What are the biomechanical principles required to efficiently produce a knuckleball shot in soccer?

What are the biomechanical principles required to efficiently produce a knuckleball shot in soccer?

What is it?
The knuckleball shot in soccer refers to a ball kicked at very low spin and high pace, which results in the ball producing an unpredictable zigzag trajectory. Due to the unpredictable movement of the ball the knuckling effect is highly troubling for many goalkeepers and is regarded as one of the most difficult shots to predict in soccer. An individual performer who has mastered this technique is Cristiano Ronaldo who has a free kick success rate three times that of the average soccer player. This blog will explore the technical elements he uses to produce this shot and discuss the biomechanical principles required to effectively produce a knuckleball shot in soccer. The elements of this skill that will be explored are; The run up and final stride, the ball to foot contact, the leg swing and follow through, the support leg and pelvis during approach and the body posture during the kick.

                                                                                                                

Run Up and final stride                                                                   Figure 1

The first part involved with this skill is the run up, for this part it is recommended that a run up of 4-6 steps would be most suitable for this type of kick. Reilly & Williams (2003) found that when performing a free kick run ups of 4,5,6 are the most successful. This is because when players use power in their run up, it is more difficult to utilise 100% of their power into the kick, therefore, they are not recommended to reach maximal speeds in the run up phase. Due to the length of run up the players now will not be using their speed to generate power onto the ball, instead, they utilise  the summation of force concept. In this skill this concept is developed through the length of the last stride and the shape of the support leg prior to the kick. As seen in the Figure 1 Ronaldo has quite a large final stride when kicking, by doing this he is able to generate high amounts of leg speed through his hips and thighs to be transferred through the ball. Lees & Nolan (2002) associated greater length of the last step with a greater degree of pelvic retraction, which in turn allowed a greater range for pelvic protraction. This action also relates to Newtons third law as explained in Blazevich’s text “Optimising human performance” (2013) which states “For every action there is an equal and opposite reaction”. By retracting the hip higher greater tension is placed on the hip tendons which react by protracting the leg with high speed to generate velocity in the kick. The positioning of the support leg in this final stride is also extremely important from biomechanical perspective. At this point Ronaldo places his support leg down with slight in the knee. What this allows him to do is absorb the force generated in the run up through his support leg and transfer this through his body and into the kick.  

Ball to foot contact.

The trajectory of a soccer ball that is kicked is influenced not only by the initial condition of release,

but also by the flow of air caused by a rotation of the ball during its flight.(Lees et al 2010)Traditionally, when taking free kicks players will try and ‘curve’ the ball over the wall, for this players utilise what is known as the Magnus effect. This effect refers to the “changing of trajectory of an object towards the direction of spin” and it occurs due to the high and low pressures around the ball, and the ball ‘grabbing’ onto air which flows past it (Blazevich 2013). For this kick the player must hit the ball off the centre of mass depending which way they want it to spin in order for the curve to be effective. The knuckleball shot is different to this technique as the objective is to place as little spin as possible onto the ball giving it its renowned zigzag effect. To produce the knuckling effect the player aims to strike the ball directly in the middle and close to the ankle joint, this reduces the likelihood of spin and will send the ball in a straight trajectory.  Neal Smith (PHD in sport biomechanics) states that striking in the middle and giving it no spin will send the ball in a straight trajectory, however throughout the kick it will move off its initial course. This due to the seams of the ball, when the balls spinning fast the seams will have no effect instead the curve is due to the Magnus effect. But when there is no spin on the ball the seams can catch in the wind on one side of the ball and not the other causing the ball to deviate. Slight deviation off trajectory can generate movement of the seams which generates a turbulence that varies from one side to the other side of the ball, thus creating the knuckling effect.

This photo is a free kick performed by Cristiano Ronaldo to show just how troubling his shots can be with a deviation of 3 meters from the original trajectory.

(Figure 2) 

Leg swing and follow through  

The next biomechanical principle in this skill progression is during the leg swing and follow through of the kick. In Ronaldos leg swing prior to making contact with the ball we can see from (figure 3) that his knee is flexed when the leg is extended to kick. What this allows him to do is generate more force and power from his legs onto the ball. Another aspect of the leg swing which may determine the effectiveness of the kick is the power generated by the the hips and pelvis. When looking at figure 3 the series of photos show a large difference in the placement of the foot, however, the thigh has very little movement. What Ronaldo does in this action is he produces large amount of power through accelerating the thigh through the hip muscles and then bringing it to a stop, “The hip flexor muscles are dominant during the majority of striking phase to the ball. The hip flexor muscles are contracting eccentrically to stop the leg’s backswing; then their activity becomes concentric to accelerate the thigh towards the ball” (Robertson & Mosher, 1985). This then has a whip like action on the knee where the food is brought through at high speeds due to the stored energy in the hips and thighs. This whip–like movement is known as the kinetic chain, which is the sequential flow of energy and momentum from bigger segments to smaller ones. (Blazevich 2013)

The follow through of this kick is also another important biomechanics aspect as this is another way maximum force is exerted on the ball. In his kick Ronaldo uses a punch like action which allows for the sudden transfer of power through the ball. In this phase of the skill rather then kicking through the ball Ronaldo ‘stabs’ at it which doesn't generate a large follow through. Instead he jumps to bring a stop to the follow through which allows the sudden transfer of power onto the ball and the ability to carry his body forward and then upwards throughout the kick . (Ahmeti 2012)

Body posture when kicking

In many sports it is important to keep the head and eyes still during the execution of a movement, this usually improves the accuracy of our movements. (Blazevich 2013) The final biomechanical aspect that will be discussed in relation to the knuckleball shot is in body posture. As mentioned maintaining a eye level throughout the progression of the skill is extremely important to maintaining accuracy in the kick. From looking at figure 3 we can see that Ronaldo maintains an almost perfect line of eye level throughout his kick. Not only this but his body posture when kicking is a factor in his effective performance. When looking at figure 4 we can see that Ronaldos centre of mass is directly above that of the ball. What this means in terms of performance is that by utilising this position he is able to bring the entire motions of his body through the ball, which generates large amounts of force on the ball. Being on top the balls centre of mass also decreases the likelihood of spin, if the body was at an angle then the ball would veer off in that direction, however, due to Ronaldos high level of balance and support he is able to produce a shot of high power and no spin

How else can we use this information?

This information can be used in many ways but most likely in the technique development of younger players. This information provides a framework on the biomechanical principles required to effectively produce a knuckleball shot. This blog also contains information which can be transferred across other sporting areas. By understanding the Magnus effect on a spherical object, cricket players could utilise the information in order to develop their swing bowling likewise tennis players to understand the basic principles of a top spin shot. The foot position and body posture are crucial when kicking a football or rugby ball and can alter performance when the correct application of these techniques is used. 

Refrences

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.

Lees, A. Asai, T. Andersen, T.B. Nunome, H. Sterzing, T. 'The Biomechanics Of Kicking In Soccer: A Review'. Journal of Sports Sciences 28.8 (2010): 805-817. Web. 15 June 2015.

Reilly, T., & Williams, A. M. (Eds.). (2003). Science and soccer. Routledge.

Robertson, D. G. E., & Mosher, R. E. (1985, January). Work and power of the leg muscles in soccer kicking. In 9th Congress of the International Society of