Associations of structured and unstructured physical activity time in kindergarten with physical fitness in preschoolers: an isotemporal substitution and time threshold analysis

This cross-sectional study aimed to examine how reallocating time across different types of activities is associated with preschoolers’ physical fitness and to identify potential time thresholds that could inform kindergarten activity planning. A total of 337 Chinese preschoolers (168 boys, 169 girls; mean age 4.41 ± 0.89 years) from nine kindergartens participated. Time allocation among sedentary behavior (SB), structured physical activity (PA), and unstructured PA was quantified using accelerometer data and video observation, and physical fitness was assessed with standardized tests from the Chinese National Physical Fitness Measurement for Preschool. Isotemporal substitution models and compositional time-allocation change point analyses were conducted using R software. Reallocating 10 min of SB to either structured PA or unstructured PA was associated with higher physical fitness scores, with structured PA showing a stronger association. These associations followed a linear dose-response pattern. Model-derived threshold estimates were identified at 53 min for structured PA and 40 min for unstructured PA; however, only the structured PA threshold showed a statistically significant difference in composite fitness scores. Structured moderate-to-vigorous physical activity (MVPA) also demonstrated positive associations with fitness outcomes when replacing other activities, with an estimated threshold of 23 min. These findings suggest that replacing sedentary time with structured or unstructured PA, particularly structured MVPA, is positively associated with improved physical fitness in preschoolers. The estimated time thresholds provide preliminary evidence-based targets for optimizing kindergarten activity curricula and inform the design of future longitudinal interventions.