RESUMEN
Background: Physical fitness optimization and injury risk-reducing require extensive monitoring of training loads and athletes' fatigue status. This study aimed to investigate the effect of a 6-month training program on the training-related stress indicators (creatine kinase - CK; cortisol - COR; serotonin - SER; brain-derived neurotrophic factor - BDNF) in youth soccer players. Methods: Eighteen players (17.8 ± 0.9 years old, body height 181.6 ± 6.9 cm, training experience 9.7 ± 1.7 years) were blood-tested four times: at the start of the preparation period (T0), immediately following the preparation period (T1), mid-competitive period (T2), and at the end of the competitive period (T3). CK activity as well as concentrations of serum COR, SER and BDNF were determined. Training loads were recorded using a session rating of perceived exertion (sRPE). Results: Statistical analyzes revealed significant effects for all biochemical parameters in relation to their time measurements (T0, T1, T2, T3). The statistical analyzes of sRPE and differences of biochemical parameters in their subsequent measurements (T0-T1, T1-T2, T2-T3) also demonstrated significant effects observed for all variables: sRPE (HKW = 13.189 (df = 2); p = 0.00), COR (HKW = 9.261 (df = 2); p = 0.01), CK (HKW = 12.492 (df = 2); p = 0.00), SER (HKW = 7.781 (df = 2); p = 0.02) and BDNF (HKW = 15.160 (df = 2); p < 0.001). Discussion: In conclusion, it should be stated that the most demanding training loads applied in the preparation period (highest sRPE values) resulted in a significant increase in all analyzed biochemical training stress indicators. The reduction in the training loads during a competitive period and the addition of recovery training sessions resulted in a systematic decrease in the values of the measured biochemical indicators. The results of the study showed that both subjective and objective markers, including training loads, are useful in monitoring training stress in youth soccer players.