From Pre-Season to In-Season: Variations in Fitness and Hematological Variables in Elite Soccer Players

Several variables are commonly used to monitor soccer players during and off-training; however, how different phases of the season influence these variables and their relationship with performance requires further investigation. This study aimed to examine how pre-season and in-season soccer periods influence physical fitness as well as hematological and inflammatory markers in elite soccer players. Twenty-one soccer players of a second-highest division team in Italy (Ternana Football Club) were recruited and tested at three time points: before the pre-season, after the pre-season and in the middle of the season (in-season). Fitness tests included countermovement jump (CMJ), 10-meter sprint, repeated sprint ability (RSA), and shuttle-running (Yo-Yo IR-2) testing. Blood markers evaluated were hematocrit, C-reactive protein (CRP), creatine kinase (CK), lactate dehydrogenase (LDH), and interleukins 6 and 10 (IL-6, IL-10). Training and matches were monitored throughout the season via GPS tracking and load indices were derived. A repeated-measures MANOVA was used to test differences in the dependent variables among the three time points. Results showed decreased performance on the 10-m sprint and RSA, along with elevated inflammatory markers (CK, LDH, IL-6) following the pre-season. In contrast, in-season data revealed improved fitness performance accompanied by a reduction in IL-6 levels. Coherently, a negative correlation was observed between IL-6 variation and RSA performance. These results could help the teams' staff to select the most sensitive markers to investigate players' fatigue during the season, offering practical insights to optimize individualized recovery strategies and enhance performance.