Total and regional skeletal muscle mass measured by magnetic resonance imaging in Japanese children aged 7–11 years

The aims of this study were to obtain representative data on total and regional skeletal muscle (SM) mass in prepubertal Japanese children using magnetic resonance imaging (MRI), and to examine sex differences in SM mass and its distribution. A total of 129 healthy prepubertal Japanese children (78 boys and 51 girls), aged 7–11 years and classified as Tanner stage 1, were enrolled. Contiguous MRI scans were acquired from the first cervical vertebra to the ankle joint. SM volume was calculated by summing digitised cross-sectional areas and dividing them into discrete anatomical regions. Age was significantly correlated with total, arm, trunk, thigh, and lower leg SM mass in both sexes. Total SM mass ranged from approximately 7 to 11 kg in boys and 6 to 10 kg in girls across the age range, increasing by roughly 1 kg per year in both groups. Boys had significantly higher intercepts in the regression lines between age and total, arm, trunk, and thigh SM mass compared to girls (p < 0.05), except for the lower leg. While no sex differences were observed in the slopes of the regression lines for total, arm, trunk, and lower leg SM mass, boys demonstrated a significantly steeper slope for thigh SM mass. This study provides detailed MRI-based reference representative data on SM mass and regional distribution in prepubertal children, contributing to a better understanding of early developmental differences between sexes.