Abstract
The purpose of the study was to analyze and quantify the ability of the goalkeeper while defending shots from left and right sides of penalty area or the goal area, which frequently occurs in an actual game situation. In other words, the study aimed at verifying the reaction ability of the goalkeeper to defend goals in two different defense positions across a 3 m width area without performing dives. Eleven male goalkeepers from a university soccer league participated in the study. The goalkeepers were assigned to react to a specific target point, i.e. high, middle, low, left and right in response to the LED signal positioned at the front. Three-dimensional motion analysis system was used to perform kinematical analysis through which variables such as the approach time of reaching specific target points and the movement used to cover the defensive area were observed. In results, no significant differences were observed in approach time of the hand to the target point in both defense positions, i.e. Type-A (defense posture of positioning the hand at knee height) and Type-B (defense posture of positioning the hand at waist height). But Type-B defense position average velocity was observed faster than Type-A for blocking shots of low-middle heights. Hence, the results obtained from the study showed a possibility that being prepared in Type-A defense position will be more accurate and stable. Moreover, goalkeepers were able to occupy wider defense range in Type-A defense position than Type-B within the time period of 0.7 ∼ 0.8 s after triggering of the LED signal. In conclusion, the results obtained from the study pointed out that Type-A defense position can be much more effective in soccer for blocking shots from the left and right sides of the penalty area. The defense posture of positioning the hand at low height is considered more effective when defending a ball shot from areas near the penalty area than the high hand position. It was also confirmed that the available defense area can be defended widely and effectively by moving the center of the body. Further studies need to be performed to consider the cognitive perspective by applying various methods.
Introduction
Goalkeeper participates in the final defense and is involved in the most crucial moment in the soccer game. The most important role of the goalkeeper is to defend shots from the opposing team. Effective defense of the opposing team’s offense is considered crucial to keeping the other team from scoring, and the psychological effects of such good defense are significant. Indeed, the defense performance of the goalkeeper is regarded as an important factor in increasing the collective efficiency of team members in field sports psychology 1 and soccer coaches are putting much effort into increasing the defense performance of goalkeepers by applying diverse knowledge related to sports science.2,3
Shin et al. 4 analyzed major Korean and international matches including the 2002 FIFA World Cup in Korea/Japan and found that the main shooting areas in a soccer game are the penalty area (52.6%), the area adjacent to the arc (28%), and the goal area (9.4%). Further, it was found that the shooting success rate is 33% in the goal area, 19% in the penalty area, and 6% in the area adjacent to the arc. This result confirms that scoring can occur most frequently in the penalty and goal areas and that shooting is the most likely to result in a goal, especially in the goal area.
When the opposing team’s attack reaches the goal area, the goalkeeper occupies his or her own position as determined in advance to minimize the defense area. 5 When the opposing team has the ball near the penalty area, the goalkeeper moves from side to side according to the location of the ball by making a fanwise move based on the central goal area and stands approximately 1 to 2.5 m ahead of the goal line. When shooting is performed at the edge of the penalty area, the width that should be defended by the goalkeeper is 4 m and the defense area decreases as the distance between the goal and the goalkeeper decreases. Thus, a ball shot by a striker from the left or right side of the goal area passes through a space of 2 m from side to side based on the body of the goalkeeper. 6
When the goalkeeper selects the appropriate location and the ball is shot at the penalty area or goal area, the possibility of a goal being scored is affected the most by the issue of how much area the goalkeeper can defend within the shortest time. 7 In other words, if the goalkeeper can block the progress path of the ball very quickly within the time required for the ball to pass through the same plane to the frontal plane of the goalkeeper, penetration of the ball into the goal can be successfully stopped.
Several studies have been conducted to identify the characteristics of a goalkeeper’s effective defense motions. Suzuki et al. 8 examined the correlation between a diving motion and the defense performance of the goalkeeper through kinematic analysis and verified that skilled goalkeepers dive toward the location of the ball more quickly and reach the ball by making a virtually straight diving path compared to less skilled goalkeepers. Graham-Smith et al. 9 classified the motions of the goalkeeper during defense of a penalty kick into seven motions and performed kinematic analysis on them, thereby confirming that the defense motions of the goalkeeper vary according to the course and distance of the ball and that rotation of the upper body and joint extension movements are important for effective defense.
In addition to the studies on the characteristics of the goalkeeper’s diving motions outlined above, studies on quantitative evaluation of the defense performance of the goalkeeper have also been conducted. Kerwin and Bray 10 modeled the available defense area of the goalkeeper through mechanical analysis of diving motions toward a ball shot for a penalty kick and confirmed that approximately 28% of the area, including the edges of a goalpost cannot be defended. Based on this study, Matsukura and Asai 7 examined changes in the defense area according to the directions and distance of diving motions and verified the type of area that can be defended by the goalkeeper during the defense of a penalty kick by mathematically estimating and quantifying a reachable range within the limited time.
