Abstract: Background: Skeletal muscle is a vital component of body composition, playing a key role in maintaining normal growth, development, and overall health in children. However, the levels of skeletal muscle development in children and adolescents differ across various social environments and ethnic backgrounds. Objective: To provide genderand agespecific reference standards for skeletal muscle development in children and adolescents, and to compare the patterns of appendicular skeletal muscle development between Chinese and U.S. children and adolescents. Design: Crosssectional study. Methods: A nationwide sample of 12,583 Chinese children and adolescents aged 318 years was drawn from the China Child and Adolescent Cardiovascular Health study (20132019). Data from the National Health and Nutrition Examination Survey (NHANES) for U.S. children and adolescents, aged 818 years from 2011 to 2018 (n=6,467), were also included. Wholebody and appendicular skeletal muscle mass were measured using dualenergy Xray absorptiometry (DXA). Generalized Additive Models for Location Scale and Shape (GAMLSS) method was utilized to estimate the genderand agespecific reference values. Main outcome measures: Appendicular Skeletal Muscle Index (ASMI), Appendicular Skeletal Muscle Ratio (ASMR), Upper Limb Skeletal Muscle Index (ULSMI), and Lower Limb Skeletal Muscle Index (LLSMI). Results: Overall, ASMI, ULSMI, and LLSMI increased with age in children and adolescents. Boys showed a gradual slowing trend of growth after 14 years of age, while girls exhibited a deceleration and even a decrease after 13 years of age. Additionally, for all age groups, U.S. children had higher values for the same Zscores compared to Chinese children, with this difference widening after 14 years of age. ASMR showed a slightly different developmental pattern, with an initial increase followed by a decrease, but the rate of increase was faster in the Chinese population between the ages of 1014 years. All indicators demonstrated significant gender differences, with boys having higher values in all measures. Conclusion: Developmental patterns and disparities of skeletal muscle mass between Chinese and U. S. pediatric population should be assessed based on the sexand agespecific skeletal muscle reference standards of their respective countries.

Key words: Skeletal muscle, Reference values, Children, Adolescents