## 4 Discussion
Regarding balance, players with greater knee flexors flexibility showed greater range on the right (\(p=0.411\); \(p=0.040\)), but not on the left side (\(p=0.364\), \(p=0.096\)). This may be explained by the fact that these players were all right-footed, presenting greater hip flexion range of movement (fundamental to the lower limb in support, during the Y balance test) and knee extension (relevant to the reach of the lower limb), which allows them to make further reach distances. In a 2020 study [8] that aimed to understand how the dynamic balance related to flexibility, sprint, strength and jump in young football players, it was found that some reach directions from the star excursion balance test and flexibility of the knee flexors had positive and significant correlations (anterior and anteromedial; \(p<0.05\)), while others were considered negatively significant (posterior and posteromedial; \(p<0.05\)). It should be noted that, according to [PERSON] et al. [48], the Y balance test can help identify flexibility deficits of the lower limbs and flexibility asymmetries in the ankle and hip regions, which points to the possible precarious relationship between balance and flexibility in the knee region, justifying this conflict of data. The knee extensors' flexibility did not show statistically significant correlations with speed, despite the most flexible players being slower in the 20 m sprint (right: \(\rho=0.360\); \(p=0.100\); left: \(\rho=0.386\); \(p=0.076\)). Similar to what was registered in the vertical jump, the greatest flexibility of this muscle group is not associated with an improvement in performance in these two skills, so a larger muscle stiffness may seem to be more advantageous. These results are also following [PERSON] et al. [7], who did not find associations between speed and flexibility both for the knee extensors (difference between groups = 0.64%; \(p=0.462\)) and knee flexors (difference between groups = 0.32%; \(p=0.890\)). In the study by [PERSON] et al. [12], the group with the highest knee flexors flexibility achieved better performance in the acceleration tests of 5 (difference between groups = 6.12%; \(p=0.003\)) and 10 m (difference between groups = 4.09%; \(p=0.007\)), and in the 20 m sprint test (difference between groups = 3.29%; \(p=0.024\)). The authors suggest that greater flexibility of knee flexors is a key factor for young football players in the performance of specific football skills such as the jump, sprint, agility and shot. However, further research should be done to clarify the conflicting results. Our work shows some correlations between flexibility and jump performance, balance and speed, which adds to the body of knowledge that underlines the importance of this ability in football performance. A practical implication of this study is that flexibility training should be included in the training schedule, particularly for the knee flexors. Nonetheless, this practical implication must be taken with caution, as the literature shows conflicting results on this matter. Some limitations of the present study should be mentioned. This being an observational study, it becomes impossible to establish causal relationships. Importantly, the sample size and the competitive level of the participants impaired the application of more robust statistical procedures, which could help to better clarify the associations between the variables analyzed, limiting, therefore, the generalization of this study's results. Additionally, it was not possible to respect a schedule that could standardize the effect of the circadian rhythms in the anaerobic performance tests. Finally, the forced interruption of competitions by COVID-19 may have caused a decrease in the physical capabilities of players, particularly amateurs, which may have led to different results.