Mediating Role of Echo-Intensity in Sex Differences of Physical Fitness among Children

Numerous studies have documented disparities in Physical Fitness (PF) between boys and girls. This cross-sectional study investigates if echo-intensity (EI) explains the relationship between sex and PF among children. The study cohort consisted of 497 children aged 4.0–10.9, including 288 boys and 209 girls. Anthropometric measures, including height, weight, and waist circumference, were assessed. PF was evaluated using age-appropriate tests: the ALPHA Fitness battery for school children and the PREFIT battery for preschool children. Ultrasound (US) was employed to evaluate EI in the rectus femoris muscle. Our analysis revealed boys exhibited significantly higher PFcan components and echo-intensity levels compared to girls (all PF p < 0.001, EI p = 0.001). Mediation analysis revealed that the total effect of sex on overall fitness (z-score) was 0.51 (95% CI, 0.36 to 0.66), with an indirect effect mediated by EI of 0.07 (95% CI, 0.02 to 0.13), and a direct effect on overall fitness (z-score) of 0.44 (95% CI, 0.30, 0.58; 15.68% mediation). EI in the rectus femoris muscle serves as a factor elucidating how a child's sex influences their PF. These findings emphasize the importance of considering sex in exercise planning and interpreting the results of fitness assessments. These insights hold potential implications for public health strategies targeting children and adolescents.