It has been hypothesized that SOD, CAT, and GPx, which represent the major enzymatic antioxidant systems involved in ROS scavenging, display different features of adaptation to training. Both CAT and GPx convert hydrogen peroxide (H2O2) produced by SOD to water and oxygen, but the threshold of H2O2 for their induction is different. During moderate exercise GPx activity is enough to scavenge low levels of produced H2O2, while after strenuous exercise there is an excessive production of H2O2 leading to an increase in CAT synthesis to compensate for the insufficient clearance of H2O2 by GPx .