA. Sex- and grade-specific regressions of PFI on BMI were done by using a linear model.

Results: For 8941 students, the PFIs on very low, low, normal, high and very high group were -1.77, -0.91, 0.32, -0.17 and -0.54, respectively, and showed an inverted U shape. The normal BMI group students presented the highest PFI. Data from Linear regression analysis revealed that PFI was significantly positively correlated with BMI, while negatively associated with BMI square, which indicated that PFI was the quadratic function of BMI. When BMI was increasing, PFI showed a parabolic curvilinear.

Conclusion: Relationships between BMI and PFI were parabolic curvilinear among the children and adolescents aged 7 to 18 in Anhui province.