As shown above, previous studies on the characteristics of defense motions and defense performance of the goalkeeper primarily focused on the goalkeeper’s motions during a penalty kick based on the defense situation accompanying diving motions. These studies can be effectively used for kinematic understanding of the effective defense motions of the goalkeeper; however, defense motions toward shooting at the penalty area or goal area where shooting is performed the most frequently, and which significantly affect the match result, were not examined. Moreover, because most of the previous studies kinematically describe the motion characteristics of the goalkeeper, they are limited in their ability to provide direct data that can be utilized in the field.
Consequently, this study quantitatively analyzed the defense performance of the goalkeeper in defense situations that frequently occur in a match, and examined a method of increasing the athletic performance of the goalkeeper by identifying the characteristics of effective defense motions based on the analytic result and increasing the defense area. To this end, this study reviewed two goalkeeper defense positions that are regarded as standard during a goalkeeper’s training, from the perspective of maximizing the defense area in an area that has the goalkeeper’s defense width within 3 m (Figure 1); this allows the goalkeeper to defend the goal without diving within the penalty and goal areas where shooting is performed most frequently and a goal is most likely to be scored.
Area that has the defense range of the goalkeeper within 3 m in the penalty and goal areas.
Methods
Subjects
Eleven goalkeepers (experience: 9.7 ± 2.3 years, age: 19.9 ± 2.1 years, height: 186 ± 5.6 cm, and weight: 79.6 ± 7.8 kg) from among the players of college football leagues were selected as research subjects in this study. The players selected did not have any goalkeeping problems associated with injury and participated in this research after being fully informed about the purpose and method of this study.
Experimental equipment
Eight infrared cameras (Qualisys Oqus500, Göteborg, Sweden) were used to analyze the response motions of goalkeepers according to their positions. And two ground reaction force platforms (AMTI OR6-7, Watertown, MA) were used to confirm the beginning of motions for preventing anticipative motion of subjects. The sampling rate of the infrared camera was set at 100 Hz and that of the force platform at 1000 Hz. A nine-bulb LED board that notified of the beginning points of motions was manufactured and used, and data were obtained by connecting all the equipment with an A/D board and a computer using a program, Qualisys Track Manager 2.3 (Qualisys, Göteborg, Sweden).
Experimental procedure
A non-linear transformation (NLT) method was used to establish three-dimensional spatial coordinates. The L frame was installed at the origin point of the global coordinate system and a T wand was moved and calibrated to include the entire experimental space for 30 s. After being informed about the task to be carried out prior to the experiment, the research subjects warmed up for 15 min to prevent possible injury, donned full body tights, and each attached 19 reflective markers all over the body. In the task to be carried out, the research subjects were asked to swiftly stretch out their hands to touch a rubber ball with the size of a soccer ball installed at target points in 10 directions (RHF: right-high-far, RHN: right-high-near, LHN: left-high-near, LHF: left-high-far, RMF: right-middle-far, LMF: left-middle-far, RLF: right-low-far, RLN: right-low-near, LLN: left-low-near, LLF: left-low-far) by looking at the LED board installed 1 m ahead of them (Figure 2). We chose the positions of targets by calculating the range that the goalkeeper must defend mathematically (Figure 1).
The 10 target points in the task to be carried out.
The subjects performed the task by taking two defense postures—A posture with the hands at the height of the knees and B posture with the hands at the height of the waist—and the task order was randomly established (Figure 3). Each task was performed two or three times and the task result in which the promptest response was provided was analyzed.
The two defense postures of the goalkeeper.
Data processing
The 3D coordinates of the reflective markers attached to the body were smoothed using a Butterworth fourth-order low pass filter to remove noise and the cut-off frequency was set at 6 Hz. An event was defined as the moment (signal) when the LED was turned on, indicating the beginning of the motion, to the moment the task was completed (finished). Computational variables included the available defense area according to time, time required for the hand to reach the target point, and the average and maximum velocities of the hand. The time required for the hand to reach the target point was within the signal to finish section; the average and highest velocity of the hand within the signal to finish section were found to calculate the maximum velocity. The available defense area was obtained by connecting the coordinates of the head, hand, and feet in each task, as shown in Figure 4.
Standard points of the available defense area.
In Figure 4, the area of the polygon generated by connecting each point can be calculated as follows:
The data obtained through the experiment and calculation were analyzed using a statistical program, SPSS Statistics Ver. 21 (IBM, Armonk, NY). A paired t-test was used to compare the two postures with the significance level set at 0.05.
Results
Changes in the available defense area over time
Available defense area over time.
p < .05.
Figure 5 shows the available defense areas of each posture over time. The movement of the hands was found in the available defense area from 0.3 s after the LED signal was given. The available defense range of the A posture was located relatively lower than that of the B posture until 0.9 s when each hand reached the target points.
Comparison of the available defense ranges over time.
Time required for hands to reach the target points
Time required for hands to reach the target points.
p < 0.05.
Average and maximum velocities of hands in each target direction
Average velocity of hands in each target direction.
p < .05.
