Introduction: An overview

Soccer is a popular worldwide sport that has great focus on the skill of kicking. The team with the most kicks on target has a greater chance of scoring and therefore winning the game (Kellis & Katis, 2007).  The instep soccer kick is essentially a foundational kick that is also known as the ‘driven’ shot due to its power and accuracy. It involves the ball coming off the top of the foot where the laces are tied (see figure 1).  This kick is primarily used for goal shots due to its power. Beginner players often have difficulty learning the instep kick due to the elements that it involves. For example, Brown & Narvaez  (2004) indicate that these reasons involve, the difficulty coordinating moving body parts, foot placement and positioning as well as previous perceptions of kicking.  This is when understanding biomechanics can be beneficial for coaches and advanced players to enhance and improve accuracy of the instep soccer kick. Acceleration, power and swing are elements which all contribute to the performance of the instep kick. This blog will look at the different biomechanics aspects involved within the instep kick and how they can be achieved for players to perform the optimal instep kick.

This poses the question of: 

What are the biomechanical principles that optimize and enhance the instep soccer kick?   

Figure 1: image showing the instep soccer kick
Retrieved online from  http://www.livestrong.com/article/464483-types-of-kicking-in-soccer/    

To answer this question the biomechanics of the instep soccer kick can be broken down into sub-headings. 

•        The approach
•        The planting foot
•       The limb swing loading
•        Hip flexion and knee extension
•         Foot contact
•        Follow through

   Kinematics

There are two patterns involved in the kinematic chain. These are the ‘push like pattern’ and the ‘throw-like pattern’ (Blazevich, 2012). It can be noted that the soccer instep kick uses the ‘throw-like pattern’.  To comprehend this, the ‘throw-like pattern’ essentially is when the joints of the body extend sequentially (Blazevich, 2012).  Figure 2 it shows the main stages of the instep soccer kick these are the approach, the planting foot, the limb swing loading, hip flexion and knee extension, foot contact and the follow through. It is important to note the importance of upper and lower body when evaluating the instep soccer kick.

The following You Tube clip provides a visual understanding of the kinetics chain in regards to the instep soccer kick.  (You Tube, video by Sutherland, 2014)

Retrieved online from You Tube: https://www.youtube.com/watch?v=byVzXHhYnDk

Figure 2: The six main technical components involved in the soccer kick- the approach, the  planting foot, the limb swing loading, the hip flexion & knee extension, foot contact, follow through
Retrieved online from: http://expertfootball.com/forum/viewtopic.php?t=15192

The Approach:

The run up begins the instep soccer kick and is important to increase forward momentum and transfer force upon the ball (refer to figure 2).  Ideally this run up will only consist of 2-4 steps (Lee, 2012). In regards to the biomechanical aspects  of the instep soccer kick Newton’s First Law can be applied. 
This is “an object will remain at rest or continue to move with constant velocity as long as the net force equals zero” (Blazevich, 2012). 
This law is also known as the law of inertia. Therefore a soccer ball will remain at rest or stationary until a force is applied to it such as the player kicking the ball. The ball will then remain moving until a force such as a player or the goals stop the ball from moving (See figure 3).

Figure 3: diagram indicating Newton's first law of inertia
Retrieved online from: https://session.wikispaces.com/1/auth/auth?authToken=05251f52635e1bd2db634b325c92e8593

 The force applied in an instep soccer kick then leads to Newton’s Second Law,“the acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of an object” (Blazevich, 2012). Essentially, the more force applied to the ball, the faster and longer distance it will travel. As the instep soccer kick is primarily used for goal kicking, distance and force are important for the ball to move quickly pass the goalkeeper.  In order to move an object of constant mass faster, the velocity and momentum need to be increased (Blazevich, 2012). As a stationary object has no velocity or momentum it means that force has to be applied to make the object such as the soccer ball to move. Understanding this concept allows the impulse momentum relationship to be applied. This essentially  provides information on how to best accelerate the body in order to create the greatest force (Blazevich, 2012). Angular velocity is also important in the approach of the instep soccer kick.  A significant misconception about kicking in soccer is that many beginner players think that they run up to the ball in a straight line. However if this occurred the leg swing would not be very powerful as less velocity would be produced decreasing the power and speed of the kick (Isokawa, & Lees, 1988). Research indicates that an angle of 43 degrees provides the maximum speed of the ball (Lee, 2012).  Masuda et al, (2005) quotes “the speed of the ball also depends of muscle strength, and the muscle potential is conditioned by the angle of running to the ball." Therefore angular velocity and momentum are important in providing an optimal instep soccer kick.  Once the player has quickly accelerated towards the ball, force is transferred into the planting foot.

