By Lachlan Scott and Christopher Skull

Introduction

The skill of shooting in soccer is the most important factor in the sport in relation to attacking play and goal-scoring. Shooting is a skill which relies heavily on correct decision making, where often, as a result of poor technique and hesitation, shooting opportunities can be missed. However, by obtaining relevant biomechanical cues and an ability to acquire an external focus, shooting accuracy will most likely become more efficient. As shown below in Figure 1, the preparation phase of Steven Gerrard’s shooting technique reveals a full range of movement that was created in the approach, coordination of all limbs and well placed foot plant, as well as the perceptual focus on the ball in order to ensure contact point between boot and ball is efficient enough to produce sufficient power in the shot. The aim of this blog is break down the skill of shooting into different stages as well as exploring specific biomechanics and certain biomechanical principles in relation to finding the optimal technique for a soccer shot.

Figure 1. (Emirates 24/7, 2011)

APPROACH

How can biomechanical principles be used to explain impulse momentum developed in the approach phase?

The approach is a pivotal aspect of the early stages of the soccer shooting motion. Figure 2 demonstrates Cristiano Ronaldo approaching the ball at an angle of approximately 45 degrees to develop the greatest possible velocity. Whilst approaching the ball on an angle, the body is inclined towards the centre of rotation, therefore reducing the impact of inertia through manipulating centre of mass. Initially, the player must accelerate to increase impulse momentum. This stage of the approach involves synergy amongst the kinetic chain, which involves the quadriceps and hamstrings flexing and extending, trunk position rotating and arms flexing and extending. This process can be explained by key biomechanical principles such as angular kinetics and angular momentum to overcome the moment of inertia and increase torque. In the final stride before reaching the ball, the stride is increased in length to increase potential hip rotation, therefore allowing for greater force production (Lees at al, 2010). Upon arrival at the ball, the opposite foot lands 10 cm away from the ball to maximise direction and velocity. This foot plant relates to Newton’s third law since, an equal and opposite force is applied to increase the amount of force produced in the latter stages of the skill.

Figure 2. (Harfield, 2014)

The Answer

Impulse momentum is critical in the approach phase of the soccer shot, since the player is looking to create momentum for the latter stages of the skill. As a result, the player should intend on increasing propulsive impulse throughout this stage of the shooting process. Linear velocity is also an essential part for the lead up phase prior to the contact point on the ball. With large horizontal propulsive impulse created in the approach, the player is able to utilise the generated velocity to gain greater angular momentum into the leg swing (Blazevich, 2012). This phase is demonstrated in figure 3, whereby Cristiano Ronaldo attempts to gain impulse momentum in the early phase of his approach. Since the approach of soccer players is typically only 4 metres, this momentum is generated through accelerating in the initial couple of steps. To develop this acceleration in the initial strides, the player needs to develop vertical impulses to propel into the air, however large horizontal impulses must be applied to maximise forward velocity. In order to do so, the player should intend on striking the ground with the forefoot. Initially, the foot should travel forwards relevant to the ground, upon contact with the ground the foot should apply a forward force allowing the ground to exert an equal and opposite braking force. Eventually, application of backward force from the foot will minimise the braking impulse and maximise propulsive impulse (Blazevich, 2012). Since impulse momentum is a combination of angular velocity and moment of inertia, these factors must be considered in the approach phase of shooting. Torque is pivotal in reducing the impact of inertia in the approach phase, whereby the body is inclined towards the centre of rotation, therefore reducing the impact of inertia through manipulating centre of mass. Manipulating torque to reduce the impact of inertia will eventually allow the player to enhance hip extension and provide stability. This angle used in the approach phase also applies angular velocity since, an inclined lower body allows for a greater range of motion. As a result, this can become pivotal in the conservation of angular momentum and eventually transfer more power into the shot (Lees et al, 2010).

Figure 3. (SrRondo, 2014)

 How can we use this information?

Figure 4. (Jenkins, 2015)

The findings demonstrated throughout this analysis are applicable to a wide range of sports. Particularly, the rugby conversion uses a similar angle in the approach to generate greater elevation, reduce the moment of inertia, allow greater angular velocity and produce stability whilst shooting. Figure 4 demonstrates Dan Carter manipulating his moment of inertia whilst approaching the ball through generating torque in the rotational swing of the kicking leg. Other skills where these key biomechanical principles can be applied include, cricket bowling, NFL kicking and AFL goal shooting.

