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

Abstract Whether anthropometrically estimated fat mass explains variation in physical fitness beyond body mass index z-score remains unclear. This school-based cross-sectional study examined these associations in 3,259 Japanese elementary schoolchildren aged 7–12 years. Fat mass was estimated as measured body weight minus anthropometrically predicted fat-free mass and analyzed with height adjustment because conventional fat mass index showed residual height dependence. The assessed locomotion-related components of physical fitness comprised the 20-m shuttle run, 50-m run, side-step test, and standing long jump. The composite fitness score was calculated as the mean of their sex- and age-standardized scores, with the 50-m run reverse-coded. Sex-stratified models adjusted for age, survey year, and school fixed effects. Higher body mass index z-score and estimated fat mass were associated with lower composite fitness scores in boys and girls. Adding estimated fat mass to models containing body mass index z-score and height increased explained variance by 4.4% points in boys and 2.7% points in girls. Estimated fat mass provided modest additional explanatory information within this sample; these findings do not establish clinical predictive utility.