Thursday, 19 June 2014

What are the biomechanical principles to efficiently perform two of Cristiano Ronaldo’s famous free kicks, the ‘Knuckle Ball’ and the 'Curve Ball'?

This blog will explain the biomechanical principles behind a soccer free kick, in particular the principles of Cristiano Ronaldo’s famous ‘Knuckle Ball’ and ‘Curve Ball’. The aim is to increase the efficiency by using biomechanical principles. This includes foot contact, the initial run up, the leg swing and the follow through. The basic principles can be used within a variety of  sports, however the specific techniques are difference. Some similarities between sports are the summation of forces which assists in generating power and speed in most sports and movements such as a tennis serve, a football kick or a cricket bowl. The Magnus effect also has a great impact on numerous ball sports, such as tennis and golf. These principles are both used within Ronaldo’s free kick and assist him in being a better overall player.
Within this video, Ronaldo’s technique has been studied for the first time. Through the use of technology and various tests, they are able to analyse aspects of Ronaldo’s game, to understand why he is one of the greatest soccer players of all time. Skill, strength, mental ability and technique were the areas that were covered within the analysis. Each aspect plays an important role within soccer. Within this blog, the main focus is on Ronaldo’s technique. This includes how he kicks a ball to beat a defensive wall, the ‘curve ball’ and the famous free kick the ‘knuckle ball’.
 
Main Answer
Principles of Ronaldo’s ‘Knuckle Ball’
What is it?
The Knuckle ball is a free kick that has only been mastered by one professional player in the world, Cristiano Ronaldo. The free kick is hit dead straight, however in the air, it wobbles side to side and veers off its normal trajectory by up to two meters. This is due to the Magnus effect. This ball is extremely difficult for a goal keeper to predict, as the wind and environment effect how to ball will travel.  
Run up
Ronaldo used a run up of 4 paces to perform his knuckle ball free kick. It has been proven that having 4, 5 and 6 paces are the most successful amount when performing a free kick (Reilly & Williams 2003). When performing the run up in soccer, players cannot run at maximum speed. This is because when players use power in their run up, it is more difficult to use 100% of their power in the kick (Reilly & Williams 2003). However, players need to generate enough power through their run up to transfer through to the ball when kicked. Ronaldo does this by taking his last step on his kicking leg with a straight leg, so he can then generate force through the ground with his standing leg.

 


Figure 1

Figure 2



 
Foot contact
Traditionally, when kicking a ball a player will use the in-step of their foot (see Figure 1). However, to produce lift and more power, the player uses the top of their foot, where laces are (see Figure 2). To produce a ‘Knuckle Ball’ free kick, at the moment of contact, the ball is close to the ankle joint and hit directly in the centre of mass. This produces a straight ball with no spin. As there is no spin, the seams in the ball catch in the wind. This then determines which way the ball will deviate.


Figure 3

Figure 4

Figure 5



 
 
Leg swing and follow through
Traditionally, when performing a free kick, a player will have a large leg swing (see Figure 3 and Figure 4), however when Ronaldo makes contact with the ball, his leg swing stops almost immediately after contact (see Figure 5). He forces his hip joint to stop, however allows the knee and ankle joints to continue. This causes a whip like action, which generates enormous power transfer through the ball. When performing this particular kick, Ronaldo does not produce much follow through. This is because most of his generated power has been transferred through his body to the ball, and his follow through allows the power to safely leave his body, to prevent injury for occurring if he came to a sudden stop.
Body Shape
When Ronaldo kicks the ball, his torso is straight. His leg is an extension of this and therefore is a powerful technique when kicking a ball. This allows Ronaldo to generate power with his kick.
Spin
When Ronaldo kicks the ‘Knuckle Ball’ he produces no spin on the ball. This is interesting because when a ball is kicked, naturally it produces spin. Ronaldo has used his knowledge of soccer and trial and error to devise a way in kicking the ball with no spin. When a ball has no spin it is then affected by what is called the Magnus effect.
The Magnus effect
The Magnus effect refers to how the ball spins through the air. This Magnus effect, is primarily influenced by the thin layer of air that ‘clings’ to the ball while it moves through the air (Dooghin, Kundikova, Liberman & Zel’dovich 1992). The seams of the ball assist with the movement of air flow across the ball surface. Knowledge of how the Magnus effect can assist a player in producing a more efficient kick, as it is more difficult for a goal keeper to read. However the player that has benefited the most from the use of the knowledge behind the Magnus effect is Cristiano Ronaldo. His free kick has little or no spin, which, according to the Magnus effect, allows the wind to determine in which way the ball will curve.