Leg Swing

How does optimal usage of the kinetic chain influence the level of force production in a soccer kick?

Figure 5. (Camw, 2010)

The leg swing involved in a soccer kick is primarily centred on force production. In order to produce the greatest amount of force, a range of biomechanical principles can be included to find the optimal technique. Figure 5 demonstrates the optimal biomechanics for the leg swing phase. Key areas of notability included in this figure include, arms out wide to maintain stability, knee flexion, hip extension, low angular velocity on the kicking leg, and the kicking foot is flexed to increase velocity. As a result, all muscles involved in the kinetic chain are working as a summation of forces from larger to smaller muscle groups. Throughout this stage of the kicking action, the opposite foot is firmly planted and the knee is flexed to conserve angular momentum, therefore decreasing the moment of inertia. Whilst decreasing this moment of inertia, the kicking motion will increase linear velocity. This phase of the kicking skill is also highly relevant to the biomechanical principle of angular kinetics. Since the pivot point in a soccer shot is the hip, the laws of radius of gyration state that whilst optimising the soccer shot, it is critical to reduce the moment of inertia in the lower limbs. Torque can be increased through having strong muscles around the pivot point and a larger moment arm, which involves the distance between the muscle and joint centre (Blazevich, 2012). Newton’s third law is the final relevant biomechanical principle, whereby this states that there is an equal and opposite force reaction, upon applying a downward force. As a result, greater amounts of angular momentum can be utilised in the latter stages of the shot.

The Answer

Optimal execution of the leg swing phase is heavily reliant on the kinetic chain. The drawback of the leg in this phase is a fundamental movement through all throw-like movement patterns. Initially, the leg is drawn backwards, however once this changes to a forward movement, large muscles at the hip accelerate the thigh, however the lower limbs are drawn back as a result of inertia. Under this high load, the knee is forced into flexion. However, simultaneous contraction of the leg ultimately results in an extension of the knee and high foot speed (Blazevich, 2012). Throughout this process, the tendons play a pivotal role in releasing potential elastic energy to recoil at a fast speed. This allows for the key muscles in the process to simultaneously contract and produce a fast extension (Blazevich, 2012). Through applying a summation of forces, greater power is eventually found in the execution of the shot.

How else can we use this information?

Figure 6. (Getty Images, 2012)

These answers can be applied to a variety of sports that require similar movement patterns. One particular skill that involves similar usage of the kinetic chain is goal kicking in the AFL. As demonstrated in figure 6, since both of these skills involve a throw-like movement pattern, the leg is drawn back in a very similar way and eventually using summation of forces to execute the skill. This application of the kinetic chain is also executed in numerous other skills such as baseball pitching, netball shoulder pass, rugby conversions and kicking in American Football.

CONTACT POINT

What certain biomechanical techniques contribute to the amount of power generated in the shot through the contact phase?

Figure 7. (Fanshare, 2014)

Newton’s second law states that the acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of the object. However, it should also be understood that mass cannot be changed through technical adaptions, but specific technical changes can be made to produce more force (Blazevich, 2012). Therefore in this stage of the soccer shot, we can assume that the acceleration of the projectile (ball) is dependent on the force and technique that is previously created from the human being. A greater acceleration of the ball can be contributed from a number of factors including; amount of linear velocity produced in approach, amount of angular velocity in the leg swing, ability to conserve angular momentum, and how the centre of mass can be manipulated to increase torque (Blazevich, 2012).

Linear velocity is an essential part for the lead up to the contact point on the ball. With large horizontal propulsive impulse created in the approach, the player is able to utilise the generated velocity to gain greater angular momentum into the leg swing. Additionally, as seen in Figure 7, the sudden break in the sequence by planting the non-kicking foot firmly on the ground creates a ‘sling-shot’ motion which in-tern creates an opposite reaction (relating to Newton’s third law) with the kicking leg, allowing it swing forwards with more angular velocity when contacting the ball.

