The soccer-specific physiological demands require players to be physically fit in terms of aerobic and anaerobic power, muscle strength, flexibility, and agility (19). A number of laboratory and field tests have been developed to evaluate players' physical abilities, determine individual strengths and weaknesses, and assess the effect of various training and other procedures expected to improve soccer performance. In general, the tests have provided valuable results due to their high reproducibility, standardized testing conditions, as well as reliable and precise equipment (14). However, laboratory measurements are less accessible and often too expensive for routine use. Furthermore, these tests are time-consuming, and as a result, laboratory testing is rarely used throughout the season (24). On the other hand, the sport-specific field tests are popular among both coaches and athletes due to their simplicity, validity, and minimal use of equipment. A number of field tests have been described in the literature and routinely used by coaches in order to assess various aspects of soccer-specific fitness (22). However, most of them assess endurance (4,9,16) rather than various aspects of motor performance, such as speed, agility, anaerobic power, or skill.
Activities like sprinting over a short distance, accelerating and decelerating with or without changing direction, as well as controlling the ball during those activities are expected to have the property of face validity when used for assessing soccer-specific performance. However, although the results of those tests have been reported in the literature (3,11,23), the reliability of the tests has not been assessed. Therefore, this study applied several tests of the anaerobic power and skill of professional male soccer players with the main aim of evaluating the tests' reliability. The selection of the tests was based not only on their soccer-specific validity, but also on their simplicity and a minimum of required equipment.
Experimental Approach to the Problem
The tests were conducted on a high-quality soccer field (i.e., natural grass) on 2 separate days without precipitation and at an ambient temperature of 24 to 26°C. On the first day, the anthropometric measurements were taken. Thereafter, the throwing-in, standing kick, 10-m sprint, and flying 20-m sprint were performed. On the second day, running 10 × 5 m, zigzag test, and zigzag with the ball were tested. Both testing protocols were preceded by a standard warm-up and stretching procedure and followed by a regular training protocol designed by the main coach.
Each test was demonstrated, and thereafter, 1 practice trial was allowed for habituation. The following 3 trials were recorded as experimental trials. Each subject had 3 or more minutes of rest between 2 consecutive trials.
The tested subjects were 20 professional senior male soccer players, excluding goalkeepers, who belonged to the first selection of a premier national league team. The average (mean ± SD) age, mass, and height for the subjects were 20.4 ± 1.8 years, 8.9 ± 6.6 kg, and 1.85 ± 0.06 m, respectively. All participants were highly trained soccer players with extensive experience in the tested activities, including various kinds of agility, due to their regular soccer activities. At the time of the study, all players had an average of over 8 years of soccer training experience. Their training programs over the year preceding the experimental testing included 8 to 10 training sessions a week, with each lasting about 90 minutes. None of the subjects reported neurological diseases or recent injuries. The data reported herein were obtained through the regular preseason testing procedure. The study was approved by the Ethical Committee of the Faculty of Sport and Physical Education of the University of Belgrade. Subjects received a complete explanation of the purpose of the testing protocol and signed the institutionally approved written consent according to the Declaration of Helsinki.
Body height and mass were measured to the nearest 0.1 cm and 0.2 kg, respectively.
This test evaluated the power of the upper body (10). The standing subjects were asked to throw the ball with both hands as far as possible in the fashion of throwing-in in a real game. The distance was measured to the nearest 0.2 m.
This test was expected to assess both the power of the lower body and the kicking skill (13,15). Subjects were instructed to kick the ball without a run-up (i.e., while the player was stationary) for maximal distance. They were standing with the nondominant leg positioned beside the stationary ball and, while using a preparatory countermovement swing of the kicking leg, kicked the ball as fast as possible. The kicking distance was measured to the nearest 0.2 m.
The ability to rapidly accelerate from a standing position was measured over a 10-m dash initiated from a standing position (1,2,11).
Flying 20-m Sprint
This test assessed the sprinting ability over a short distance, which should be of particular importance for soccer (2,11). The running time along 20 m following the 10-m maximal acceleration (see previous test) was measured. As a consequence, the subjects were instructed to run with maximal speed over 30 m, and both the 10-m (i.e., acceleration) and the 20-m (i.e., maximal speed over a short distance) tests were obtained from the same trial.