Principles of Ronaldo’s ‘Curve Ball’
What is it?
The 'Curve Ball' is traditionally used to curve a ball over a defensive wall when attacking. This is done by hitting the ball off centre, to generate spin. The ball then spins over the wall, and dips into the back of the goal net, over the goal keepers head. This ball is difficult for a goal keeper to defend against, as they are unable to see the ball until the last minute.
Run up
Ronaldo uses a similar run up with his ‘Curve Ball’ and his ‘Knuckle Ball’. The most efficient amount of step in a run up if 4, 5 or 6, as they allow the player to generate enough force to push through the ball, but not too much that they are unable to correctly perform the skill (Reilly & Williams 2003). In Ronaldo’s run up, he performs a small skip one the kicking legs last step. This allows him to generate power through the ground in his next step, which can then transfer through his body to the ball (Lees & Nolan 1998)
 
Foot contact:
To kick a 'Curve Ball', the placement of the foot is a vital component as it allows for spin on the ball to generate curve. This is achieved by hitting the ball off its centre of mass, for example if you want the ball to curve to the left, the foot contact will be to the right hand side of the centre of mass. Once the spin is generated on the ball, different air pressures are what allow the ball to curve of its natural course, to then curve over a defensive wall. When the ball travels, there is low and high pressure around it, the ball will spin towards the direction of low pressure (Blazevich 2013). When tested, Ronaldo was able to curve the ball off its normal trajectory by more than three meters (Ahmeti 2012)
Leg swing and follow through
When Ronaldo performs his ‘Curve Ball’, he does not use much swing. He used a punch like motion, which allows sudden transfer of power through the ball. Biomechanically, a larger leg swing allows for more power, however in Ronaldo’s case, he generates power from other sources, such as his standing foot which transfers power through the body into the ball. Due to the punch like motion Ronaldo uses when kicking, he does not have much follow through. Instead, he jumps to stop his follow through (Ahmeti 2012). This allows his leg to suddenly transfer power to the ball, and expel any unused power through his jump.
Spin
The Magnus effect is also what generates spin when curving a ball around a defensive wall. This is due to the low and high pressure around the ball. This is due to the ball ‘grabbing’ the air that flows past it (Blazevich 2013). On one side of the ball, the air pressure is fast, which is low pressure. The other side is slower causing the ball to rotate, or curve to this side, which is high pressure (Blazevich 2013). This also provides the ball flight path with a curve shape. A well known example of this is famous soccer player David Beckham’s free kick, which is known as ‘Bend it like Beckham’. Beckham is able to bend a ball over the defensive wall and off its normal trajectory, by using the spin created by the Magnus effect.
Secondary Questions:
How does a player’s knowledge of the spin of a ball affect the final outcome of the kick?
When a player knows how to spin a ball around a defensive wall, it assists their team as they are more likely to score a goal. Knowledge of this skill can also be useful when attacking, as a player positioned wide, can cross a ball in with spin, and it is more difficult to defend against. This increases the attacker’s chance of scoring a goal.
How does a player’s knowledge of the Magnus effect, affect the final outcome of the kick?
Knowledge of the Magnus effect can increase the player’s chance of scoring a goal. This knowledge of the Magnus effect itself is not the only crucial part of the kick. The player must also understand how to create the power behind the kick, as well as the knowledge of the technique used.
How does a player’s knowledge about summation of forces affect the final outcome of the kick?
Knowledge about the summation of forces is important for all athletes. The summation of forces allows a player to generate more power, which in turn increases the speed and is therefore difficult to defend. Summation of forces is not just important when performing a soccer free kick, it is important within every skill present in soccer. It is also important across numerous different sports such as a football, basketball, cricket and volleyball.
How else can we use this information?
This information can be used to develop the technique of young soccer players. This information provided also relates to a variety of other sports. The foot position is crucial when kicking a football, soccer ball, or rugby and can alter how the ball moves through the air. The summation of forces also relates to an enormous amount of sporting skills. If a player wants to generate more power, being able to transfer it through the body is one way in which this can happen. This is also apparent with the Magnus effect. The Magnus effect relates to any sport that involves a round ball, such as tennis or golf.
 













 
References
Ahmeti, A. [Andin Ahmeti]. (2012, April 15). Cristiano Ronaldo – Tested To The Limit [Video File] retrieved from http://www.youtube.com/watch?v=vSL-gPMPVXI
Blazevich, A. J. (2013). Sports biomechanics: the basics: optimising human performance. A&C Black.
Dooghin, A. V., Kundikova, N. D., Liberman, V. S., & Zel’dovich, B. Y. (1992). Optical magnus effect. Physical Review A, 45(11), 8204.
Lees, A., & Nolan, L. (1998). The biomechanics of soccer: a review. Journal of sports sciences, 16(3), 211-234.
Reilly, T., & Williams, A. M. (Eds.). (2003). Science and soccer. Routledge.
 

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