Introduction
Kicking in soccer is one of the most important and
fundamental skills used in a game and is highly researched due to the
popularity of the game. The instep kick is the most powerful kick in soccer and
requires the correct technique to achieve the greatest distance (Kellis, Katis, & Gissis, 2004). There are different
variations of the in-step soccer kick, which are often used, these include
passing the ball at medium to long distance, shooting at goal, and performing
penalty kicks. In a game of soccer the in-step kick is used when the goal
keeper has a goal kick, the goal keeper puts the ball on the 6 yard line and
kicks it as far down the field over the half way line as they can. Another
situation in the game when the in-step kick is used is when the defence gets a
free kick close to the half way line; the player pits the ball down and kicks
it towards the goal.
Understanding the optimal biomechanical techniques for
coaches and player is significant in improving mechanical effectiveness in
execution, and identifying factors that influence successful performance. This
blog will focus on the optimal biomechanical principles of executing an in-step
soccer kick. There are six major movement patterns in which achieve the optimal
biomechanics of the instep kick in soccer. These include the approach to the
ball, force production during the foot plant/ supporting leg, Limb swing,
hip/pelvis flexion and knee extension, foot contact to the ball, and the follow
through.
The action of kicking can be described as a “throw like”
movement pattern that involves movement through all of the joints in the
kinetic chain in a sequential order to successfully execute the kick in a
smooth and fluent manner (Blazevich, 2010). This creates high velocities due to
momentum generated in the proximal segments with the production of large
muscles forces impacting the distal segment as the muscles forces are
transferred through (Blazevich, 2010).
The kinetic chain for a soccer in step kick is “open” as the body is
able to move freely when executing the kick (Blazevich, 2010).
The skill begins with an approach that involves both flexion
and extension of the hips and knees along with plantar and dorsiflexion of the
ankles (Eleftherios and Athanasios, 2007). The approach creates momentum and kinetic
energy with the use of swinging arms, which create speed (Blazevich,
2010). The hips rotate forwards allowing
a flow on effect for the rest of the movement phases of the soccer kick to be
executed smooth and efficiently (Eleftherios and Athanasios, 2007). This
movement requires summation of forces allowing the sequential movement of the
kicking action (Blazevich, 2010). Many
biomechanical aspects are applied in the kicking action to help distance,
accuracy and power and are discussed in more detail below.
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THE APPROACH
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| Figure 1 (a) |
The
approach is the first movement phase of a soccer kick and is an important
aspect for developing momentum and therefore velocity (Wang and Griffin, 1997).
The approach is traditionally 2-4 steps however; a shorter approach can mean
that the ball is struck sooner allowing faster attack and less time for
defensive structures to assemble (Wang and Griffin, 1997). Potential energy is
associated with a position and can be used to increase speed as adopting a good
starting position will result in less steps and a faster attack (Blazevich,
2010). This starting position can be seen in figure 1 (a) and has the potential
to gain kinetic energy (Blazevich, 2010).
In the
initial stages of the approach as seen again in figure 1 (a) the chest is
leading forward, where the centre of gravity is outside the centre of mass.
However this changes throughout the approach as the player moves closer to the
ball at the final curve of the approach, the centre of gravity shifts towards
the inside of the curvilinear path as displayed in figure 2. This helps
create forward momentum and power due to the summation of forces and an overall
increase in acceleration speed, resulting in a greater overall velocity of the
action. (Blazevich, 2010) Hip flexion in this direction enables for a greater
range of motion later in action, at ball/foot contact (Blazevich, 2010). This
allows for greater hip flexion to occur, thus facilitating increased torque
about the hips, translating through the kinetic chain into a faster and more
powerful kick (Blazevich, 2010).
 |
| Figure 2 |
Many
players favour an angled approach as scientific studies show a 45-degree angle
is optimal for facilitating maximum ball speed (Eleftherios and Athanasios,
2007). This angled approach is also known as a curvilinear path in
biomechanical terms (Blazevich, 2010). This angled approach enables greater
pelvic rotation and limb-swing velocity creating a greater range of motion and
overall increases speed (Eleftherios and Athanasios, 2007). This curved
approach assists in facilitating the leading foot to be planted perpendicular
to the ball, increasing the time and range of motion for the kicking leg to be
rotated about the body (Blazevich, 2010). Therefore, greater torque forces are
able to generated at the hip and knee flexion points (Blazevich, 2010).
 |
| Figure 1 (c) |
The
approach is most successful when executed on the balls of the feet as this
allows players to increase their propulsive impulse and reduce breaking
impulses in the lead up to making contact with the ball (Blazevich, 2010). All
steps should be positioned reasonably close to the body with high hip extension
reducing contact time with the ground to create fast explosive steps
(Blazevich, 2010). If strides are extended in their approach (refer in
figure 3) it will create an increase breaking impulses and reduce
acceleration and speed, (Blazevich, 2010). However the final step should
be extended in front of the body and can be seen in figure 1 (c) in order to
increase range of motion in the swing pathway of the kicking leg (Blazevich, 2010).
