We also analyzed levels of LDH, CK, and CK-MB that have been confirmed as useful serum markers of muscle injury. During exercise the increase in ROS production can induce lipid peroxidation which in turn alters membrane permeability causing the release of protein from myocells. Previous animal studies reported a positive correlation between increased lipid peroxidation and CK as well as aspartate-aminotransferase, suggesting that biomarkers of muscle damage during exercise are indicative of oxidative stress . Serum CK concentrations may change in response to eccentric exercise too. Water polo is considered a noneccentric exercise mode; nevertheless there is evidence of relevant cocontraction muscular intervention modes, possibly implying also eccentric actions of the antagonist muscle . An adaptive behavior of trained muscle depending on type and duration of exercise has been demonstrated. Peak values in serum CK have been recorded after endurance event  as well as in eccentric exercise . Also markers of myocardial damage, such as the CK-MB isoenzyme, myoglobin, and cardiac troponins, are affected by prolonged or strenuous exercise . However, Dahlqvist et al.  suggested that cardiac troponin represents a marker of more severe alterations associated with sarcomeric damage. Indeed, it has been demonstrated that cardiac troponins appear in blood of well-trained athletes competing in ultra-endurance events such as Ironman Triathlon or exercise of even longer duration, while they are not detected in blood after intermittent or short duration (<90 min) exercise [30, 31]. Accordingly, in water polo players after training we reported an increase in LDH, CK, CK-MB, and myoglobin plasma levels, while no changes were observed for troponin. These results indicate that intermittent high intensity activity in water polo athletes causes a moderate muscle injury.