Objective: To examine the relationship between 24 hour movement behavior and physical fitness of pupils by using compositional analyses, so as to provide a theoretical basis for physical activity promotion and physical fitness improvement. Methods: By random cluster sampling method, 120 students of grade 4 and 5 from the Second Experimental Primary School of Yingze District of Taiyuan were selected. Physical fitness was evaluated by the indicators of the Protocol of National Physical Training Standard and 20 meter shuttle run. The movement behavior was measured by accelerometer. The relationship between each behavior and physical fitness was analysed by component multiple linear regression, and the effect of replacement of components on physical fitness was discussed. Results: Girls 1minute situps were lower than boys（24.79±7.77，28.21±6.52），and sitting forward flexion was higher than boys[9.00（5.00，14.00），5.20（1.00，9.75）cm]（ t/Z =2.60，-3.15， P ＜0.05）. Boys showed higher light physical activity(LPA), moderate to vigorous physical activity(MVPA) and lower sedentary behavior(SB), while girls showed lower LPA, MVPA and higher SB. MVPA was positively correlated with the performance of 30 second rope skipping and 20 meter shuttle run ( β= 13.19, 7.90, P <0.05). Sleep(SLP), SB, and LPA were not significantly correlated with physical fitness. After re allocating 10 min MVPA to SB and SLP，the performance of 30 second rope skipping and 20 meter shuttle run increased by 2.25，2.28 and 1.28 ,1.34 times，respectively，while significantly decreased after reverse reallocation ( P <0.05). MVPA replaced LPA，the 20 meter shuttle run performance increased 1.46 times，while decreased significantly in reverses ( P <0.05). Conclusion MVPA is positively correlated with the speed and endurance of pupils. There are gender differences in movement behavior patterns and physical fitness. Children, especially girls, should be encouraged to raise exercise awareness and increase PA, especially MVPA.

Objective: To systematically review the associations of physical activity (PA), sedentary behavior (SB) and sleep with obesity, as well as isotemporal substitution effects among behaviors, determined with compositional data analysis methods, to provide a reference for obesity interventions among children and adolescents. Methods: Studies in the CNKI, Wanfang, PubMed, SPORTDiscus, Web of Science and Medline databases were searched from January 1, 2014 to May 1, 2022. Two experienced reviewers independently completed document screening, data extraction and quality assessment. Results: Sixteen articles were included, with a methodological quality score range of 7-12 points. Moderate to vigorous physical activity (MVPA), sleep and obesity were negatively correlated, and substituting other behaviors with MVPA decreased obesity risk. The substitution time ranged from 1.5 min/day to 60 min/day. Light physical activity(LPA) and SB were positively correlated with obesity. Conclusion MVPA is the primary focus of obesity interventions in children and adolescents, and extra 60 min of MVPA per week on the existing level of physical activity may be the minimum necessary to decrease the risk of obesity.

Abstract As the compositional nature of "constant sum" of physical activity, sedentary behavior and sleep are widely recognized, the methodological and result limitations of previous studies that considered physical activity, sedentary behavior, or sleep as independent influences on physical fitness and health have been highlighted. At present,physical activity guidelines of various countries have shifted from local recommendations based on physical activity amount to global recommendations based on 24 h time frame. From the perspective of time use, the content of daily schedule of children and adolescents also has the attribute of "constant sum" of compositional data, and the reduction of students homework burden and extracurricular discipline training after "double reduction" inevitably means an increase in the time allocated to other activities. From the perspective of time use, the paper focuses on physical fitness and health promotion strategies in the context of "double reduction".

Objective: The method of compositional data analysis was used to explore the relationship between 24 hour movement behavior and obesity indicators, and to examine the difference of quantitative effect on obesity indicators when one behavior replaced another behavior, so as to provide specific movement behavior advice for weight control in children and adolescents. Methods: In June 2021, 231 students from eight classes in a primary school and a middle school in Taiyuan City, Shanxi Province were voluntarily recruited by using random cluster sampling. ActiGraph GT3X+ accelerometer was used to measure 24 hour movement behavior and Inbody J20 body composition analyzer was used to measure body composition. The relationship between each component and obesity indicators was analyzed by compositional multivariate linear regression model. In addition, 30 minutes of one behavior was used to replace another behavior to predict the effect difference of the outcomes. Results: After adjusting for covariates such as height, weight, age, and sex, compared with time spent in sedentary behavior(SB), sleep (SLP) and light physical activity (LPA), time spent on moderate to vigorous physical activity (MVPA) was positively correlated with fat free mass index (FFMI) ( β= 0.40, P ＜0.05), negatively correlated with waist circumference (WC) ( β=-2.50, P ＜0.05) and waist hip ratio (WHR) ( β= -0.04 , P ＜0.05). Compared with SB, SLP and MVPA, time spent on LPA was positively correlated with WHR ( β=0.06, P < 0.05 ). If MVPA of 30 min/d replaces SLP, SB, and LPA respectively, WC and WHR decrease 1.10,1.10,1.34 cm and 0.02, 0.02 ,0.02 respectively, and FFMI increases 0.19,0.19,0.15 kg/m 2 respectively. Conclusion In 24 h movement behavior, with consistent level of sedentary behavior, sleep or low intensity movement behavior, maintaining a high level of MVPA and replacing sedentary with active activities are crucial for optimal abdominal fat and fat free mass in children and adolescents.