The maximum velocity was found to be significantly high when the posture in which the hands are placed at the height of the waist was assumed in directions close to the upper right side; in the case of low target directions, the maximum velocity was significantly high when the posture in which the hands are placed at the height of the waist was assumed in the remaining target directions (specifically, RLF, RLN, and LLN), with the exception of the directions of the far lower left side.
Discussion
In a soccer game, defending a goal is virtually the same as scoring a goal. For this reason, as much effort should be made to prevent the opposing team scoring as that made to score a goal through various tactical and technical methods in order to win the game. All the players in the game are devoted to their roles to keep the opposing team from scoring, but goalkeepers have the most significant effect on the possibility of the opposing team scoring, which is also affected by the area that can be defended by the goalkeeper within the given time up to the final moment. 7 This study compared the defense performance of goalkeepers according to the location of their hands by applying the concept of the time taken for their hands to reach certain target points, movement velocity of their hands, and available defense area under the defense situations that occur the most frequently in an actual game in which the goalkeeper selects the most appropriate location and shooting is performed from the right and left sides of the penalty area or goal area.
The results of this study indicate that as for the available defense area, the time for the hands to reach the target points close to the body after the LED signal is given is distributed within approximately 0.6 s. In the section between 0.7 and 0.8 s, the available defense area is wider when A posture, in which the hands are placed at the height of knees, is assumed than when B posture, in which the hands are placed at the height of the waist, is assumed. Approximately 0.6 s is required for a ball that is swiftly kicked at 100 km/h by a striker in the penalty area to reach the goalkeeper. Consequently, the fact that the available defense range showed a significant difference in the section after 0.6 s implies that goalkeepers are likely to prevent the striker from scoring by placing their hands at a low level when shooting is performed in the area adjacent to the penalty area. Given the study of Sainz de Baranda et al., 11 which reported that shooting was performed more frequently in the area adjacent to the penalty area than in the area near the goalpost during the 2002 World Cup, our results have important implications for goalkeeping for short range shoots.
In terms of the total amount of time required to reach the target points, the difference between the two postures was insignificant. However, less time was required to reach the high target point than to reach the low target point. External force is needed to move the center of body as opposed to a part of the body and gravity is the only external force that can move the center of body to a lower direction. In this regard, acceleration to move the center of body to a lower direction is limited to 9.81 m/s2. Conversely, when the body is moved to an upper or side direction, the acceleration can be 9.81 m/s2 or higher by using ground reaction force and frictional force.
The reason for the longer time to reach a lower point can also be explained by examining the intuitive reactions of the subjects. In this experiment, the target points of the lower parts were in contact with the ground. Consequently, to keep their hands from hitting the ground, the subjects should inevitably reduce the velocity of their hand movement before their hands reach the target points. On the other hand, in the case of the high target points, their hands are unlikely to be injured even when they reduce the velocity of hand movement after their hands reach the target points, thereby leading to less time required.
Because less time is required for a ball shot from the ground to reach the goalkeeper in the case of the lower part, the best way to keep the opposing team from scoring is to effectively defend the lower part. Sainz de Baranda et al.
11
reported that approximately 45% of the balls shot head for a lower part, thereby suggesting a necessity to focus on the lower defense by placing the hands at a lower level for the defense posture. The result of this study confirms that the average velocity of the hands for reaching the lower target points is higher when the posture in which the hands are placed at the height of the knees is assumed, thereby indicating that this posture is effective for defending a low ball more accurately and easily. It was also found that the posture in which the hands are placed at the height of the waist is more effective in the high areas (RHN and LHN) close to the body, whereas the posture in which the hands are placed at the height of the knees is more effective in the other areas (Figure 6).
Advantageous defense areas according to the height of the hands.
On the basis of the results of this study, it can be concluded that the defense performance of goalkeepers will be higher if their hands are placed low at the height of the knees than when their hands are placed high at the height of the waist.
Conclusions and suggestions
This study conducted a kinematic analysis of the advantages and disadvantages of two goalkeeper postures, regarded as the standard postures during goalkeeper training, to quantitatively examine the defense performance of goalkeepers in a defense situation that occurs frequently during a match. The following conclusions can be drawn.
A goalkeeping posture in which the two arms are lowered broadens the available defense area between 0.7 s and 0.8 s after the signal is given compared to a posture in which the two arms are raised. Consequently, it is considered more effective when defending a ball shot from areas near the penalty area. It was also confirmed that the available defense area can be defended widely and effectively and that goalkeepers are likely to defend balls shot from all directions, including the low direction, more accurately when their hands are placed at the height of the knees.
However, as this study used a LED board to inform the goalkeepers about the directions in the experiment. Some of the limitation to the study were the type of information the goalkeeper should obtain regarding the striker or the ball and timeframe to accurately estimate the direction of the ball. Thus, further studies need to be performed to consider the cognitive perspective by applying various methods.
Footnotes
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, authorship, and/or publication of this article.