The arms play a significant role in producing speed
but also maintaining balance as the swinging of the arms increases leg speed
and creates body rotation (Blazevich, 2010). This technique involves backwards
rotation of the arms along a sagittal plane in opposition to the legs to create
speed and power (Blazevich, 2010). This is because the torque created by the
ground reaction force changes, therefore the arms must also adapt and change
(Blazevich, 2010). When the foot is out in font of the body the arm is bent
creating an acute elbow angle and can be seen in figure 3, however when the
foot strikes the ground the arm will lengthen, almost extending straight out
which increases it’s angular momentum (Blazevich, 2010). When the arm is
extended it increases the moment of inertia, which increases angular velocity
and translates through the kinetic chain overall creating a faster approach
(Blazevich, 2010). As the running action continues the foot falls behind the
body, this is where the arm begins to shorten again and reduces its angular
momentum. This technique allows angular momentum of the upper of lower body to
work in an equal and opposite manner creating forward momentum and speed
(Blazevich, 2010). Overall the quicker the arms the more angular momentum it
possesses and greater speed can be produced.
 |
| Figure 3 |
Foot plant / support
leg
The
support leg is a critical aspect of success when executing a soccer kick,
however significantly less research has been conducted on the support leg
compared to the kicking leg (Inoue et al.,
2014). Balance in this movement phase is critical for success with the optimal
technique displayed in figure 4. In
this image we can see the trunk of the body leaning slightly backwards with the
centre of gravity behind the centre of mass. A Player in this phase of the
movement pattern should replicate the support leg flexion of a 26-degree knee
bend at foot ground contact (Lees &
Asai et al., 2010). When the kicking leg
begins to come through in a fast forward motion the support leg flexion should
increase approximately 42- degrees (Lees & Asai et al., 2010).
This knee bend lowers the centre of gravity creating a strong balanced and
stable position, allowing for greater velocity of the kicking leg as the hip
and pelvis are now at an optimal angle for the limb swing (Inoue et al., 2014).
The
support leg receives the most external force, hence its important role in
resisting these forces and stabilising the body on execution of the kick,
allowing for greater distance and accuracy (Inoue
et al., 2014). These external forces can also be referred to as ground
reaction forces and are an element of Newton Third Law stating “For every
action, there is an equal and opposite reaction” (Blazevich, 2010). Therefore
when the foot plant occurs from the support leg a downward force is applied,
where the ground exerts and equal and opposite reaction force back, stopping
the foot from sinking into the ground (Blazevich, 2010). The support legs role
is to stabilise the body when the ground reaction force occurs. The knee bend
technique is a key element of this stability (Blazevich, 2010).
 |
| Figure 4 |
As seen in many expert players and also in figure 4 a common theme is for the
final step to be executed on the heel foot, meaning the heel will contact the
ground first. This creates a short breaking impulse, however allows for greater
body balance on contact and allows more time for the hip/pelvis and limb of the
kicking leg to swing through and make perfect contact with the ball, overall
increasing accuracy of the kick.
Another key
technique to the foot plant is the ideal placement, which is suggested to be
5-10 cm behind the ball and between the window of 5-28cm to the left of the
ball (assuming a right-footed kicker). (Eleftherios and
Athanasios, 2007). Studies show that
any more than 10cm behind the ball compromises both direction of the kick and
balance of the player (Eleftherios and Athanasios,
2007).
LIMB SWING, HIP/PELVIS FLEXION AND KNEE EXTENSION
 |
| Figure 1 (b) |
The next phase within the biomechanics of the kick is the
back swing of the kicking limb in preparation for the downward motion towards
the ball. The foot plant initiates the backwards phase. Figure 1 (b) shows the backswing
phase of the kicking leg as the foot makes contact with the ground. As the
action begins the player’s non-dominant arm, the opposite arm to the kicking
leg it abducts and horizontally extends as this helps counter balance the
rotating body (Lees, Asai, Andersen, Nunome, & Sterzing, 2010).
By
raising the opposite are the athlete is producing speed but also maintaining
balance as the swinging of the arms increases leg speed and creates body
rotation (Blazevich, 2010). The backswing of the
leg starts to occur at the hips and initiates an open kinetic chain throw like
movement. This causes a rotational
propulsive impulse through the sequential extension of the hip, knee and foot.