The Planting Foot:

      The planting foot is referred to the non-kicking foot. The planting foot indicates the relationship of the ball and which direction it will travel. The foot should be perpendicular to the centre of the ball in order to remain balance and maximum velocity (Hay, 1996). The breaking force created from the momentum achieved in the acceleration provides energy to be transferred up the body (Blazevich, 2012). In order for the planting foot to break without sliding along the ground, soccer boots are worn. This correlates with Newton's first law of inertia as the studs on the boots make it harder for the player to move due to friction. The grass and the boots interlock with one another to make it harder for the player to move. 

    The Limb Swing Loading:

      The limb swing coincides with the planter foot. This is when the backwards leg swing is about to make contact with the ball. The players opposite arm to kicking leg is raised and pointed in the direction the ball is intended to go. The kicking leg is then extended with the knee completely flexed.  This is due to Newton’s third law “for every action, there is an equal and opposite reaction force” (Blazevich, 2012). When the knee is flexing it allows elastic energy to be stored to then transfer a greater force when the ball makes contact with the foot.

   

    Hip Flexion and Knee Extension:

      The knee goes from a bent position to a flexed position in order to transfer the maximum force on the ball (Lees, 2012).  As this is happening the hips are flexed in order to increase angular velocity of the thigh. The knee is  also extended in order to rotate the lower leg and foot to create momentum for the leg swing.  Torque is a moment of force causing the rotation of an object. Hip flexion helps rotate the hips and torso through the ball that increases the velocity of the ball  (Bauer et all, n.d). Torque assists the moment of inertia on the body and on the ball as it gains momentum into the motion of the kick (Blazevich, 2012).

     Foot Contact:

       The foot then makes contact with the ball at a quick speed. An estimated 15% of kinetic energy is released to the ball while the other 85% is transferred up the leg to assist with the follow through action (Gainor, Pitrowski, & Puhl 1978 ). The ball has the highest velocity when the area of contact is close to the foots centre of gravity (Lees, Asai, Andersen, Nunome & Sterzing, 2010).

       Projectile motion refers to the motion of an object projected into the air at an angle (Blazevich, 2012). This is an important aspect to look at in regards to the instep soccer kick. There are many factors that influence projectile motion such as gravity, air resistance, projection speed and height of projection (Blazevich, 2012). Objects, such as a soccer ball can move at a horizontal or vertical angle. For example the vertical angle is the height above the ground from which the ball is released and the horizontal angle is the distance from where the ball lands from where it was released (Blazevich, 2012). Ideally, the optimum projectile angle for a soccer kick is at 45 degrees as it provides an equal amount of vertical and horizontal velocity (McGuinis, 2005). However, if the soccer player is aiming for goal the projectile angle should be altered depending on the height of the goalkeeper in order to prevent the ball being defended (Blazevich, 2012).

      Coefficient resolution can be a biomechanical aspect that occurs when the ball makes contact with the foot. Coefficient resolution is the energy that remains in objects after they collide (Blazevich, 2012). It can be considered that a soccer ball will travel further when the foot makes contact with the ball with a greater force and with the ankle locked to create a harder surface area (Blazevich, 2012). Soccer boots can also provide a solid surface are and prevent injury when the ball comes off the foot.  Coefficient resolution can be measured between a figure of 0-1 with 0 being all energy that is lost and 1 being all energy retained. High coefficient resolution on balls will generally be of higher temperatures, lighter weight, harder surfaces and higher velocities (Blazevich, 2012). This will allow less energy to be transferred into heat  and sound and more energy to be transferred into kinetic energy.

Figure 4:  Magnus force
Retrieved online  from: http://www.soccerballworld.com/Spinning_Ball.htm

       When a soccer player kicks the ball into the air it is subjected to Magnus force. Magnus force is a lift force acting on a spinning object (see figure 4). It can be considered as a complex system that has many factors involved within it (Blazevich, 2012). Wind speed, ball speed and rotation speed impact the curve of a ball (Blazevich, 2012). If the ball is spinning in a forward direction friction is created by the air and ball surface (Real physic problems, 2009-2015). If a soccer player kicks the ball left of the centre the ball will spin clockwise causing the ball to curve right. If the player kicks the ball right of the centre it will spin counter clockwise and curve left (Real physic problems, 2009-2015). This is an important factor to consider for goal kicking as the ball can potentially avoid the goalkeeper by confusing them with the direction of play. Professional soccer players such as Beckham (figure 1)  greatly rely on the spinning curl of the ball  in a free kick  to curve the ball (Ireson, n.d).  

     

    Follow Through:

Figure 5: Digram illustrating body position in follow through
Retrieved online from http://www.sportsinjurybulletin.com/archive/biomechanics-soccer.htm

     The follow through is an important component to finish the instep kick. It allows the kicking foot to remain in contact with the ball for the longest time possible which allows greater momentum for the ball to travel through the air (Blazevich, 2012). The follow through allows balance to be remained, which in turn allows for a smooth accurate kick. On top of this, the follow through also prevents injury as it decreases the kinetic energy created by the leg swing (Barfield, 1998).  The follow through position can be illustrate in figure 5.