More specifically, as seen on Figure 7, Wayne Rooney’s centre of mass is clearly leaning onto the side of his non-kicking leg when he makes contact with the ball. By placing more mass onto his planted foot, he is able to control the impulse momentum that was developed in the lead up stages. Additionally, by manipulating his centre of mass to his non-kicking side, his kicking leg has the opportunity to swing forward with high angular velocity and contact the ball without striking the ground. This use of manipulation with his centre of mass also allows for a greater moment of force, allowing for increase amount of rotation in the leg swing. Increasing the amount of angular rotation in the kicking leg will in-tern increase the range of movement in the swinging leg, allowing Rooney to make contact with the ball with an extended knee. Through an extension of the knee, Rooney is able to utilise more leg muscles in a sequential order to complete a ‘throw like’ which will assist in increasing the amount of power generated in the shot.

The Answer

Linear velocity developed in the approach and the positioning of planting foot to create a ‘sling-shot’ action with the swinging leg are essential factors that contribute to the amount of power generated in a soccer shot. However, as we have found specifically for the contact phase, the manipulation of centre of mass is crucial in terms of increasing the amount of angular rotation. Through a wider range of movement that the manipulation of centre of mass provides, the swinging leg is able to make contact with that ball with a fully extended knee, allows for greater amount of power to be developed through the kinetic chain, and prevent cramping of the limbs throughout the shooting sequence. However, it should be understood that the amount of power generated in the contact phase is not only reliant on correct biomechanics but can also be dependent on a variety of external factors such as coefficient of restitution of the projectile, boot weight, and boot design. Coefficient of restitution describes the amount of energy that remains in an object after the collision (see Figure 8). For example, a ball with higher coefficient of restitution will be able to obtain more kinetic energy after contact and therefore more power will remain. In this instance, a soccer ball which been filled with higher amounts of air pressure is more beneficial than a ball with low amounts of air pressure. Furthermore, the ball will be able to travel further due to its ability to use energy that was transferred from the collision. Boot weight and design is also another contributing factor to the amount of power generated through the contact phase of the soccer shot. For example, a boot designed with lighter mass will allow for the foot to swing at a higher angular velocity. It has been found that in order to reduce moment of inertia the lower limbs, more mass is required in the upper leg muscles (quadriceps, gluteus maximus, and hamstrings) (See Figure 9), while less mass is required in the lower leg muscles (calves). By utilising a boot with lighter mass, the swinging leg will ultimately be able to rotate with increase angular momentum and transfer more power into the shot.

Figure 9. (Hendrie, 2015)

Figure 8. (Allain, 2011)

How Else Can We Use This Information?

Figure 10. (Ludbey, 2012)

This information can be used for a variety of athletes in sports which require repeated kicking sequences. For example, as seen on Figure 10, the same concept of shifting the centre of mass to the planting-foot side is utilised by Lance Franklin in an AFL goal-kicking situation. Similarly, Figure 11 shows the resemblance of an NFL goal-kicker with that of a soccer shot, both using a technique which enhanced angular rotation in the kicking leg to generate power. However, similar biomechanical concepts can be transferred into different sporting situations, with many baseball pitchers thrusting their centre of mass in a forward direction to use a summation of forces which generates more power in the kinetic chain through the sequential throw-like movement (see Figure 12).

Figure 12. (Janish, 2014)

Figure 11. (Rogash, 2015)

FOLLOW THROUGH PHASE

How can soccer players cope with the amounts of linear and angular velocity developed in the approaching stages in the follow through stage? And what techniques can players adopt to ensure projectile motion is on target?

Figure 13. (Doyle, 2012)

The follow through stage of the soccer shot provides a representation as to the amount of impulse momentum that was developed in the previous stages. As seen on Figure 13, Cristiano Ronaldo boasts a wide range of motion through the finishing point of the kicking leg. This stage of the shooting sequence is recognised as the finishing point of the throw-like movement in the kinetic chain, with the sequential order of power from different leg muscles transferred into the projectile. In contrast from the approaching stages, the follow through stage sees the flexion of the hip joint of the kicking leg, and extension of the knee joint of both kicking and non-kicking leg. As seen on Figure 8, as a result of high angular rotation, the kicking leg uses the developed momentum to swing in a vertical direction, allowing Ronaldo to ‘hang’ in the air momentarily.

