24-H movement behaviors and physical fitness in preschoolers: A compositional and isotemporal reallocation analysis

Background/Objectives: This study examined the relationships between 24-h movement behaviors and physical fitness (PF) in preschool children. Methods: The study was conducted on 474 children aged 3–6 years in Zhuhai. Physical activity (PA) and sedentary behavior (SB) were collected by the accelerometer, and sleep time was assessed through the parent-report questionnaire. Balance, cardiorespiratory fitness (CRF), flexibility, muscle strength, muscular endurance, and speed-agility were measured using a balance beam test, 20 m shuttle run test, sit and reach test, handgrip test, sit-ups, and 4 × 10 m shuttle run test respectively. The compositional data analysis was used to examine the association between 24-h movement behaviors and PF, and the compositional isotemporal substitution analysis was used for the time reallocation. Results: The daily composition, adjusted for age, gender, and body mass index (BMI), was significantly associated with CRF (p < 0.001, r² = 0.20), flexibility (p < 0.001, r² = 0.07), muscular strength (p < 0.001, r² = 0.37), muscular endurance (p < 0.001, r² = 0.26), and speed-agility (p < 0.001, r² = 0.26). The addition of moderate-to-vigorous PA (MVPA) at the expense of SB and sleep, MVPA at the cost of sleep, was associated with significant muscular strength and speed-agility improvements respectively. The impact of SB and sleep replacing MVPA is stronger than MVPA replacing SB and sleep on muscular strength. Conclusion: These findings offer useful insight for the replacement of movement behaviors within the recommended range to facilitate PF development in early childhood.