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.
