In recent years, soccer has been characterized by dynamic changes in the physical activity of players during the game. There are continuous, but unpredictable, changes occurring at different intensities throughout the match. Players must perform different types of speed-related movements (in place or moving), rapid movements of different parts of the body (e.g. the lower limbs), and must respond to various, potentially unpredictable situations occurring during the match. Speed-related movements include: the individual attack, returning to a defensive position, or, in one-on-one interactions, passing the defender, and unpredictable movements by the attacker to surprise the defender. In most cases these actions are preceded by starts from different positions and different directions.
Speed, as a motor function and ability in soccer, is based on running at a maximum sprint speed for less than a 30-m distance (96% of sprints during a soccer match), accelerating for less than 10-m (49% of sprints occurring during the match), and sprints with change of direction (COD) movements (every 2-4 seconds). CODs involve movements in different directions (left, right, forward, backward) and at different angles (Comfort et al., 2014; Little and Williams, 2005; Sporis et al., 2009; Stolen et al., 2005). Akenhead et al. (2013) found that the English Premier League soccer player, on average, sprints (>5.8 [ms.sup.-2]) for 2% of the overall distance traveled in the match, engages in high-speed running (>6.78 [ms.sup.-2]) for 5% of the total distance traveled in the match, covers 10% of the total distance in acceleration motions, and spends 8% of the match in stopping movements. In turn, professional players from the English FA Premier League can perform on average more than 8 CODs per minute during a match. Therefore, an effective soccer player (the attacker or defender) should have the ability to move at high speeds in a straight line, or most frequently, while performing COD movements or changes of running pace (Carling et al., 2008; Hachana et al., 2014). In contrast to short sprint episodes, multiple accelerations over short distances, frequent stop-movements, and repeated COD movements all require different repositioning techniques that are key with respect to soccer playing ability. Most often, these movements occur as a reaction to stimuli that the players are constantly experiencing throughout the game (Goncalves et al., 2015; Simonek et al., 2016).
The impact of COD on a soccer players' movements as their physical activity increases during the match remains a topic of current research and analysis, as researchers seek to identify the determinants of these movements. Numerous soccer studies have focused on the relationship between sprints, CODs, and jumping abilities; however, the results have been inconsistent. Some studies showed a strong correlation between these variables, while others showed poor relationships. In a study by Little and Williams (2005) on a group of 106 English soccer players, it was found that between COD speed (evaluated using the Zigzag test), starting speed (acceleration), and maximum speed achieved, there were statistically significant correlations. However, the determination coefficients between the tests were low (from 0.119 to 0.388). Based on these findings, the authors concludes that COD speed and straight sprints are characteristic and independent motor skills among soccer players (Little and Williams, 2005). Similarly, strong and moderate correlations between acceleration, maximum speed, and results of the Zigzag agility test were reported by Koklu et al. (2015), based on their investigation of 16-year-old soccer players.
The relationship between COD movements and vertical and horizontal jumps has also been explored. In the studies of Struzik et al. (2017), they investigated 12-year-old soccer players. They found no significant links between counter-movement jumps (CMJ) and CODs during sprints. Rouissi et al. (2017) found no significant correlation between standing broad jump (SBJ) and COD performance. In contrast, Young and Farrow (2006) and Lockie et al. (2014) indicated that leg power was an essential component of COD speed. It is believed that team players who achieve higher jump results will probably achieve faster times in multidirectional speed tests. The relationship between speed and jumping ability has also been investigated. However, according to Marques and Izquierdo (2014), the relationship between speed and strength of the lower limbs should be interpreted with caution.
