Estimated fat mass and body mass index z-score in relation to locomotion-related components of physical fitness in Japanese elementary schoolchildren

Excess adiposity may impair performance in weight-bearing fitness tasks, but whether fat mass index estimated from routine anthropometry provides information alongside body mass index z-score remains unclear. This school-based cross-sectional study examined associations of body mass index z-score and prediction equation-based fat mass index with locomotion-related physical fitness in 3,259 Japanese elementary schoolchildren aged 7–12 years. Height, weight, and field-based fitness tests were assessed during annual school-based assessments in Miyagi Prefecture, Japan, in 2018, 2019, 2021, and 2022. Fat mass index was estimated using validated prediction equations. Locomotion-related fitness was assessed using the 20-m shuttle run, 50-m run, side-step test, and standing long jump, and summarized as a sex- and age-standardized composite score. Sex-stratified models were adjusted for age, survey year, and school fixed effects. Children with overweight or obesity had lower adjusted composite scores than those with healthy weight. Higher body mass index z-score and fat mass index were inversely associated with the composite score in boys and girls, with descriptively larger coefficients for fat mass index. Both indicators were also associated with greater odds of lower fitness quartiles. Prediction equation-based fat mass index may provide complementary adiposity-related information for school-based health and fitness assessment.