## 4 Discussion
Our findings indicate a mild muscle damage of the participants, something that it is also validated if we take into account the regular reference values for CK and LDH. Nonetheless, equivalent CK and LDH values have been linked to bone and muscle injuries [52; 53], highlighting the importance of continuing to investigate the mechanisms behind these procedures and to achieve a deeper understanding of the functions of CK and LDH activities. Additionally, CK and LDH had proven to be effective markers for a better planning of the recovery strategies, which are very important for maximizing athletic performance [54; 55]. Consequently, their variations following a pre-season training period should be carefully analyzed, particularly to minimize potential injuries and to organize the appropriate recovery strategies. No significant results were found after the pre-season period or between the teams, regarding the PC measurements, while in the case of GPx, Teams 1 and 3 showed significant increases after the end of the preparation period: (GPx: Team 1 \(\rightarrow\) 10 \(\pm\) 11.73 U/L) and (GPx: Team 3 \(\rightarrow\) 6 \(\pm\) 6.32 U/L). Oxidative stress can manifest when the generation of reactive oxygen species exceeds the capacity of the antioxidant defense system [56]. Also, it is already established that high oxidative stress values can impair athletic performance through various metabolic paths [56]. PC and GPx are two of the most approved markers with antioxidant status, and that is the reason they were chosen for the biochemical assessment of this study [56,57]. The increases that we reported for Teams 1 and 3 indicate a mild oxidative stress presence, which is maybe directly related to the delayed inflammatory response that originated from the muscle damage, and can unfavorably contribute in the development of fiber strains, or negatively affect the reconstruction of the damaged muscle fibers [56,58]. On the other hand, low oxidative stress values may also have a positive effect on athletic performance, as the low number of free oxygen radicals plays an important role in the exercise-induced adaptation of the muscle fibers [56]. The exact number of free radicals that are directly related to positive or negative effects on athletic performance varies from person to person and is based on many factors, such as gene polymorphism [59]. Finally, our results showed a mild increase in GPx for Teams 1 and 3 but not for Team 2. This finding can be explained from the outcomes of previous studies, which have demonstrated that the type, the frequency and the intensity of training can significantly affect the production of free oxygen radicals, which in turn leads to different oxidative stress values for every athlete [57,60,61]. The positive correlation between CK and LDH confirms an increased muscle damage of the participants after the end of the pre-season period. This finding comes in agreement with the study by [PERSON] and [PERSON], 2018 [62], which showed a similar increase in CK and LDH enzymes in female basketball athletes after a single exercise session. Another significant correlation that was found between GPx and speed has shown that as the oxidative stress increases, as it is expressed by the GPX values [63], so do the times of the athletes in the 20 m sprint. This increase in the sprint times means that their speed decreases as the levels of oxidative stress are increasing. Oxidative stress can cause impairment to the metabolism of lipids, proteins and DNA procedures and therefore reduce athletic performance [64]. Regarding the performance markers, aerobic capacity has been positively correlated with the maximum strength of the hamstring muscles. In a recent study in female cyclists, a positive correlation between maximum oxygen intake and muscle strength [65] was also highlighted. Strength training improves the maximum strength of athletes and anaerobic ability, but it has also been found to improve their aerobic capacity.