Since the majority of COD runs in soccer matches occur at angles between 1[degrees] and 90[degrees], which are specific to soccer, it is important to determine COD speeds at these angles (Faude et al., 2012; Hader et al., 2015). In addition, the defending player often moves away from the attacker for a short distance using controlled backward running followed by forward sprinting (Hammami et al., 2016). A high-level of skill is required to combine COD movements, changes of movement type, and re-accelerations. The backward, run as an unorthodox movement, occurred over 5.3% ([+ or -] 2.4%) of the total match time, according to one study (Krustrup et al., 2009). Some researchers suggest that speed, COD and strength of the lower limbs should be measured and evaluated using separate tests (Buchheit et al., 2012; Cardoso de Araujo et al., 2018; Sheppard and Young, 2006; Silva-Junior et al., 2011). In turn Castagna et al. (2003) indicated that a strong differentiation between forward, backward, and sideways motion exists. Presumably, COD movements should be measured with various tests in which the subject would perform forward, backward, and sideways motions.
There are a small number of studies that have used multiple regression models to identify associations between sets of variables such as vertical and horizontal jumps (e.g., CMJ, SBJ), linear speed (e.g., 10-m sprint, 30m sprint), COD sprints, and other patterns of movements in young soccer players. Determining the models of these variables will provide increased knowledge of the key determinants of motor behavior in young soccer players. It is helpful to evaluate motor ability levels using a battery of tests to optimize the training process (Northeast et al., 2017). Such models may contribute to a more effective selection and specification of the game performed by each player through enhanced physical preparation and skill acquisition (Koklu et al., 2015). Therefore, the purpose of these studies was to investigate the relationship between linear speed (straight-line sprinting) and jumping ability (lower extremity explosive power measured through horizontal and vertical jumping) on COD performance measures (COD-speed) in young soccer players. We aimed to determine the variables that can explain the effectiveness of COD speed using the 30-m ZigZag (cutting maneuver) under 60[degrees] and the 30-m sprint divided into forward-backward-forward movement. This is the first study defining the performance of change-of-directions under 60[degrees] based on times measured at 5m, 10m, 15m, 20m, 25m and 30m in the test. Additionally, the first research on the determination of performance change-of-direction forward-backward and backward-forward during the 30-meter distance was conducted.
We used a multiple regression model to identify associations between the set of variables to be developed that could determine the efficiency of COD movement patterns. We hypothesized that there is a strong relationship between the results of straight-line sprinting and jumps and the two tests defining COD speed in young soccer players.
To determine COD-speed (Ruscello et al., 2013), the 30-m sprint test in two different patterns was applied. The first test, the 30[CODS.sup.1], comprised six 5-m sections, wherein the CODs were less than 60[degrees]. In practice, this test is often recognized as a ZigZag. A 60[degrees] cutting maneuver, as recognized by Faude et al. (2012), is specific to moving the soccer ball during the match. Another test, the 30[CODS.sup.2], involved 15-m forward sprinting, stopping, 5-m backward sprinting, stopping, and then 10-m forward sprinting. This is a modified form of the sprint with backward and forward running (SBF) test. This test was also chosen for its specificity with regard to soccer practices and matches. For the maximum speed measurements, 30-m sprint tests were performed. To determine the total time and the time required for each 5-m segment of a particular test, the 30-m [COD.sup.1] sprinting test, 30-m [COD.sup.2] sprinting test, and the 30-m sprint test were performed with the Fusion Smart Speed System (Fusion Sport, Coopers Plains, QLD, Australia). The jumping ability of each player, based on the CMJs and SBJs, was determined using the Optojump System (Microgate Engineering, Bolzano, Italy)
Soccer players at different competitive levels were recruited to better generalize the results of the study. The research was conducted during the soccer season to avoid additional confounding variables. It included a wide battery of tests comprising maximal speed (30-m sprint), COD movements, and horizontal and vertical jumping tests (CMJs and SBJs), which are commonly used in soccer training. The experiment was carried out in the Chair of Ball Games Research Laboratory in the University School of Physical Education in Wroclaw, with an ISO 9001:2009 certification, in collaboration with the Lower Silesia Sport Federation in Wroclaw, Poland. The study was conducted over one afternoon.
We included 60 male soccer players from the Lower Silesian sports clubs, who participated in the Central League of Junior Championships in...