INTRODCUTION

Kicking in soccer is one of the most important and fundamental skills used in a game and is highly researched due to the popularity of the game (Barfield, 1998). The instep kick is the most powerful kick in soccer and requires the correct technique to achieve the greatest distance. There are different variations of the in-step soccer kick, which are often used, these include passing the ball at medium to long distance, shooting at goal, and performing penalty kicks Kellis & Katis, 2007). In a game of soccer the in-step kick is used when the goal keeper has a goal kick, the goal keeper puts the ball on the 6 yard line and kicks it as far down the field over the half way line as they can. Another situation in the game when the in-step kick is used is when the defence gets a free kick close to the half way line; the player pits the ball down and kicks it towards the goal.

Understanding the optimal biomechanical techniques for coaches and player is significant in improving mechanical effectiveness in execution, and identifying factors that influence successful performance. This blog will focus on the optimal biomechanical principles of executing an in-step soccer kick. There are six major movement patterns in which achieve the optimal biomechanics of the instep kick in soccer. These include the approach to the ball, force production during the foot plant/ supporting leg, Limb swing, hip/pelvis flexion and knee extension, foot contact to the ball, and the follow through.

THE APPROACH

(b)

The approach is the first movement phase of a soccer kick and is an important aspect for developing momentum and therefore velocity (Wang and Griffin, 1997). The approach is traditionally 2-4 steps however; a shorter approach can mean that the ball is struck sooner allowing faster attack and less time for defensive structures to assemble (Wang and Griffin, 1997). Potential energy is associated with a position and can be used to increase speed as adopting a good starting position will result in less steps and a faster attack (Blazevich, 2010). This starting position can be seen in figure 1 (a) and has the potential to gain kinetic energy (Blazevich, 2010).
 
 

In the initial stages of the approach as seen again in figure 1 (a) the chest is leading forward, where the centre of gravity is outside the centre of mass. However this changes throughout the approach as the player moves closer to the ball at the final curve of the approach, the centre of gravity shifts towards the inside of the curvilinear path as displayed in figure 2. This helps create forward momentum and power due to the summation of forces and an overall increase in acceleration speed, resulting in a greater overall velocity of the action. (Blazevich, 2010) Hip flexion in this direction enables for a greater range of motion later in action, at ball/foot contact (Blazevich, 2010). This allows for greater hip flexion to occur, thus facilitating increased torque about the hips, translating through the kinetic chain into a faster and more powerful kick (Blazevich, 2010).



Many players favour an angled approach as scientific studies show a 45-degree angle is optimal for facilitating maximum ball speed (Eleftherios and Athanasios, 2007).  This angled approach is also known as a curvilinear path in biomechanical terms (Blazevich, 2010). This angled approach enables greater pelvic rotation and limb-swing velocity creating a greater range of motion and overall increases speed (Eleftherios and Athanasios, 2007). This curved approach assists in facilitating the leading foot to be planted perpendicular to the ball, increasing the time and range of motion for the kicking leg to be rotated about the body (Blazevich, 2010). Therefore, greater torque forces are able to generated at the hip and knee flexion points (Blazevich, 2010).
 

(c)


The approach is most successful when executed on the balls of the feet as this allows players to increase their propulsive impulse and reduce breaking impulses in the lead up to making contact with the ball (Blazevich, 2010). All steps should be positioned reasonably close to the body with high hip extension reducing contact time with the ground to create fast explosive steps (Blazevich, 2010).  If strides are extended in their approach it will create an increase breaking impulses and reduce acceleration and speed,  (Blazevich, 2010). However the final step should be extended in front of the body and can be seen in figure 1 (c) in order to increase range of motion in the swing pathway of the kicking leg (Blazevich, 2010).
 


The arms play a significant role in producing speed but also maintaining balance as the swinging of the arms increases leg speed and creates body rotation (Blazevich, 2010). This technique involves backwards rotation of the arms along a sagittal plane in opposition to the legs to create speed and power (Blazevich, 2010). This is because the torque created by the ground reaction force changes, therefore the arms must also adapt and change (Blazevich, 2010). When the foot is out in font of the body the arm is bent creating an acute elbow angle and can be seen in figure 4, however when the foot strikes the ground the arm will lengthen, almost extending straight out which increases it’s angular momentum (Blazevich, 2010). When the arm is extended it increases the moment of inertia, which increases angular velocity and translates through the kinetic chain overall creating a faster approach (Blazevich, 2010). As the running action continues the foot falls behind the body, this is where the arm begins to shorten again and reduces its angular momentum. This technique allows angular momentum of the upper of lower body to work in an equal and opposite manner creating forward momentum and speed (Blazevich, 2010). Overall the quicker the arms the more angular momentum it possesses and greater speed can be produced.

Figure 4