MAJOR QUESTION
What are the optimal biomechanics for scoring a top corner penalty kick in soccer?
Soccer is one of the most popular sports in the world, and the objective of the sport
is to win by scoring goals. One way to
score a goal is from a penalty kick.
A penalty kick occurs under two circumstances in soccer. If there is a foul inside the 18 yard box, the
opposing team (who did not commit the foul) will be awarded a penalty
kick. A penalty shootout will also occur
in a knock out game or a final if it is a draw after full time and extra time,
to determine the winner of the match. Penalty
kicks are taken from the penalty spot, which is 11.005m directly in front of
the goals (Figure 1) (Bauer, Fryer, Levesque & O’Brien, n.d.). The aim of a penalty kick is to kick the ball
past the goal keeper and into the net to score a goal. A curved ball that is moving at a high
velocity means the goal keeper has minimal time to react, and may be deceived by
the swing on the ball; therefore, the kicker is more likely to score. Aiming towards the top corner of the goals also
creates a higher chance of scoring, as it is harder for the goal keeper to
reach the ball.
Figure 1: The Penalty Spot (Shergold, 2013). |
When
taking a penalty kick the approach is important
for the player to overcome their moment of inertia. The approach
should be between 3-5 steps; this enough to gain a reasonable amount of speed
and overcome the moment of inertia, however it would not be functional to approach
the ball from much further away and reach an extremely high speed, as this could
have a negative effect on balance and control upon reaching the ball (Lees et
al., 2010). The approach is the
beginning of the kinetic chain, and a foundation to build momentum which is
later translated into the kick, having an effect on ball velocity. Novice players often begin their approach directly
behind the ball, however this method is not effective as it inhibits swing limb angular velocity and leads to a less powerful kick (Lees et al., 2010). Instead, the approach should be curved and begin
on a 45 degree angle on the non-kicking side; this can be seen in Figure 3 (Kellis & Katis, 2007). Studies have shown that a curved approach is
more effective as it produces greater swing limb velocity and encourages a
centre of rotation upon reaching the ball, creating angular velocity and torque. This in turn leads to a faster, curved ball
that is harder for the keeper to save (Lees et al., 2010). The final stride in the approach before the foot
plant has an effect on the penalty kick itself.
A lengthier final stride enables more pelvic rotation to occur during the
swing through stage, which creates greater angular velocity and torque, and hence
increased ball velocity (Bauer et al, n.d.).
These biomechanical aspects will be discussed in more detail in their
relevant stages.
Figure 3: Curved 45 degree approach to penalty kick (LSM, 2014).
|
The Foot Plant and Swing-Limb Loading
During
stage two, the non-kicking foot is planted adjacent to the ball. Simultaneously, the kicking leg is flexed in
preparation for the swing through to kick the ball. The foot plant is a crucial aspect of the
penalty kick as it determines accuracy and transfer of momentum from the
approach into the kick. Therefore, a
good foot plant provides a good foundation or a link in the kinetic chain. When the support foot is planted, it supports
the total body weight. To maintain stability
the foot should be planted adjacent to, and in line with the ball,
approximately 5-10cm away (The Sports Injury Doctor, n.d.). The aim is to plant the foot as close as
possible to maintain a stable centre of gravity and hence produce a more
accurate kick, but not to plant it so close that it is in the path of the
kicking leg (Barfield, 1998; Bauer et al., n.d.). A close foot plant also allows the kicking
foot to make better contact with the underside of the ball; the importance of this will be discussed in the ‘ball
contact’ stage. There should be a slight
knee bend in the plant leg to absorb kinetic energy and maintain balance, and
the plant foot should be facing the direction the player wishes the ball to go
for accuracy purposes. Expert players
can sometimes face their plant foot away from where they are kicking to deceive
the goal keeper, however this requires a rapid last minute hip rotation to produce
adequate angular velocity and torque, and an accurate kick. Whilst
the non-kicking foot is planted, the kicking leg is flexed and close to the hip. This is known as the swing-limb
loading phase (Lees et al., 2010). During
this phase, elastic energy is stored in the kicking leg and this allows
for the transfer of more force onto the ball in the swing through phase. In this phase the centre of mass should be
located as close to the hip as possible.
This decreases the radius of gyration and moment of inertia about the
hip joint, and produces greater angular velocity in the swing phase, which is
important for producing a more powerful kick. Figures 4 and 5 highlight the importance of a good foot plant; Figure 4 shows a novice player who plants her foot too far away, resulting in an unstable centre of gravity and less transfer of momentum, whereas Figure 5 shows an ideal foot plant for producing power and accuracy.
Figure 4: Novice player plants foot too far back and loses balance and momentum (Bauer et al., n.d.). |
Figure 5: Good foot plant maintains centre of gravity and enables transfer of momentum into kick (Bauer et al., n.d.). |
Hip Flexion and Knee Extension
This
phase incorporates the downward swing of the leg towards the ball. Because the moment of inertia about the hip
is decreased from the previous stage, there will be greater angular velocity
produced at the hip during the swing through stage. This leads to greater acceleration and linear
velocity of the foot, hence producing a greater force acting upon the ball and greater
ball velocity, which is the aim of the penalty kick (Bauer et al., n.d.). Angular velocity and torque are important in
this phase for translating momentum and producing maximum ball speed (Figure 6). The curved approach at the beginning
facilitates torso rotation simultaneous with the downswing of the kicking leg. Torque is produced by the rotation of the
torso about the hip of the plant leg (Bauer et al., n.d.). The torques and angular momentum produced in
this phase are transferred down the leg and into the foot, resulting in a more
powerful kick; this can be seen in Figure 6 below.
