Molecular and Physiological Adaptations to Seasonal Training in Elite U18 Ice Hockey Players

Monitoring adolescent team-sport athletes may benefit from combining performance and molecular markers, but empirical evidence supporting this approach in youth team sports remains limited. Objective: Our study investigated molecular and physiological adaptations to seasonal training in elite U18 ice hockey players, focusing on aerobic capacity, salivary cortisol, serum irisin, and cell-free DNA (cfDNA) dynamics. Methods: National-level U18 players were enrolled in our study (<i>n</i> = 23 for cross-sectional analysis, <i>n</i> = 12 longitudinal) during the pre- and early-competition season. Aerobic performance was assessed via graded treadmill VO₂max testing, and the biochemical markers quantified using ELISA-based assays. Results: From pre- to early-season (paired <i>n</i> = 12), VO₂max increased by 10.6% (g = +1.00, <i>p</i> = 0.003) and irisin by 14.7% (g = +0.83, <i>p</i> = 0.010). cfDNA decreased by 60.8% (g = −0.54, <i>p</i> = 0.070; moderate effect, not statistically clear), while cortisol remained stable (+11.3%; <i>p</i> = 0.667). Inter-individual variability increased for VO₂max and irisin and decreased by 82% for cfDNA. Exploratory cross-sectional positional analysis indicated higher irisin levels in forwards and elevated cfDNA in defensemen, although differences did not reach statistical significance. Conclusions: These preliminary findings provide cohort-size limited longitudinal evidence of chronic irisin elevation in ice hockey players and highlight the possibility of combining VO₂max + irisin + cfDNA to assist individualized load/recovery in elite youth ice hockey.