## 4 Discussion  
There were no significant correlations between the total times taken in the CST and the RSST, between the total times taken in the CST and the RSCD, and between the times taken in the curved sprint test, the repeated straight sprint test, and the repeated sprint test involving changes in directions. These findings are in agreement with [PERSON] et al. [25], who found a coefficient of determination of \(\sim\)35% between linear and curve sprinting. The authors reported that the curved sprint test is highly reliable and that curvilinear and linear sprints are different and independent actions. In addition, [PERSON] et al. [31] found no correlation between the linear 10, 20, and 30 m sprint and the \(t\)-test with a change of direction.  
There was no significant correlation between the time taken in the CST and the RSST. These findings are in contradiction with [PERSON] et al. [32], who discovered an eminent correlation between speed in the zigzag test (sprint involving changes in direction) and speed measured by a 30 m straight sprint (\(r\) = 0.560) (in 15-year-old soccer players). In addition, other authors [33; 34; 35] have reported that the times measured in tests involving changes in direction are in correlation with times in straight sprint tests. This significant relationship between the times taken in individual tests indicate that they influence the same abilities to some extent.  
Additionally, there was no significant correlation between the total times taken in the curved sprint test and in the repeated sprint involving changes in direction. These findings are at variance with [PERSON] and [PERSON] [33], who found a correlation (r \(\geq\) 0.600) between the times taken in Illinois and Pro-agility tests involving changes in direction.  
The possible explanation is the diversity in test tracks, the number of repetitions, and the number of changes in direction. The CST contains of two changes in direction. There are many more changes in direction in the remaining tests, making them more difficult in terms of coordination. The critical part is not only speed endurance but also whole-body coordination and the ability to change direction. Developing these skills is as important as improving sensory and cognitive functions. Video observation/motor imagery training may be useful for individuals who need to simultaneously develop a fast response to different types of stimuli [36].  
The standard distance during a match is closely linked to performance in the curved sprint test and the repeated straight sprint test [14]. For that reason, these tests are appropriate for assessing the speed endurance of soccer players. Their advantage lies in simultaneously assessing the maximal performance as well as the performance in repeated  
Figure 5: Correlation between the total times taken in the CST and the RSST.  
sprints. The highest sprint speed achieved in a match is in correlation with the highest speed achieved in the RSST [14]. That means the player performance in this test can be used for estimation of the highest sprint speed performed during the match.  
The limitation of this study was that the research included only young soccer players. Further investigations should involve elite, professional soccer players. It would also be worth to address the relationship between test performance and match performance. This could be valuable and useful information for trainers in practice.