Soccer at the elite level is a physical demanding game, and the physical challenges of the players have increased during the later years as the game has become more intense and matches more frequent. Among managers, physical coaches and the medical staff surrounding the modern professional soccer teams, there is a constant quest for even small adjustments that might improve the players` condition and performance. Changes in training type and amount, match participation, restitution, diet and various other aspects of the players` daily life can be made in order to maximise physical performance of the team or an individual player. Such changes can be based on general knowledge of physiology as well as empiric scientific experiments or sometimes pure intuition. Nevertheless, soccer is a very complex sport that requires physical skills related to endurance, strength and agility. Although "simple" physical parameters such as aerobic level, muscle strength and running speed can relatively easily be tested for, the underlying physiological reasons for unexpected drop in physical performance is not revealed through the often extensive test batteries used. Thus, monitoring tools that might explain decrements in physical capacity as well as predicting the players` risk of injury or overtraining/overreaching is in demand (Budgett, Hiscock, Arida, & Castell, 2010; Bury, Marechal, Mahieu, & Pirnay, 1998; Ekstrand, Hagglund, & Walden, 2011; Ekstrand, Waldén, & Hägglund, 2004; Hartmann & Mester, 2000; Meister, Faude, Ammann, Schnittker, & Meyer, 2013). One such tool could potentially be frequent analysis of blood samples obtained from the players.
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