Soccer-specific skill tests are contemplated as objective and reliable measures of skill proficiency (see comprehensive reviews (Ali, 2011; Russell and Kingsley, 2011)), while being able to discriminate young soccer players according to playing position, current level or future success (Coelho e Silva et al., 2010; Huijgen et al., 2009; Reilly et al., 2000; Vaeyens et al., 2006). Recently, dribbling speed, shooting accuracy and passing tests were acknowledged as important discriminating factors between goalkeepers and outfield players, while fat-free mass and ball control entered the model in middle and late adolescence, respectively (Rebelo-Goncalves et al., 2015). Available data comparing goalkeepers of different playing standards (Gil et al., 2007; le Gall et al., 2010) are limited to anthropometric and physiological characteristics with the exception of one study (Rebelo et al., 2013), where elite U-19 goalkeepers were largely differentiated from their non-elite peers in ball control skill. However, the distinctive technical demands of goalkeeping claim the need for test measures regarding the performance-related characterization of soccer goalkeepers (Ziv and Lidor, 2011).
While their movement patterns are mainly characterized by long periods of low intensity, soccer goalkeepers are required to perform moderate-high intensity multidirectional movements and a number of skilful actions. Accordingly, the average number of sprint actions was reported to be 2 [+ or -] 2 with a total distance range between 0 and 15 m, with a higher prevalence of sprints of 0 - 5 m reported in sixty-two goalkeepers from 28 teams in the English Premier League (Di Salvo et al., 2008). One of the most critical movements involved in goalkeeping is the diving action. During the 2002 FIFA World Cup in Korea and Japan (De Baranda et al., 2008), goalkeepers performed up to 17 dives (6.2 [+ or -] 2.7) per match. According to the same authors, the dive is associated to the lateral save and situations of maximum intensity in which the goalkeeper performs a parry or a fly. Diving motion was analysed in four goalkeepers and it was found that the more skilled players dived faster (4 m x [s.sup.-1] as opposed to 3 m-s-1) and more directly to the ball (Suzuki et al., 1987). The kinetic and kinematic characteristics of goalkeeper making diving saves showed that asymmetries exist in the movement patterns of goalkeepers according to the preferred or non-preferred side due to over rotational differences in the transverse plane (Spratford et al., 2009).
Hardly any information regarding goalkeeperspecific skills can be found with the exception of a recent study of Knoop et al. (2013) designed to evaluate the reaction and action speed test (RAS) among thirty-four German goalkeepers of different age groups and competitive levels. Although the results of the RAS test have successfully differenced the first goalkeepers and their substitutes, the instrumental apparatus used in this study (Knoop et al., 2013) is not easily accessible to coaches and trainers. Therefore, simpler protocols are needed to evaluate the particular technical skills involved in goalkeeping.
The overall purpose of the current research was to develop and evaluate two soccer-specific tests designed to examine goalkeeper-specific technique. The study was divided in two parts: 1) to evaluate the reproducibility of the Sprint-Keeper Test and the Lateral Shuffle-Keeper Test; and 2) to examine the construct validity of the applied tests. The assessment procedure included measures of test-retest reliability. Validity was ascertained by comparing two groups of young soccer goalkeepers of different competitive levels, hypothesizing that elite goalkeepers would perform better.
Participants and procedures
A total sample of sixty-six young male goalkeepers, all Caucasians, participated in the current research (Table 1). In the first part of this study, twenty-six goalkeepers (chronological age: 14.49 [+ or -] 2.52 years; accumulated soccer training: 5.62 [+ or -] 2.42 years; weekly volume of training: 5.8 [+ or -] 1.6 hours) completed the goalkeeper-specific tests on two separate occasions to determine test-retest reliability, with a week of interval between tests. In the second part, a subsample of eighteen elite goalkeepers (chronological age: 13.81 [+ or -] 1.81 years; accumulated soccer training: 6.00 [+ or -] 1.82 years; weekly volume of training: 6.1 [+ or -] 1.5 hours) and twenty-two non-elite goalkeepers (chronological age: 15.04 [+ or -] 1.43 years; accumulated soccer training: 6.64 [+ or -] 2.44 years; weekly volume of training: 5.8 [+ or -] 1.3 hours) was used to examine the construct validity. Elite goalkeepers belonged to the youth department of two professional clubs and played at a national level, while non-elite players were part of amateur clubs and competed at a regional level. Chronological age (CA) was calculated to the nearest 0.01 year by subtracting birth date from date of testing. Soccer experience, i.e. accumulated soccer training, and number of hours per week devoted to soccer training and preferred diving side were obtained by interview. None of the subjects reported recent injuries.
Clubs and coaches were contacted and fully informed about the nature of the study and the procedures involving data recording. All subjects received a complete explanation about the testing procedures and that they could withdraw from the study at any time before giving their verbal consent to participate. The study received ethical approval from the Scientific Committee of the University of Coimbra taking into account the standards for sports medicine and the recommendations of the Declaration of Helsinki.
The protocols for the specific tests for soccer goalkeepers proposed and used in this study included typical movements of goalkeeping (sprint, lateral shuffle, change of direction and dive) that better mimic a game situation. Therefore, an acceptable ecological validity can be assumed. Besides, due to their simplicity and a minimum of required equipment, the testing procedures were designed to be easily available to coaches and trainers. The final version of the applied tests resulted from an original straight sprint and diving protocol. Adaptations occurred during three experimental sessions with an independent sample of six goalkeepers until the proposed tests emerged to assess goalkeeper-specific diving technique.
All field protocols were performed between 15:30 and 19:30 hours, on artificial grass with the players equipped with appropriate goalkeeper wear and soccer boots. A standardized 10 minutes warm up, which included jogging, a series of increasing intensity sprints, dynamic stretching and specific goalkeeper ball drills, was performed before the commencement. No static stretching exercises were allowed before any test (Fletcher and Monte-Colombo, 2010). Afterwards, a familiarization tryout preceded the two trials. Each subject was instructed and verbally encouraged to give their maximal effort during all trials. The subjects had 2 or more minutes of rest between 2 consecutive trials. Experimental conditions were controlled for air temperature (14-23[degrees]C) and relative humidity (30-67%).
Short-term muscle power
The subjects performed two vertical jump protocols. The first jump test consisted in the standardized counter movement jump (CMJ) during which the subjects were asked to keep their hands in their hips, to maintain their body vertical throughout the jump, and to land with their knees fully extended. The second vertical jump test was a free counter movement, jump during which the players freely swing the arms (CMJ-free arms). The vertical jump performances were evaluated by means of an optical acquisition system (Optojump, Microgate, Bolzano, Italy), developed to measure with [10.sup.-3]-second precision all flying and ground contact times. The Optojump photocells are placed at 6 mm from the ground and are triggered by the feet of the subject at the instant of take-off and are stopped at the instant of contact on landing.
Then calculations of the height of the jump were made (Komi and Bosco, 1978).
Acceleration was evaluated using two sprint tests, involving straight sprinting of 5-m and 10-m as fast as possible from a standing start position. Time was recorded using a system of dual infrared reflex photoelectric cells (Polifemo; Microgate). Players began from a standing start, with the front foot 0.5 m from the first timing gate. The best result of the two trials for both vertical jump and acceleration were retained for the statistical analysis.
The first protocol was proposed to examine diving technique, involving moving as fast as possible in the direction of a stationary ball after performing a change of direction, in a total distance of approximately 10 m. The Sprint-Keeper Test (S-Keeper) was composed by the following successive phases: a)...