Outcome-specific associations of whole-body and segmental body composition with physical fitness in Chinese youth

Body composition is associated with physical fitness in youth, but task-specific relationships of absolute and relative muscle-related measures remain unclear. We analysed cross-sectional data from a national multicentre survey of Chinese children and adolescents aged approximately 6.9–18.0 years: 7,171 for handgrip strength, 7,667 for standing long jump, and 7,648 for the 50 m × 8 shuttle run. Twenty-four anthropometric and body-composition indicators were prioritized using site-grouped cross-validated XGBoost with held-out SHAP values. The highest-ranked indicator for each outcome was then examined using exploratory, post-selection restricted cubic spline models with cluster-robust inference conditional on the selected exposure. Skeletal muscle mass (SMM) ranked highest for handgrip strength, whereas skeletal muscle mass-to-weight ratio (SMMR) ranked highest for standing long jump and shuttle-run time. Complete-model pooled out-of-fold R² values were 0.793, 0.567, and 0.288, respectively. P90-versus-P50 contrasts were + 11.8 kg for handgrip, + 18.4 cm for jump, and − 8.27 s for shuttle-run time. Evidence of nonlinearity was strong for handgrip strength, modest for standing long jump, and absent for shuttle-run time. Predictive performance was substantially lower for shuttle-run time. These observational findings support outcome-specific associations but do not establish independent biological importance or causal effects.