Monday, April 22, 2013

The Instep Soccer Kick
The instep soccer kick is a vital skill in the sport, and is undertaken numerous times by each player throughout the game. It plays a vital role for shooting at goal or playing a long ball trying to hit a target (ie. A team mate). There are six phases that are involved in the instep kick in the world game of soccer. These six phases include:
·         The Approach
·         Plant-Foot Forces
·         Swing-Limb Loading
·         Hip Flexion and Knee Extension
·         Foot Contact
·         Follow-through
These six phases are vital for completing the instep kick in soccer, and all are a part of many biomechanical principles which they can be related to, which can explain why it is performed in a certain way (as shown in figure 1).This discussion will look a different biomechanical principles of the skill and help to understand how to optimize the instep kick in soccer.
Figure 1. Phases of instep kick
Coefficient of Restitution
The Coefficient of Restitution of two colliding objects is a fractional value representing the ratio of speed after and before an impact, which is taking along the line of impact. In other words the COR measure the bounciness of an object. 
The higher the elasticity of a ball, results in a higher coefficient of restitution. The more elasticity of the ball it can retain a higher amount of kinetic energy upon impact and therefore can exert a greater amount of force after impact as shown in Figure2.
Figure 2. Coefficient of Restitution (Foot Contact Phase)

Figure 2 expresses the stages of a kick in relation to the coefficient of restitution. When the impact is first made upon the ball it compresses, storing kinetic energy and at the moment the ball leaves the object of impact is it released almost like a spring action.
In relation to an instep soccer kick, when making contact with the ball there would be a higher coefficient of restitution if contact was made in the centre of the ball. At the moment of impact the ball is compressed, if the ball is hit in the centre the ball is able to compress allowing the kinetic energy and forces to be equally distributed for an optimal outcome.
During this “Foot Contact” phase, the time your foot has contact with the ball is only around 16 milliseconds (Reilly, 2003), this a general assumption however factors such as inflation levels can affect this. Furthermore at the point of impact around 15% of the kinetic energy of the swinging limb (lever) is transferred to the ball (SIB, 2010). There rest of the kinetic energy is used by the swinging limbs hamstring muscle group to be able to slow the lever down. Therefore it is vital in this stage to perform the technique correctly and generate as much kinetic energy through the backswing/forward motion of the kick to transfer as much kinetic energy to the ball.
Projectile Motion
When a soccer ball kicked with an instep kick the ball travels in the air before touching the ground. However during the flight of the ball there is air resistant, which can affect the flight of the ball. However in many cases air resistance and drag which would affect the ball could be very minimal and in this case are disregarded, but if there a noticeable winds or other factors it can significantly affect the projectile motion of the object. When the ball is moving through the air it can experiences a drag force which is exerted by the passing air. This can be examined by taking the ball being stationary and replicating the air which would pass and flow over the ball, the term used to define this action is called the Magnus effect.
Figure 3. The Magnus effect

The Magnus effect is cause by an interaction between the spinning ball and then passing air (Figure 2). When the ball is spinning the passing air is moving in the same direction as the surface of contact on the other side. Which means the relative speed of the air is smaller at the side where it makes contact with the rotating the surface of the ball. As a result of this it means the airflow pattern on each side of the ball is not symmetric, therefore the ball is pushed to the side that the air is moving, opposite to the rotating surface (Wesson, 2002).
The Magnus effect is relevant to the instep soccer kick when attempting to complete the kick with adding spin onto the ball to hit a target, whether it is a shot at goal or passing to a team mate. Figure 3 is a video example of the Magnus effect in action with a kick in soccer and helps express when and how it is relevant in a game situation. In this case it is a player taking a free kick aiming to score a goal.
In the case of the instep soccer kick the lever is the preferred kicking leg. The longer the lever the greater the velocity at impact and the greater the momentum developed by the object (Jones, 2008).  The velocity is greater at the end of a long lever than at the end of a short lever, as the longer lever has a greater range of motion and is able to generate a greater amount of force. Therefore biomechanically a person with a longer leg (in this case the lever) should have a greater range of motion, in turn should be able to generate more force upon the ball compared to a person with a smaller lever.  Figure 4 gives an example of a lever in action in relation to a soccer kick, and can help depict and image of what a lever looks like in motion.
Figure 4. Instep Kick Lever

With the instep soccer kick during the hip flexion and knee extension phase (when the lever is in motion) there is a lot of force generated. The high is swung backward and then forward with a forward extension of the lower leg as demonstrated with the extension of the lower leg in figure 5. As the legs descends and makes its way back toward the ball for contact the thigh movement slows, and the leg/foot begins to accelerate because to get the optimal outcome it is vital to transfer momentum and release the stored elastic energy in the knee extensors, then the knee extensors powerfully contract the swing the leg and foot towards the ball (Wahrenburg, 1978).
Figure 5. Knee Extension phase of the instep kick

By applying all these biomechanical principles into the instep soccer kick and having background knowledge of these principles can improve and aid in optimizing this skill for the optimal outcome. Understanding levers for example can help you understand where some of the forces are generated and understand why parts of the technique are vital in completing the skill successfully.
These biomechanical principles can be applied specifically to the instep kick in soccer, however these biomechanical principles can be relatable to many other sports and the knowledge and relativity can be transferred for an advantage when performing in other sports.
For instance with the coefficient of restitution understanding the entire principle is hard to comprehend, however after engaging and understanding the knowledge of the particular force can help you apply it to other sports. For example tennis, by understanding the coefficient of restitution can help understand the principles in which when the tennis ball collides with racquet and how that influences how much force is generate.
Also relating to tennis is the Magnus effect, in regards to top spin and back spin. If you understand the Magnus effect it may help you understand how much spin to put on the ball and may help your decision making in what angle and type of spin you may want to incur on the tennis ball itself. However it is all not just relatable to tennis, for example the Magnus effect is also very relevant in Cricket, in particular with spin bowlers, again another way of transferring and using the knowledge from the instep kick.  
In relation to levers, this biomechanics principle or concept can be widely transferred through a variety of sports; generally all sports which involve kicking of a ball with have the biomechanical principle relevant to generating force. For example in football (AFL) when kicking at anytime the levers are relevant for generating force, understanding where and how the force is generated through the levers in the body can help maximise and allow for an optimal outcome in AFL. Other sports such as rugby, American football, Gaelic football can all be applied to this principle as well.
Another way in which this information could be used is by sports coaches/teachers. If the sports coaches/teachers have an understanding of the biomechanical principles involved with different sports when coaching or trying to teach a skill you can elaborate and help the student or player better understand how, what and why certain outcomes are occurring. If the teacher/coach knows the knowledge of the biomechanical principles more effective feedback and instruction can be taught and a greater knowledge can be transferred to the students/players.
Wesson, John, The science of soccer. Bristol: Institute of Physics Pub., 2002.
De Proft, E, Cabri, J, and Dufour, W (1988), Strength training and kick performance in soccer players. In Reilly, T, and Williams, M. 2003), Science and Soccer (2nd ed). Routledge: London.
Jones (2008). What are Levers, Examples of Levers. Week 8 (Levers In Sport) URL:
Wahrenburg, H, Lindbeck, J, and Ekholm, J (1978), Knee muscular moment, tendon tension force and EMG during a vigorous movement in man. Scand J RehabMed. 10:99-106.