Abstract: Background: Current research mostly explored the effect of single behavior on muscle development under traditional regression models, ignoring the temporal relationship between behavior changes. The iso-temporal substitution model can accurately quantify the comprehensive effect of all behavior changes on muscle development. Objective: To explore the association between different physical behaviors and muscle development in children and adolescents in Guangzhou based on the iso-temporal substitution model. Design: ross-sectional survey. Methods: A total of 7,767 children and adolescents aged 6-17 years were enrolled in Guangzhou between November 2020 and June 2021. Questionnaire survey was conducted to collect information on physical activity, sleep, and sedentary behaviors. Body compositions were measured using bioelectrical impedance analysis. Based on linear regression model, iso-temporal substitution model was conducted to evaluate the effect estimates for substitution of sedentary behavior with moderate-vigorous physical activity(MVPA), light physical activity(LPA), or sleep on muscle development, with 30 min·d-1 as the substitution unit. Main outcome measures: Fat-free mass index (FFMI), fat-free mass percentage (FFM%), and appendicular skeletal muscle mass percentage (ASMM%). Results: Replacing 30min of sedentary behavior with MVPA per day significantly increased the Z-scores of FFMI, FFM%, and ASMM% by 0.08, 0.04, and 0.07 units (all P<0.05), respectively. Replacing sedentary behavior with 30 min·d-1 of sleep duration was significantly associated with decrease of FFMI Z score (β=0.04, P<0.05) and increase of FFM% Z score (β=0.03, P>0.05). The stratification analysis by gender and grade level showed similar results, with no significant heterogeneity between subgroups. Conclusion: Replacing sedentary behavior with 30 min of MVPA per day is associated with the increase of FFM and ASMM. The iso-temporal substitution of sedentary behavior with sleep may also be positively associated with muscle development.

Key words: Iso-temporal substitution model, Physical activity behavior, Muscle development, Children and adolescents