Purpose: Despite the global emphasis on talent identification and development (TID), longitudinal evidence on physical development in Asian youth remains scarce. Using Japan's local TID (L-TID) multi-sport matching model, this study examined longitudinal trajectories of physical fitness in talented Japanese youth athletes categorised by maturity status. Methods: Data were analysed from 96 participants (43 males, 53 females) in a Fukuoka L-TID programme, followed from Grade 4 to Grade 9. Somatic maturity (Early, Average, or Late) was based on the timing of peak height velocity (PHV). Measurements included height, body mass, 25-m sprint, standing broad jump (SBJ), reaction time (RT), and 20-m shuttle run (SR). Results: In males, improvements in SBJ and sprint speed synchronised with the PHV period (p < 0.05), supported by medium-to-large interaction effects. Conversely, females demonstrated a significant post-PHV decline in SR (p < 0.05), suggesting a transient physiological constraint. RT improved independently of maturation across both sexes (p < 0.05), reaching a plateau between Grades 8 and 9. Correlation analysis revealed substantial performance ranking instability between programme entry and exit, particularly within the Average maturity group (r = 0.09-0.22). Conclusion: Biological maturation acts as an accelerator for males but a constraint for females. Observed ranking instability suggests proximity to PHV disrupts motor coordination, requiring neuromuscular re-integration. Therefore, these findings suggest that early-stage TID programmes could consider integrating neurocognitive development to establish a stable foundation before the onset of maturation-related physical fluctuations.
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