The moderating effect of body mass index on the association between lung function and physical fitness among adolescents.

OBJECTIVE: To investigate the moderating effect of body mass index (BMI) on the association between lung function and physical performance in middle school students, providing quantitative empirical evidence for the implementation of precision physical education. METHODS: A total of 493 eighth-grade students from a middle school in Beijing were recruited for this study. BMI, vital capacity, and various physical fitness indicators were assessed, and the participants were categorized into four groups: normal weight, overweight, obese, and underweight. Spearman rank correlation analysis with Bonferroni correction was employed to evaluate the relationship between lung function and physical fitness across different BMI groups. Following mean-centering of the variables, hierarchical regression models combined with Bootstrap resampling were constructed to examine the interaction and moderating effects. RESULTS: The association between lung function and physical fitness exhibited significant BMI-stratified heterogeneity. In the normal-weight group, vital capacity correlated with the standing long jump (boys: r = 0.329, p < 0.001; girls: r = 0.252, p < 0.001) and 50-m sprint (boys: r = -0.257, p < 0.001; girls: r = -0.230, p < 0.01), as well as the boys' 1,000-m run (r = -0.192, p < 0.01). In overweight boys, it negatively correlated with the 50-m sprint (r = -0.557, p < 0.05) and 1,000-m run (r = -0.698, p < 0.001). Global quadratic interaction terms lacked overall significance (p > 0.05). However, the boys' quadratic BMI_c2 term showed a non-linear trend in the 50-m sprint [95% CI (0.002, 0.025)] and significance in the 1,000-m run (β = 0.340, p = 0.002). Conversely, girls' running events only showed a BMI_c main effect (β = 0.391, p < 0.001); their standing long jump maintained a significant Vital Capacity_c main effect (β = 0.221, p = 0.014) and a linear interaction effect [Vital Capacity_c × BMI_c: 95% CI (0.019, 0.434)]. CONCLUSION: The moderating effect of BMI on adolescent vital capacity and athletic performance demonstrates significant stratified heterogeneity. The translation of vital capacity into physical fitness diverges once specific BMI thresholds are surpassed. Accordingly, physical education should abandon 'one-size-fits-all' approaches in favor of precision interventions integrating individual physiological profiles.