Running 10 × 5 m
This test assessed running agility from required rapid changes in direction (18). Subjects were instructed to run as fast as possible between 2 parallel lines positioned 5 m apart, back and forth consecutively, 10 times. Each time, they were expected to step over the line with at least 1 foot.
This test assessed running agility from changes in direction. A zigzag course consisted of 4 5-m sections set out at 100° angles. The selection of this test was based on rapid acceleration, deceleration, and balance control required for short running time, which represented the result of the test (11).
Zigzag With the Ball
The ability to control the ball while changing direction was assessed. Subjects were instructed to run with the ball as fast as possible along the same zigzag path used in the previous test.
The ratio of the results obtained from the zigzag test without and with the ball was calculated. A higher index (i.e., a smaller relative increase in the zigzag running time when the ball had to be controlled) was interpreted as a higher skill of controlling the ball.
The distance between the standing position (i.e., throwing-in) or initial ball position (i.e., standing kick) and the point where the ball had reached the ground was measured by a 50-m tape. Instructions for both tests were to throw, or kick, the ball as far as possible.
The time in all running tests was recorded to a resolution of 0.01 s by photocells (Newtest Oy, Oulu, Finland). The photocells were positioned approximately 0.8 m above the floor, which typically corresponded to the hip level. The positions of the photocells in the 10-m and flying 20-m sprint are shown in Figure 1A. The first pair was positioned along the starting line, the second 10 m apart, and the third pair along the finish line. The individual trials were self-initiated by subjects starting with their front foot positioned 0.3 m behind the photocell. Poles 1 m high were used as obstacles for the zigzag test, and the subjects were not allowed to touch them as they ran around them.
All data are reported as mean ± SD. The precision of population estimates is shown as 95% confidence intervals (CIs). One-way analysis of variance (ANOVA) with repeated measures and a Tukey post hoc multiple-comparison test were used for detection of possible systematic bias between 3 consecutive trials of each particular test. Statistical significance was set at p < 0.05. Relative variability was expressed as an intraclass correlation coefficient (ICC) (25). Absolute (i.e., within-individual) variability was assessed by typical error of measurement as well as by coefficient of variations (CV) (5). Sample size was estimated from the retest correlation by using the following formula:
The statistical analysis was performed by using SPSS for Windows, version 12.0 (SPSS, Inc., Chicago, IL).
Table 1 depicts the results of all evaluated tests through the descriptive statistics calculated across the subjects. The results of individual tests are based on the best of the 3 consecutive trials, and in all running tests, shorter times represent a better result.
Table 2 depicts the performance averaged across the subjects for each recorded trial, together with the corresponding measures of reliability. When applied on each particular test, ANOVAs revealed F(2,60) = 0.16-0.74 (p > 0.05), suggesting that the differences between consecutive trials proved to be small and inconsistent.
The reliability, as depicted by ICC, for most of the tests was high and exceeded 0.80. The only exception was the standing kick (ICC, 0.76). Similarly, the throwing-in and standing kick revealed considerably higher within-subject variation expressed as CV (7.1% and 9.2%, respectively) than the 1.2% to 3.9% variation obtained from the remaining tests.
Based on CV, the estimated sample size required to detect a 2% change in assessed variables for most applied tests was close to the sample size of the current study (range, 2-30). Again, the exceptions were the throwing-in and standing kick, in which the estimated sample sizes were 98 and 165, respectively.
The purpose of this study was to examine the reliability of selected soccer-specific field tests. Regarding their validity, throwing and kicking the ball are parts of a soccer game per se. Sprinting over various distances is also an important demand in soccer, including rapid changes in direction in particular (22). During a soccer game, a sprint bout occurs approximately every 90 seconds, with each lasting an average of 2 to 4 seconds and covering up to 11% of the total distance (22). Therefore, one could claim a face validity of the selected tests. Their various versions have been often applied in soccer practice as well as frequently described in the professional literature. Finally, the tests belong to the tests of performance of rapid movements, which do not require normalization for body size (6,7) and thus add to their simplicity. Therefore, the remainder of the article is focused on the tests' reliability as well as on some particular test-specific issues.
