Both tactics and player position on the soccer field are essential for the organization of a soccer match (Ruas et al., 2015) and for improving the successful outcomes of the matches. Specifically, the field role determines the plan of action and game strategies. Genetic predispositions influence the potential role selection: anthropometric characteristics indicate that, for example, taller players are the most suitable for central positions and for certain "target" positions among the forwards, where body size provides an advantage (Perroni et al. 2015; Rebelo et al., 2013; Reilly et al., 2000). Nevertheless, no positional differences in young soccer players have been reported in previous studies, in terms of their anthropometric characteristics, physical capacity or mental confidence (Fiorilli et al., 2013; Coelho et al., 2010). Physiological demands also vary with regard to the rates of physical exertion required for each position (Macarthur and North, 2005; Payne and Montgomery, 2004), although players may develop certain skills as reactive strength, endurance and balance (di Cagno et al., 2013; 2014), based on the demands of their specific position (Weber et al., 2010). The selection of young players for a specific field position based on their anthropometric, physical and physiological profile may not be appropriate because the differences in the length of time taken for boys to mature favours average and early maturing boys (Deprez et al., 2015; di Cagno et al., 2014). Consequently, cognitive skills and tactics may be a more suitable and stable indicator for appointing young players to different roles. Moreover, we must take into account that cognitive skills are an integral part of the abilities required in soccer in reaction to the opponents' actions (i.e., stimuli) (Battaglia et al., 2013). Players differ in their ability to 'read and react' to stimuli, which is the definition of Reactive Agility (RA). Agility is an essential component in most field and team sports and it is fulfilled by changes of direction every 2-4 seconds, with 1200-1400 changes of direction throughout a game (Sporis et al., 2009). Traditionally, agility was simply defined as speed with directional changes (Draper and Lancaster, 1985). Currently, agility is considered an open skill and was recently defined as a change in velocity or direction in response to a stimulus that cannot be pre-planned (Sheppard et al., 2006). The stimulus may be the movement of the ball or the movement in response to the actions of an opposing player (Moreno, 1995). RA is "a multi-planar or multidirectional skill" that combines acceleration, explosiveness, and reactiveness. This definition suggests that RA consists of both cognitive and physical abilities. An examination of the physical factors that influence agility demonstrates that running technique can play a key role in Change of Direction Speed (CODS). Several studies, that examined the performance of unfamiliar techniques in a time-stressed situation, have highlighted the fundamental role of RA training (Bradshaw et al., 2011; Serpell et al., 2011). Young et al., (2002) suggested that foot placement, trunk lean, stride adjustment, and running posture are important technical components of CODS and RA and also facilitate perception of an offensive player's deception in one-on-one situations (Jackson et al., 2006). It is well known that in soccer, athletes who have anticipatory expertise are able to recognize and attend to different stimuli faster after the presentation of a stimulus (Williams and Davids, 1998), compared to novice players, who may require the entire skill to be executed by the opponent (e.g., cross-over, step and direction change) before making the correct decision and responding to the stimulus. In invasion sports, such as soccer, RA skill is beneficial for forwards, as it allows them to evade their opponents' pressure or tackles and to gain and maintain possession of the ball. It is also beneficial for defenders to reduce space on the field or court to limit attacking movements and to prevent the opposition from scoring (Spittle, 2013).
The purpose of this study was to determine the relationship between CODS and RA in young soccer players according to their field position in order to improve training methods and optimize interventions on the basis of player's roles.
Experimental approach to the problem
This is a cross-sectional study in which the authors assessed CODS, RA performance and ball control techniques in three groups of high-level young athletes who play in different soccer field positions. The assessment involved two tests, each one performed in two different ways: a reactive and a pre-planned Y-test and the Illinois Change of Direction Speed Test (ICODT) with and without the ball. The two Y-tests were similar. The CODS test, assessed with a pre-planned Y-test (Y-PLAN test), was designed to replicate the movement of the agility test (Y-REAC test), but participants were instructed to move in a certain direction in advance. In the agility tasks, the athletes chose the appropriate direction in response to a stimulus. The ICODT with and without the ball assesses the speed in change of direction, taking into account ball control skill and management. The differences in the CODS and RA performances and ball control techniques between the three groups were investigated.
Ninety-two elite soccer players (average age was 15.18 [+ or -] 0.98 years) belonging to two youth leagues, the Italian First and Second Divisions, volunteered for this study. They were classified into three soccer roles: defenders, midfielders, and forwards. Detailed characteristics of the participants are shown in Table 1. Their training regimen consisted of three training sessions and one game per week.
To be involved in the study, they had to meet the following inclusion criteria: participation in at least 80% of the training sessions of their respective clubs, a minimum of 7.5 years of experience in competitive soccer, no injuries that had occurred in the previous year, and no presence of relevant diseases. The following exclusion criteria were applied: presence of injuries, pathologies, or other conditions (temporary or not) that could influence the correct execution of the tests proposed in this study. The players and their parents were informed about the purpose of the study, and all of them gave their written informed consent to the procedures described in the study. The study was designed and conducted in accordance with the Declaration of Helsinki and approved by the bioethical committee of the University of Molise.
At baseline, the participant's weight, height, and Body Mass Index (BMI) were measured.
The three groups of soccer players performed two tests with and without the ball to assess CODS and RA: the Y-pre-planned agility test, the Y-reactive agility test, for assessing agility and the ICODT with and without the ball for assessing ability to change direction (Figure 1). Goalkeeper performances were not analysed because there is a clear...