ABSTRACT: The aim of this study was to make a comprehensive gathering of consecutive detailed blood samples from professional soccer players, and to analyze different blood parameters in relation to seasonal changes in training and match exposure.Blood samples were collected five times during a six months period and analyzed for 37 variables in 27 professional soccer players from the best Danish league. Additionally, players were tested for body composition, VO2max and physical performance by the Yo-Yo intermittent endurance sub-max test (IE2).Multiple variations in blood parameters occurred during the observation period, including a decrease in hemoglobin and an increase in hematocrit as the competitive season progressed. Iron and transferrin was stabile, whereas ferritin showed a decrease at the end of the season. IgA and IgM increased in the period with basal physical training and at the end of the season. Leucocytes decreased with increased physical training. Lymphocytes decreased at the end of the season. VO2max decreased towards the end of the season whereas no significant changes were observed in the IE2 test.The regular blood samples from elite soccer players reveal significant changes that may be related to changes in training pattern, match exposure or length of the match-season. Especially the end of the preparation-season and at the end of the competitive season seem to be time points were the blood-derived values indicate that the players are under excessive physical strain and might be more subjected to a possible overreaching/overtraining conditions.We suggest that regular analyzes of blood samples could be an important initiative to optimize training adaptation, training load and game participation, but sampling has to be regular and a database has to be build for each individual player.
Journal of Strength and Conditioning Research, 2013, Vol 27, Issue 5, p. 1260-1271