Due to the backward movement occurring the angular velocity of the thigh is low
and the trunk velocity is negative (Kellis et al., 2004).
Towards the end of the backswing phase, but before the foot contact, the
hamstrings are maximally active to slow the leg eccentrically (Lees
et al., 2010). Studies have shown that elite athletes kick the ball
further with less muscle activity being more relaxed during the swinging phase,
but with a larger eccentric antagonistic muscle activity than novices
(Lees et al., 2010). More elite and skilled
kickers have more efficient use of their motor systems and biomechanical
control during the kicking actions than novices (Lees
et al., 2010). Completion of the backswing phase occurs when the support
foot makes contact with the ground.
 |
| Figure 1 (d) |
The thigh is then swung forward and downward with a simultaneous
forward rotation of the lower leg and foot. As the forward thigh movement slows,
the leg and foot begins to accelerate because of the combined effect of the
transfer of momentum and release of stored elastic energy in the knee extensors
(Kellis et al., 2004). The knee extensors then
powerfully contract to swing the leg and foot forwards towards the ball. As the
knee of the kicking leg passes over the ball, it is forcefully extended while
the foot is forcefully plantar flexed. This exposes the inside top part of the
foot (medial dorsum), which is propelled at the ball. This sequential extension
of the joints allows the player to generate high force at contact with the
ball, with a more cramped up backswing and a large last step maximising impulse
momentum and force generation (torgue). The non-dominant are adducts and
horizontally flexed to the ball contact this helps the shoulders and hips
straighten up to where the athlete wants to kick the ball. At the point of contact figure 1
(d), the hip, knee, and foot joints are straight (plantar flexed) creating a
push like movement. This facilitates for consistent contact between the foot
and the ball, and increases accuracy, distance, and speed of the kick.
As can be seen in figure 1 (b) the last step before
the foot plant is the longest step in the run up. This is because the larger
the step the more ground reaction forces (specifically braking impulses) occur.
These forces play a large role in determining the maximal velocity and momentum
of the instep kick. A high initial vertical force with the foot plant leg,
while controlling the knee and hip flexion on contact, would lead to increased
ball speed (Kellis et al., 2004). These brake forces in motion together reduce
the velocity of the hip and suggest a slowing down of body motion during the
kicking action (Kellis et al., 2004). Therefore larger the last stride in the
approach phase the more ground reaction forces will occur when maximizing the
velocity of the instep kick and the further the ball will go.
FOOT CONTACT
 |
| Figure 5 |
This movement phase occurs the instant moment the kicking
foot come in contact with the ball. The major technique requirement in this
phase is contacting the centre of the ball with the top of the foot avoiding
direct contact with the toes, as this will avoid ankle injuries e.g. strains or
impingements. Figure 5 shows the
optimal contact point for foot to ball contact. Contacting the centre of the
ball is known to have a positive impact on the projection angle, which is a key
element of increasing distance covered during the flight of the ball
(Blazevich, 2010). If players are to strike the ball lower it will create a
high ball trajectory, therefore contacting the ball high creates a low
trajectory (Blazevich, 2010). As seen in figure 6 the projection angle of 45-degrees will create an equal magnitude of
horizontal and vertical velocity overall having a maximal effect on distance
(Blazevich, 2010). Players should aim to create this 45-degree angle at foot to
ball contact and follow through.
 |
| Figure 6 |
Coefficient of restitution can be described as the
proportion of total energy that remains with the colliding objects after
collision (Blazevich, 2010). An example of restitution can be seen when
increasing the inflation of the ball as this increases the soccer balls
restitution; therefore less energy is lost after collision. In soccer the less
kinetic energy lost during collision the greater the speed and distance covered
during ball flight (Blazevich, 2010). Contacting the centre of the ball will
allow for equal distribution of kinetic energy to the ball, creating a higher
coefficient of restitution and allowing for an optimal outcome (Gainor &
Pitrowski et al., 1978).
Studies have shown that players remain in contact with the ball
for approximately 10 milliseconds (Eleftherios and
Athanasios, 2007). Upon contact the hip and knee of the kicking leg are
slightly flexed as the foot begins to move upwards. Plantar flexion of the
ankle occurs and is the major joint movement during this phase. On contact
approximately 15% of kinetic energy is transferred from the kicking leg onto
the ball (Gainor & Pitrowski et al., 1978).