Figure 6: Angular velocity and torque produced in swing through to ball (Bauer et al., n.d.). |
Ball Contact
To
produce an accurate penalty kick, a controlled stable body position is
important in the ball contact stage. The
close foot plant discussed earlier allows the player to contact the underside
of the ball with the kicking foot. Projectile
motion is relevant when considering the path of the ball. Contacting the underside of the ball facilitates
the lift of the ball which is necessary when aiming for the top corner of the
net. To contact the underside of the
ball and produce more force, players lean backwards during ball contact (Bauer
et al., n.d.). Newton’s third law of
motion states that for every action there is an equal and opposite reaction; therefore,
players may extend the non-kicking side arm to account for this backwards lean,
distribute mass evenly and maintain their centre of gravity (Lees et al., 2010). The backwards lean also allows for the kicking leg,
which is acting as a lever/force upon the ball, to fully extend. Full extension of the lever/leg produces
greater angular velocity and torque, and a greater force acting upon the ball,
therefore due to Newton’s first law this leads to a faster and more powerful
kick.
Figure 9 shows a right
angled triangle formed between the penalty spot, the top corner of the goals,
and the desired flight path. To score a
goal in the top corner, the optimal flight path would see the apex of the ball’s
arc (where the vertical velocity is zero) occurring just before the crossbar;
the ball will dip into the goals just out of reach of the goal keeper’s hands, because
gravity is increasing friction, which becomes the opposite force (Newton’s
third law) that begins to pull the ball back towards the ground. Because
it is known that the distance between the penalty spot and the goals is
11.005m, and that the goals are 2.44m high, trigonometry can be used to calculate
the optimal angle of projection which is approximately 13°, and this is
relevant for a maximal ball speed of 25m.s-1. During ball contact, 15% of kinetic energy is transferred
to the ball, whist the remaining 85% is dispersed through up the leg which slows
it down during the follow through stage (Gainor, Pitrowski & Puhl, 1978). The coefficient of restitution refers to the
proportion of total energy that remains in the ball after the kick. It is represented by a number between zero one
one; zero meaning all energy is lost, and one meaning all energy is
retained. FIFA regulations state the
coefficient of restitution must be between 0.82-0.88 at 20°c (Wiart et al.,
2011). A higher coefficient of restitution
has been suggested to lead to a faster ball speed, and the coefficient of
restitution can be affected by the mechanical properties of the ball, the
temperature and the soccer boot material (Kellis & Katis, 2007; Wiart et
al., 2011).
Figure 7: Foot should contact underside of ball for lift and spin (Bauer et al., n.d.). |
Figure 8: Foot should contact ball in this area to produce power and spin (Bauer et al., n.d.). |
Figure 9: The optimal angle of projection for the ball to dip under the crossbar is 13 degrees (Bauer et al., n.d.). |
Follow Through
The
follow through is crucial for maximising the time the foot is in contact with
the ball (which is generally only 10 milliseconds), and for reducing the risk
of injury (Bauer et al., n.d.; Lees et al., 2010). Studies have shown that with optimum force
production, ball velocity can range from 18-35m.s-1 however for
accuracy purposes an effective penalty would be around 25m.s-1 (Asai,
Carre, Akatsuka & Haake, 2002; Kellis & Katis, 2007; Lees et al.,
2010). Maximising foot ball contact time
by fully extending the kicking leg produces greater force and speed on the
ball. This is because it allows for the
transfer of momentum and kinetic energy that has been produced as a result of
the kinetic chain in the lead up to the kick.
The full extension of the leg also enhances accuracy
as the ball is inclined to follow the path of the leg. During the follow through, the hip continues
to flex and the torso rotates as the opposite force to maintain a stable centre
of gravity (Newton’s third law).
Newton’s third law is also relevant to the injury prevention component
of the follow through. The follow
through allows the body to disperse the remaining 85% of the build-up of
elastic and kinetic energy; if the player came to a sudden stop instead, there
would be an increased risk of injury to the hamstring or surrounding muscles due to equal and opposite reactions (Gainor,
Pitrowski & Puhl, 1978).
Figure 10: Follow through is important for accuracy and injury risk reduction (GBDCT, 2015 |
The Magnus Effect
Figure 11: The Magnus effect (Aviation for Kids, n.d. |
Figure 12: The Magnus effect (Wander, n.d. |
HOW ELSE CAN WE USE THIS INFORMATION?
Figure 13: AFL drop punt consists of a similar movement pattern ( |
The following video provides an excellent summary of the optimal biomechanics for a soccer instep kick, and it is included as an additional resource to further explain and demonstrate each of the stages discussed in this blog.
REFERENCES
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Blazevich, A. (2012). Sports
Biomechanics The Basics, Optimising Human Performance (2nd ed).
London: Bloomsbury.
Barfield, B. (1998). The biomechanics
of kicking in soccer. Clinics in Sports
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Bauer, L., Fryer, E., Levesque,
C., & O’Brien, S. (n.d.). Qualitative
Biomechanical Analysis of a Penalty Kick in Soccer. Retrieved from
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E., & Katis, A. (2007). Biomechanical
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