The reliability of a test depends on a number of factors, such as the number of subjects, the number of performed trials, the subjects' skills, and the homogeneity of the sample (5,13). The current study included 20 professional soccer players from a first national division. This number approximately corresponded to the number of players on a professional soccer squad. Since the participants also performed 3 trials in each of the tests after familiarization with the test procedure, it could be concluded that the tested sample of participants and number of performed trials was appropriate for the evaluation of the reliability of the selected soccer-specific tests (25).
A reliable performance test is one that has small changes in the mean (i.e., no significant systematic bias), a small within-individual variation, and a high test-retest correlation (21). The tests evaluated in this study demonstrated no significant differences among consecutive trials, while the within-individual variations, as expressed by CV, were rather low, except for the throwing-in and the standing kick. Since most of the ICCs were high, one could conclude that most of the evaluated tests could be highly reliable. An additional argument in favor of the evaluated tests is the relatively low number of subjects required to detect a worthwhile effect. Namely, for most of the tests, that number corresponds to the size of a soccer team, suggesting that changes in soccer-specific performance associated with training and other procedures could be detected by the evaluated tests.
It has been suggested that the agility tests could discriminate elite soccer players from other populations better than any other field test of strength, power, or flexibility (17,20) and that the agility tests should be used in conjunction with single-sprint tests to obtain a thorough indication of a player's speed capacity (11). Based on these studies, a recent review of testing methods applied in soccer concluded that the agility tests could be the best single indicator of overall soccer performance and provide the best differentiation among nonplayers and elite and recreational performers (24). Surprisingly, although a number of studies recommend the agility and running tests (2,8) as highly valid, there is no evidence regarding their reliability. From that perspective, the running 10 × 5 m and the pair of evaluated zigzag tests could be of particular importance. In addition to running agility (i.e., running 10 × 5 m and zigzag test), they also measure running with the ball (i.e., zigzag with the ball), which could be a highly valid soccer-specific test for assessing dribble performance and control of the ball. Finally, the skill index that was evaluated is a novel one. It revealed a high reliability, while its validity should be justified by the movement tasks. Simply, when the ball has to be controlled, the zigzag running time should increase less in individuals more skilled in controlling the ball. Nevertheless, due to the novelty of this test, further studies are needed. For example, if future studies show that the skill index can distinguish among various levels of players, the test could become a widely used one.
Several kicking tests have been recently evaluated by Markovic et al. (13). The maximal ball speed was measured by means of a radar gun, and the reliability appeared to be exceptionally high. This finding implicitly suggests that a relatively low reliability and, consequently, a high CV and relatively large number of subjects required to detect 2% changes obtained in the throwing-in and standing-kick tests could be partly caused by the selected method of the performance assessment. Specifically, the distance that was recorded in both tests could be considerably confounded by the elevation angle (12). After also taking into account the low cost and simplicity of use, using radar guns could be recommended in future tests of ball speed instead of the distance travelled, as measured in this study.
Although the evaluated tests could need further evaluation (e.g., assessment of independent groups of participants with different levels of athletic proficiency and establishing standards for them), of considerable practical importance is that they are based on a simple experimental protocol and require relatively inexpensive equipment. They could also be performed close to the training environment and easily administered within a relatively short time. Nevertheless, the results obtained suggest a high reliability for most of the evaluated tests as well as a relatively low number of subjects needed to detect expected changes in physical performance associated with training and other procedures aimed toward improvement of sport performance. Because of their face validity, the evaluated tests are recommended not only for sport-specific profiling and early selection of young athletes, but also for the regular assessment of sport performance that is sensitive enough to detect effects of various intervention procedures. The novel set of zigzag tests could be particularly promising for performance assessment in soccer players. However, direct measurement of the maximal ball velocity (e.g., with a standard radar gun) is recommended in the throwing-in and standing-kick tests, instead of the velocity assessed by the distance travelled.
This study was supported in part by Serbian Research Council grant 145082.
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