Background. Simultaneous tiredness of two or more muscles around a joint can be defined as coordinated fatigue (co-fatigue) and might occur between agonist and antagonist muscles, and vary according to the level of sporting activity levels or gender. Objectives. The aim of this study was to determine the levels of coordinated fatigue in agonist and antagonist muscles during squat-press exercise. Methods. Twenty athletes and twenty sedentary subjects participated in the study. Surface electromyography signals of the rectus femoris, vastus lateralis obliquus, biceps femoris and semitendinosus muscles were recorded at the squat press position for 15 seconds during isometric contraction. Measurements were repeated five times and a 2-minute rest period was allowed between repetitions. After erroneous EMG elimination, movement artefacts were removed by using a 20 Hz high-pass Butterworth filter. Then, as a well-recognized fatigue index, the median frequency (MF) of each filtered middle part of the EMG signal (5 to 10 s. of contraction) was calculated, given that it is known that the MF decreases during isometric contractions. Finally, each MF-based co-fatigue index was calculated by dividing the mean RF and VLO median frequencies by the mean ST and BF median frequencies. The cumulative co-fatigue values of "male vs. female" and "sedentary vs. athlete" comparisons were performed by using a two-sided Student t-test with a Bonferroni correction. Results. There was a statistically significant (Bonferroni corrected p-value < 0.05) difference between the mean female (1.57 +/- 0.53) and the mean male (1.23 +/- 0.17) co-fatigue values, while there was no statistically significant difference between the mean co-fatigue values of sedentary (1.51 +/- 0.52) and athlete (1.29 +/- 0.27) subjects. Conclusion. The offered co-fatigue indices might be useful for other sports, physiotherapy and related areas if sufficient scientific proof is accumulated.