Introduction:
Soccer is a sport with high intensity intervals, heavily relying on aerobic energy mechanisms, leading to various neuromuscular and endocrine responses.

Aim of the study:
The purpose of this study was to examine the changes in intermittent endurance performance, testosterone and cortisol concentrations, and their T/C ratio in elite professional soccer players during a soccer season.

Material and methods:
Ten elite soccer players from a soccer club of Greek Super League were evaluated for intermittent endurance, using an interval shuttle run test (ISRT), testosterone (T) and cortisol (C) concentrations, and T/C ratio at the beginning of the pre-season period (T1), at the beginning of the competitive period (T2), at the start of the second round of the competitive period (T3) and at the end of the competitive season (T4).

Results:
The distance covered at ISRT increased by 26% (p<0.001) after the pre-season. T increased by 8.6% after the pre-season, while a statistically significant decrease (24.4%, p<0.005) in T was observed between T1 and T3, and between T1 and T4 (p<0.005). C increased significantly by 90% (p<0.001) from T1 to T2, while a significant decrement by 15% (p< 0.001) was observed between T4 and T3. T/C decreased by 47% (p<0.005) from T1 to T2, while T2 values of T/C indicated a further 30% (p<0.05) until T3. T/C ratio increased by 13% (p<0.005) from T3 to T4.

Conclusions:
This study shows significant changes in intermittent endurance performance, testosterone, cortisol and T/C ratio over the season. Conclusively, coaches of professional soccer teams shall monitor endurance and hormonal changes over the season so that players successfully improve their aerobic fitness, optimize peak performance, prevent injuries and overreaching.