Smoothed percentile reference curves for health-related physical fitness in children and adolescents aged 9–17 years: a multi-regional study in Kazakhstan

BACKGROUND: Population-specific physical fitness references are essential for monitoring growth, health, and functional capacity during childhood and adolescence. AIM: To develop sex- and age-specific smoothed percentile curves for health-related physical fitness indicators among children and adolescents in Kazakhstan. SUBJECTS AND METHODS: Data were collected from 8,158 children and adolescents aged 9-17 years from 20 schools located in 10 regions of Kazakhstan between October 2023 and October 2025. Standardised field-based protocols were used to assess anthropometry, muscular strength, speed, coordination, and cardiorespiratory fitness. Percentile curves (P3-P97) were generated using the LMS method within a GAMLSS framework to ensure biologically plausible smoothing across age. RESULTS: Most performance indicators increased with age in both sexes, whereas relative aerobic capacity showed a gradual decline. The resulting references provide regionally representative normative values that may assist educational and health professionals in monitoring youth fitness and identifying deviations from expected developmental trajectories. Because the sample covers approximately half of Kazakhstan's regions and consists predominantly of ethnic Kazakh participants, the results should be interpreted as multi-regional reference standards rather than nationally representative norms. CONCLUSION: These findings contribute to global comparative datasets and highlight the importance of population-specific fitness references influenced by environmental, cultural, and behavioural factors.