Holistic Training Load Monitoring In Elite Youth Soccer

PURPOSE: The search for valid tools to holistically monitor training load and the psychophysiological response in elite sports is steadily growing. Here, we explore relationships between objective tracking data and a comprehensive set of internal load measures including blood biomarkers, questionnaires, and neuromuscular performance in soccer players. METHODS: 25 male soccer players (age: 16.6 ± 0.9 years, VO2max: 59 ± 4 ml·min-1·kg-1) competing in the first national league and the UEFA Youth League were subjected to a three-month monitoring approach. Standardized procedures with daily questionnaires (covering rate of perceived exertion (RPE), sleep, well-being), blood (twice weekly to determine e.g., creatine kinase (CK), lactate dehydrogenase (LDH), urea, cortisol, TNF-alpha, transferrin) and neuromuscular testing (countermovement jumps, CMJ) were implemented. Training sessions and games were monitored using a local positioning system. We calculated acute workload (AL), exponential chronic workload (CL) over seven days and the acute:chronic workload ratio (ACWR) as training load measures using total distance, high metabolic power distance (HMPD), and training impulse (TRIMP). Associations between workload and the subsequently measured internal load variables were quantified using Spearman's rank correlation as a mean value and for all individual players. Alpha level of significance was set to 0.05. RESULTS: AL measures (distance, HMPD, TRIMP) were most strongly reflected in RPE, with significant mean correlations in the range of r = .30 - .43. Few weak correlations between blood variables (i.e., urea, CK, LDH) and AL measures (r = .17 - .27) were found, while CMJ variables were not related. CL was most strongly correlated with RPE (r = .19 - .34) with weak negative relations to CMJ variables (r = -.16 - -.24). LDH was weakly associated with CL measures (r = .20 - .21). ACWR variables were again most strongly reflected in RPE (r = .13 - .29). Weak associations were found between ACWR and transferrin (r = -.11 - -.19) as well as CK (r = .17 - .22), while CMJ was negatively related to ACWR. CONCLUSIONS: Questionnaires showed the closest relationship to AL, CL and ACWR. Despite significant associations, future studies should focus on defining clinically relevant changes that warrant appropriate load management. The study is a cooperation between the Universities of Salzburg and Mainz and the Red Bull Athlete Performance Center. The study receives funding from the Red Bull Athlete Performance Center for the scientific accompaniment of the monitoring concept.