## 1 Introduction  
The majority of sports require an ultimate or nearly ultimate performance of players, lasting 1 to 7 s with a short recovery time over the course of 60-90 min [1]. This intermittent kinetic action typical for soccer is referred to as intermittent sport [2, 3, 4, 5, 6, 7]. The player's repeatedly performs short-term but high-intensity exercise during the whole match [8]. This places higher energy demands on competitive soccer players compared to those playing soccer at a recreational level [9].  
So far, several methods have been designed for testing repeated short-term, high-intensity exercise in soccer players [2, 10, 11, 12, 13, 14]. The protocols often differ in terms of length, number of sprint repetitions, and rest intervals [15]. However, there is only one test that contains active rest involving changes in direction. It is the well-known curved sprint test [14], also known as the Bangsob sprint test [16, 17, 18] or the sprint test [2]. Its remarkable feature lays in a track consisting of three changes in direction with length approximately 30 m, which simulates sprints in the match [14]. On the basis of the fact that the covered distance in a match is closely linked to the performance in the repeated short-term, high-intensity exercise, the curved sprint test is considered applicable for soccer players [14].  
The test results are the time of the fastest sprint, the average sprint time, and the fatigue index. The collected data might be compared with the data of elite soccer players [14].  
The curved sprint test is an effective tool for differentiating between players with different performance levels as well as for detecting changes in kinetic performance during a match. [PERSON] et al. [19] showed that the performance of top-level soccer players is significantly better than that of second-rate players. [PERSON] et al. [20] demonstrated that test performance is closely connected to the age of participants and the performance level in soccer. The performance on non-linear sprints can also help differentiate between professional and young soccer players [21].  
The optimal design and implementation of training strategies that increase repeated sprints of young talented players is of special importance for soccer coaches. The number of high-intensity runs in matches has gradually increased over the years [22], especially in crucial situations [3, 23]. Approximately 85% of all actions performed at maximum speed by soccer players consist of curvature sprints [24]. In spite of their critical importance, investigations related to curve sprints have been largely overlooked by sport scientists [25]. The performance in curvilinear runs has been evaluated by footballers [26, 27] but only at minimal speeds (e.g., jogging). Most studies on young players in team sports have investigated the effect of high-intensity aerobic training on repeated sprint ability [28, 29, 30]. However, the authors of these studies were not concerned with the relationship between particular sprint times of players in these tests that could lead to stating striking differences in participants' performance.  
Taking into account a similar track and a small number of changes in direction, we assumed a significant correlation between the total times taken in the curved sprint test and in the repeated straight sprint test but not times taken in the repeated sprint test involving changes in direction. Verification of these assumptions was accomplished by investigation of the relationship between the times taken by soccer players in the curved sprint test, the repeated straight sprint test, and the repeated sprint test involving changes in direction.