Analysis of some biomechanical variables influencing the accuracy of direct free kicks in soccer

1

Keywords:

• Soccer
• biomechanical variables
• accuracy
• direct free kick

1. Introduction

Football kicking is described as a complex motor skill consisting of six important steps: Approach angle, supporting leg forces, swinging of the striking leg, hip flexion and extension at the knee level (Alcock et al. 2012), foot contact with the ball, and follow-up (Barfield 1998), the kicking procedure can also be divided into five essential aspects: kicking. Leg kicks, impact between the foot and ball (Andersenet al. 1999), and ball flight (Lees et al. 2010). The direct free kick is an integral part of football and is considered a crucial skill, especially for those who attempt to score goals, such as strikers and other attackers (Bray and Kerwin, 2003). Because the performance’s level of the direct free kick is different between attackers and other players_ playing style influence (Haceini 2015). The objectives of this study are to identify the biomechanical variables that influence the accuracy of the direct free kick, to determine the important phases of the execution of the free kick shot and the parameters that affect the trajectory of the ball during the direct free kick, and to make the hypothesis that there are correlations and statistically significant contribution rates between the biomechanical variables and the accuracy of the direct free kick.

2. Methods

2.1. Participants

Five (5) senior semi-professional players (Three rightfooted and Two left-footed) were chosen as the subjects in this study (Age: 22.8 ± 2.59 years old, Experience: 12.4 ± 1.82 years old, Body Mass: 67.8 ± 4.27 kg, Size: 1.75 ± 0.04 m).

2.2. Data collection and analysis

Markers were applied in three places on the lower legs (hip, knees, and ankle), and players were asked to kick a ball with their dominant leg towards a football goal (FIFA regulations; 2.44 m high and 7.32 m wide) placed 20 metres away. The data collection instrument is an accuracy test of direct free kick shot on three (03) different positions (Right, Centre, and Left) over a distance of 20 m. as shown in the Figure 1. The ball was placed 20 meters in front of the goal. The ball impact filmed by two video cameras, Camera 1: Kodak Easy Share DX6490, video resolution: 4 megapixels (2304x1728 pixels), recording speed: 30 fps, 2 m perpendicular to the front plane of the ball, Camera 2: Homday X-PERT, video resolution: 1808p/720p, recording speed: 25 fps/50 fps, 5 m perpendicular to the front plane of the ball (Figure 1). The video analysis was done by Dartfish TeamPro 5.5 software.

Figure 1. Direct free kick test and data collection.

2.3. Statistical analysis

Using the central trend (arithmetic mean) and dispersion (standard deviation) parameters for the descriptive part, and the calculation of Bravais Pearson’s correlation coefficients (r) for the analytical part, the calculations were performed by the IBM SPSS statistics 20 software analysis utility.

3. Results

The results revealed that there are correlations as well as statistically significant contribution rates between the accuracy of the direct free kick shot and the biomechanical variables of the various component phases; the approach phase: the player approach time has a statistically significant correlation of 0.616 and a rate of 38% (p < 0.01), the player approach distance has a significant correlation of 0.638 and a rate of 41% (p < 0.01), the player approach speed has a significant correlation of 0.483 and a rate of 23% (p < 0.01). The placement phase of the support leg and swaying footstrike: the thigh speed has a significant correlation of 0.350 with a rate of 12% (p < 0.01), the leg speed has a correlation of 0.424 and a rate of 18% (p < 0.01), the foot speed has a significant correlation of 0.488 and a rate of 24% (p < 0.01), while the study of (Shinkai et al. 2009) indicated the mean of foot velocity before ball impact was 20.5 ± (1.0) m.s⁻¹, and after ball impact was 13.7 ± (0.9) m.s⁻¹. Foot-to-ball contact phase: the momentum of the foot has a statistically significant correlation of 0.455 and a contribution rate of 21% (p < 0.01), the kinetic energy of the foot has a significant correlation of 0.416 and a rate of 17% (p < 0.01). The flight of the ball phase: the ball flight time has a statistically significant correlation of 0.781 and a contribution rate of 61% (p < 0.01), the take-off angle has a correlation of 0.608 and a contribution rate of 37% (p < 0.01), the ball speed has a significant correlation of 0.582 and a rate 34% (p < 0.01), while (Hong et al. 2012) claim that the average ball speed during direct goal kick is 28.3 m.s⁻¹ for a floating ball shot, the speed is 25.8 m.s⁻¹ (De Luca et al. 2017) estimates that the ball’s initial velocity = 20.4 m.s⁻¹ so that the ball travels along its flight trajectory during the free kick shot in a minimum amount of time. The momentum of ball has a significant correlation of 0.582 and a rate 34% (p < 0.01), the ball kinetic energy has a significant correlation of 0.590 and a rate 35% (p < 0.01). This is explained by the negative correlation between ball speed and accuracy. The transfer of the amount of movement is made through the kinetic chain of all bodies (Alcock 2010), starting with the support foot up to the leg, then the thigh (left for a right-handed shooter), then down to the trunk and down to the thigh, then the leg and finally the shooter’s right foot after impact with the ball (Tsaousidis and Zatsiorsky 1996).

4. Conclusions

The demands of modern football are steadily increasing. It is known that team sports like football are very complex because of the parameters they bring into play. Indeed, the work of this paper focused on the incoming biomechanical parameters involved in the skill of kicking in football. By studying the correlations of the latter with the precision of the shot of the free kick, the aim of this study was to identify the biomechanical variables influencing the accuracy of the direct free kick, while defining the important phases of the execution of the shot of the free kick, in order to provide biomechanical indices to the players and coaches to succeed the direct free kick. The results show the kinematic and kinetic variables, presented, described and interpreted above, having a statistically significant correlation 99 and significant with the success of the free kick, which reveals to us that it is clear that the analysis of the performance of the free-throw shot from the phases that compose it, demonstrates the importance of the constituent variables of each phase, all linked together to maximize the fluidity of the segmental sequence of the player throughout the shot franc. For optimal performance, a player must aim to develop a more fluid kinetic chain to maximize the power and accuracy of the shot. The fluidity results from the maintenance of the stability, a greater degree of freedom of the segments and the articular mobility. During the flight of the ball, the initial angle and speed have a very important role in the success of the shot from the direct free kick, just as gravity and aerodynamics are responsible for the diversity of the balloon’s trajectory; at low speed gravity dominates and only the Galilean parabola is observed, at high speed the aerodynamics becomes preponderant and the trajectory starts to depend on the rotation of the balloon, without rotation it goes straight or zigzags, depending on the laminar nature or turbulent of its boundary layer. This almost rectilinear trajectory lasts until the balloon is slowed enough for gravity to drop it, giving rise to truncated parabolas. The last regime is that of rotating balloons that leads to circles or even pieces of spiral when the distance traveled is large enough.