It is noticeable in Figure 13 that during the follow through stage, both lower limbs and upper body are thrusted forward in a horizontal direction. This effect has a relation to Newton’s third law: for every angular action, there is an equal and opposite reaction (Blazevich, 2012). Specifically, once contact on the ball is made and the developed linear and angular momentum forces the swinging leg forward in the direction of the travelling projectile, it is recognisable that an opposite reaction occurs in the upper body, concurrently forcing the upper body forwards in the same direction of the swinging leg. This effect gives an indication of the amount of impulse momentum developed in the previous stages of the soccer shot, and is evident in Ronaldo’s shot in Figure 13. Additionally, as a result of the combination of linear velocity developed in the approach and angular momentum developed in the kick, Ronaldo has the opportunity to follow through with a large range of movement even after the collision with the ball and recognise whether the projectile motion is on target.

Another important aspect of the follow through stage is that it allows players to track the trajectory of the projectile once that shot has been taken. Projectile motion refers to the motion of an object projected at an angle in the air (Blazevich, 2012). In this instance, soccer players need to recognise the optimal angle of trajectory that the ball needs to travel so that there is an increase chance of scoring. Generally, soccer players aim to place the ball in the top corner of the goal (see Figure 14). By doing this, it gives the goalkeeper less chance of saving the goal, however, difficulty of this is technique is high with an emphasis on correct angle trajectory of the projectile. In order to get the ball up in the air and closer to the top corner of the goal, contact between boot and projectile needs to allow for positive relative height projection that will emphasise an upwards movement to get the ball off the ground. However, many soccer players are susceptible to blazing the ball over the crossbar due to leaning back. In order to reduce chances of kicking the ball over the target, it is noticeable in Figure 7 and 13 that Rooney and Ronaldo place their head and upper body over the ball to ensure that the trajectory of the ball will remain at a lower angle than that of the top of the target.

Figure 14. (EPA, 2009)

The Answer

Dealing with the amount of linear and angular momentum developed in the approaching stages is dependent on the player’s ability to conserve angular momentum. As mentioned, conservation of angular momentum can be done through the angular rotation of the kicking leg which in-tern flexes the hip joint and extending the knee joint. Simultaneously, the angular rotation should create both horizontal and vertical movement on the body, allowing the player to ‘hang’ in the air. Another way to conserve angular momentum and decrease moment of inertia is to allow the opposite reaction of the upper body in relation to the kicking leg. Assuming that the linear and angular velocity was high in the approaching stages, there should be an opposite angular reaction to the kicking leg in the upper body. Players need to recognise the optimal angle of trajectory for increase success of scoring. By establishing the difficulty for goalkeepers saving a shot in the top corner, shooting sequences should cater to these needs. However, in order to improve chances of meeting the required trajectory angle, players must adopt a technique which will reduce chances of kicking the ball over the target. As mentioned, this can be accomplished by placing the head and upper body over the ball, almost assuring that the projectile motion will hit the target.

How Else Can We Use This Information?

Once again, this information can be catered to a variety of different athletes in their respective sports. Angular rotation and effect of angular opposite reaction is highly recognisable in the sport of AFL and basketball. As Figure 15 reveals, the slam dunk shows how the upper body movement in the jump and finish forces the legs to swing in a vertical direction due to the amount of angular velocity that was developed throughout the skill sequence. Whereas,  similarities can also be found between the kicking movements between AFL and soccer. In this instance AFL kicking leg can also creating enough angular momentum to swing  body in an upwards directions, as well as thrusting the upper body forwards. 

Figure 16. (Shaw, 2016)

Conclusion

This blog has demonstrated how each component of soccer shooting can be applied to the fundamental biomechanical principles. The components covered throughout this blog include, the approach, leg swing, contact point and follow-through phases of a soccer shot. Upon completion of this blog, readers should understand how biomechanical principles apply to these phases to maximise efficiency, power and direction whilst shooting in soccer.

Reference List

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Jenkins, T. (2015, Oct 24). Dan Carter about to slot his conversion [Photograph]. Retrieved from https://www.theguardian.com/sport/live/2015/oct/24/south-africa-new-zealand-rugby-world-cup-semi-final-live

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Ludbey, W. (2012, May 14). Hawk star Lance Franklin kicks for goal against the Demons at the MCG. Retrieved from http://www.heraldsun.com.au/sport/afl/hawthorn-utility-shaun-burgoyne-is-confident-lance-franklin-can-fix-his-goalkicking-inaccuracy/story-e6frf9jf-1226355048300

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