Individual-Specific Relationship Between External Training and Match Load and Creatine-Kinase Response in Youth National Team Soccer Players

Background: Previous studies have examined the relationship between external load and creatine-kinase (CK) response at the team level. This study aimed to build individualized CK prediction models for elite youth national team soccer players. Hypothesis: The CK response of youth soccer players can be categorized as being sensitive to micromovements (MM), high-velocity (HV) parameters, or the combination of both, measured during training sessions and matches. Study Design: Prospective cohort study. Level of Evidence: Level 4. Methods: A total of 25 U16-U17 youth national team soccer players were monitored during training sessions and matches using global positioning system (GPS) units. Individual CK values were measured every morning from whole blood. The data set consisted of 57 +/- 17 individual datapoints per player. Individual prediction models were used to examine the relationship between external load and consecutive CK changes. Numerous models were built for each player using MM, HV parameters, or the combination of both. The performance of the models was described by the R-2 and the root mean square error (RMSE, U/l for CK values). Results: The MM models were superior for 8 players (R-2 = 0.68; RMSE = 113 U/l), followed by HV (8 players; R-2 = 0.69; RMSE = 88 U/l) and the combined models (2 players; R-2 = 0.64; RMSE = 141 U/l). For the remaining 7 players, the R-2 of the models was <0.5. The recovery time between efforts was more important in the HV model. Conclusion: Players could be categorized on sensitivity to MM, HV movements, or the combination of both.