Association Between Outdoor Physical Activity and Height Growth Velocity in Chinese Children Aged 9–15: A Secondary Analysis of a National Population-Based Cohort

Background/Objectives: The potential of outdoor physical activity as an intervention strategy to promote height growth velocity via stimulating growth hormone secretion and vitamin D synthesis has been scarcely investigated. The present study aimed to investigate the associations between outdoor physical activity duration and height growth velocity, and differences in gender, age, exposure time period (daily, school days vs. weekends), and body mass index (BMI) category. Methods: We performed a secondary analysis of longitudinal data from the 2019–2020 Chinese National Survey on Students’ Constitution and Health. The analytic sample included 5029 adolescents aged 9–18 years. High or low height growth velocity was defined as sex- and age-specific percentiles. Associations of high height growth velocity with outdoor activity duration (≥1 h, ≥2 h) on school days and weekends were investigated using multivariable logistic regression models. Analyses were stratified by sex, age group (9–12, 13–15, 16–18 years), and BMI category (normal weight, overweight, and obese). Results: Results from this cross-sectional analysis indicate that ≥1 h of daily outdoor physical activity is significantly associated with higher height growth velocity among normal-weight boys aged 9–15 years (OR range: 1.71–2.01) and girls aged 9–12 years (OR = 1.68). The positive association increased with ≥2 h (ORs up to 7.96). Consistently positive associations were found for activity during the school day compared to weekends. No significant associations were found in overweight and obese children. Conclusions: Ensuring adequate daily outdoor physical activity—especially on weekends—for at least two hours may be an important potential strategy to promote height growth in normal-weight children and adolescents. Interventions should consider differences in weight status and timing of activity.