FOLLOW THROUGH
 |
| Figure 1 (e) |
The final step of the in-step kick is the deceleration of
the kicking leg in the follow through phase. The primary objective is for the
kicker to keep the contacting foot in contact with the ball for as long as
possible. In kicking the ball longer the foot can keep contact with the ball,
the greater the momentum that can be generated (Garrett & Kirkendall, 2000) shown in figure 1 (e).
Secondly, follow through protects the body from injury, in particular the
swinging limb. The muscle and elastic forces that have been generated during
other phases of the kick are dissipated during the follow-through (Garrett
& Kirkendall, 2000). The follow-through increases the time component
of the impulse side of the impulse momentum equation, thereby reducing injury
possibility. The Magnus effect justifies the motion of the swing or back spins
that the ball makes during flight.
 |
| Figure 7 - (Blazevich, 2010) |
Figure 7 shows the spinning ball dragging the boundary
layer of air with the ball. On the left side the air spins with the ball
colliding with the oncoming air. The slower the velocity air is associated with
high relative pressure. (Blazevich, 2010). The opposite happens to the right
side of the ball creating pressure from the left and right, therefore the ball
swings (Blazevich, 2010).
HOW ELSE CAN WE USE THIS INFORMATION?
In order to improve the soccer in-step
kick coaches, teachers and players should spend time breaking down these
biomechanical aspects. The knowledge from the coaches and the phases above can
be transferred into different sports, which also follow a throw
like Kinetic chain movement. Other sports include Australian Rules football,
tennis, basketball, cricket, and volleyball.
For example the Magnus effect is related to the jump
shot in basketball, in regards to the backspin. When shooting at the ring the
athlete flicks his/her wrist and the ball spins backwards and holds up in the
air. As the basketball picks up speed, air on the front side of the ball is
going the same direction as it spins, and gets dragged along with the ball and
deflected back (Mazza, 2016). Air on the other side is moving opposite to the
ball’s spin, the flow separate from the ball instead of getting deflected
(Mazza, 2016). This results in the ball pushing the air one way so the air
applies an equal force on the ball the other way (Mazza, 2016). This is also linked in with newton’s 3rd
law of motion as every action, there is an equal and opposite reaction force.
By understanding the principles of the coefficient of
restitution this can help athletes understand the knowledge of the particular force
to other sports. For instance for tennis by understanding the principle of
coefficient of restitution can help the athlete or teacher understand when the
ball collides with the racquet and how that influences how much force is
generates. Another way in which this information could be used is by sports
coaches and teachers. If sports coaches and teachers understand the
biomechanical principles involved in different sports, they are able to teach
younger students and athletes in helping them improve in their techniques and
outcomes of the sport. They may help them understand why certain outcome occur
and teachers may give them effective feedback and instructions to improve.
Reference List:
Blazevich, A. J. (2010). Sports
biomechanics: the basics: optimising human performance. A&C Black.
Eleftherios, K. and
Athanasios, K. (2007). Biomechanical Characteristics and Determinants of Instep
Soccer Kick. Journal of Sport Science and Medicine, 6(2), pp.154-165.
Garrett, W. E., & Kirkendall, D. T. (Eds.).
(2000). Exercise and sport science. Lippincott Williams & Wilkins.
Hearn, A. (2013). Science in Soccer: The
Biomechanics of Kicking in Soccer. Scienceinsoccer.com. Retrieved 1
June 2016, from
http://www.scienceinsoccer.com/2013/06/the-biomechanics-of-kicking-in-soccer.html
Inoue, K., Nunome, H.,
Sterzing, T., Shinkai, H. and Ikegami, Y. (2014). Dynamics of the support leg
in soccer instep kicking. Journal of Sports Sciences, 32(11), pp.1023-1032.
Kellis, E., Katis, A., and Gissis, I. (2004). Knee
biomechanics of the support leg in soccer kicks from three angles of approach. Medicine
and Science in Sports and Exercise, 6, pg 1017–1028.
Lees, A., Asai, T., Andersen, T. B.,
Nunome, H., & Sterzing, T. (2010). The biomechanics of kicking in soccer: A
review. Journal of sports sciences, 28(8), 805-817.
Mazza, E. (2016). Incredible Basketball Trick Shot
Shows Why Physics Rocks. The Huffington Post Australia. Retrieved 10
June 2016, from
http://www.huffingtonpost.com.au/entry/basketball-magnus-effect_us_55a86f93e4b0896514d0f4c6?section=australia
Wang, J. and Griffin,
M. (1997). Kinematic Analysis of the Soccer Curve Ball Shot. STRENGTH AND
CONDITIONING JOURNAL, 19(1), p.54.
Britt Kendall and Sarah Willacy
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