   How Can We Use This Information:

      There are many biomechanical aspects involved in the instep soccer kick. It is important to understand these aspects so that it can have a positive impact on the performance of the kick.  These biomechanical aspects can be broken down into sections. These are: the approach, the planting foot, the limb swing loading, hip flexion and knee extension, foot contact and the follow through. Each of these sections involves multiple different biomechanical understandings that need to be considered. 

  

      All the biomechanics aspects can be seen to relate back to Newton’s three laws. All these laws affect the accuracy of the instep soccer kick and the body positioning involved. For example, Newton’s first law provides information about inertia and how it can be overcome for the player and the ball. The second law indicates the speed/acceleration of the kick and how mass impacts the speed of the ball. The third law indicates that all actions have opposite and equal reaction, which is important to consider with body positioning and the release of the kick in regards to the Magnus force.

      Coaches can use this information to improve the overall technique of a players instep soccer kick.  They can break down the instep soccer kick into these biomechanical aspects. A lot of these aspects can then be transferred to other games that follow similar movement patterns. For example, as the instep soccer kick uses a throw like kinetic chain other sports that use this pattern can be seen to have similar movements. Other sports that use the throw like pattern include cricket and tennis.

      The projectile motion, which involves understanding angles, is a skill that can be easily transferred from the instep soccer kick to the AFL kick. It is a common understanding that objects that travel through the air are subjected to resistant forces such as air. Magnus force refers to the curve of the ball when the soccer ball travels through the air. The Magnus force can be seen to have a similar effect in tennis when the ball is spinning/travelling over the net. Another biomechanics aspect is the coefficient resolution that essentially is when an object such as a ball makes contact with something. For example, in soccer and AFL the ball makes contact with the foot while in cricket the ball makes contact with the bat.

   

      Once players understand and become more familiar with the science behind the biomechanics of a soccer kick then they can have the skills and knowledge to self correct themselves and understand the complexity of skills.  This essentially allows players to become more sport literate and apply themselves to the best of their abilities. 

References:

Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728.

Bauer, L., Fryer, E., Levesque, C. & O’Brien, S., (n.d) Qualitative Biomechanical Analysis of a Penalty Kick in Soccer. Retrieved online 15 July 2015, from folioz.ca/artefact/file/download.php?file=24233&view=315

Blazevich, A, (2012), “Sports biomechanics, the basics: Optimising human performance,”London: A&C Black

Brown E., Narvaez M.,  (2004)  ‘Coaches information service, Teaching the instep kick to beginner soccer players,’ retrieved online 13 May https://www.msu.edu/~narvaezm/CIS_soccer.pdf

Gainor, B., Pitrowski, G., & Puhl, J. (1978). The kick. Biomechanics and collision injury. Am J Sports Med. 6, 185-193.

Hay, J., (1996), Biomechanics of Sport Techniques. Prentice Hall: New Jersey.

Hay, J. (1993) The biomechanics of sports techniques. Englewood Cliffs, N.J.: Prentice Hall.

Ireson G., (n.d), Beckham as physicist, Department of education, Loughborough University, Lecis, LE11 3TU, UK.

Isokawa, M., & Lees, A. (1988). A biomechanical analysis of the in-step kick motion in soccer. In Reilly, T, and Williams, M, (2003),Science and Soccer (2nd ed) pp. 449-455. Routledge: London.

Kapidžić, A., Huremović, T., & Biberovic, A. (2014). Kinematic Analysis of the Instep Kick in Youth Soccer Players. Journal of Human Kinetics, 42, 81–90. doi:10.2478/hukin-2014-0063

Kellis, E., & Katis, A. (2007). Biomechanical characteristics and determinants of instep soccer kick. Journal of sports science & medicine, 6(2), 154.

Lees, A. (2012). Biomechanics applied to soccer skills. Science and Soccer: Developing Elite Performers.

Lees, A., Asai, T., Andersen, T. B., Nunome, H., Sterzing, T. (2010). The biomechanics of kicking in soccer: A review, Journal of Sports Sciences, 28(8), 805:817

Masuda K, Kikuhara N, Demura S, Katstuta S, Yamanaka K. (2005), Relationship between muscle strength in various isokinetic movements and kick performance among soccer players. J Sports Med Phys Fitness. ;45:44–52.

McGinnis, P. M. (2005). Linear Kinetics: describing objects in linear motion. In. L. D, Robertson (Eds). Biomechanics of sport and exercise. Human Kinetics: New York.

Real physic problems, (2009-2015), the physics of soccer, accessed online  11 June 2015, http://www.real-world-physics-problems.com/physics-of-soccer.html

Sutherland, J., (2014) Biomechanical aspects of the instep soccer kick. You Tube, retrieved online 26 May 2015, https://www.youtube.com/watch?v=byVzXHhYnDk