What is the Optimal Biomechanical Form Required to Maximize the Distance Reached for a Goal-Kick in Soccer

Major question 

The basis of this biomechanical report will be using soccer as our chosen sport for the biomechanical analysis. Therefore the aim and objective that was created for this blog focuses on the major question that will be analysed and reviewed through this blog report is, What is the Optimal Biomechanical Form Required to Maximise the Distance Reached for a Goal-Kick in Soccer? In soccer, a goal kick is achieved through the ball fully going over the goal after the opposition player touches the ball last. The play then proceeds with the goal keeper placing the ball on the front line of the “6-yard box”, who is then allowed to play the ball to anyone within the soccer pitch. Usually, most goal keepers will look to play the ball short to either their centre backs or defensive midfielders. If the goalkeeper finds that he is not comfortable and not confident in his team’s ability to keep the ball, he can send his team up the field to execute a long goal kick to reach into their attacking third. The true biomechanics of a long soccer goal kick require great strength and precision, as a professional soccer pitch can reach up to 110 metres in length, making the halfway point at 55 metres. Having certain essential biomechanical movements is required for achieving an optimal goal kick, as goalkeepers will aim to hit a certain part of the ball with a specific contact point on their foot. To reach this point there are multiple stages of this motor skill that need to be performed in order to reach the end outcome. Firstly there is the original positioning and run-up to the ball. Research states that most skilled soccer players will take a position that is diagonal and off-centre to the ball, which will allow them to make a curved approach to the position of the ball. The most common angle of position that skilled players establish to the ball is 43 degrees, but previous research done provides information that the most power that was achieved in a goal kick is from a 45 degree angle (Nunome et al., 2002) . The next movement to this skill is the approach run-up to the soccer ball. This stage involves running up to the ball with a specific number to allow the player to precisely place their non-dominant planting next to the side, in line with the ball. This stage is used for the keep to build an optimal amount of momentum to the ball in order to increase the amount of power that they exert into the ball. The second to last movement is the contact that the keeper makes with his foot to the ball. This is a significant part of the movement as the overall trajectory and spin of the ball is dependent on where the foot hits the ball. The final stage of the goal kick being the follow through, allows the keeper to ensure that the velocity of his foot during the kicking action has not decreased, and therefore would also decrease the power and force in his kick. The follow through is also an important part in putting spin on the soccer ball to create a smoother and longer trajectory.

Breaking down the kick - Different forces at play - Biomechanics

Length of stride on approach (pendulum force) - The length and number of strides are essential to create and transfer force from the run-up to the ball during a goal kick, professional footballers will take 2-3 shorter strides after a few longer more forceful strides. This allows force to be transferred from running momentum into one point of the ball to generate work as the ball is sent away from the player. Using the pendulum force from a run-up angle of 45 degrees increases the momentum of the player towards the ball and space. Approaching from a 45 degree angle also allows the player to strike at the lower half of the ball to create backspin.

Placement of foot (stability) - The placement of the non-kicking foot is important as the player needs to strike through the ball to transfer more force and lose less momentum through the strike. The best placement of the non-kicking foot for distance for a goal kick is between 6 and 30 cm horizontally from the ball and between 0 and 10 cm in front of the ball to step through the ball, transferring force greater. Another important factor is to follow non-kicking foot through as to release increased force into the follow through.

Rotation of hip flexor (torque) - The rotation of hip flexors works to create angular momentum through the lower body and allows for a pendulum to be created for force. By rotating the hip flexors, the ball is able to be struck at an angle, allowing the foot to reach under the ball and create backspin. To create torque through the hips and legs, it is essential to step powerfully through the non-kicking leg and transfer the weight and momentum of the pendulum swing through the kicking leg. This is accomplished by flexion of the hip, adduction by bringing the leg through the ball diagonally and using internal rotation. These different hip flexor movements create torque which is thrust into the bottom of the ball.

Knee flexion and extension (pendulum) - Using the flexion and extension of the knee during a goal kick creates force and momentum through the leg to transfer into the ball to create as much force as possible into a singular point. The range of motion of knee flexion through the final step of the kick increases the time and range that maximum force can be input. This input of maximum force and velocity through the extension of the knee decreases the wind up time and lowers the inertia that air resistance places on the pendulum swing of the leg. As the lower leg is acting against less inertia and thus gravity due to the momentum the lower leg is experiencing, the force placed into the ball is greater.

Arm swing counterbalance (torque) (rotational force) - Using a counterbalance during the swing motion will increase core stability thus increasing accuracy and conservation of momentum during the follow through motion. Using the arm swing will increase stability and increase the balance of the kick, it also increases core stability to transfer momentum through the hips. The act of swinging the arm also can increase the force pushed through the hips similarly to how arm swing is used to increase momentum within a vertical jump due to mass being forced vertically. When the arm is not used within a goal kick, the player may lose balance and fall over the ball due to the run-up angle coming from 45 degrees, this will lessen the force placed through the ball thus decreasing the effectiveness and distance of the goal kick.

Core stability and power transfer (power transfer) - The position of the core in goal kicks is important as leaning over or backwards from the ball will affect the angle of release of the kick, such as for a power shot, players will step through and lean over the ball to keep the ball low and avoid kicking the ball over where they are aiming. For a distance aimed goal kick, leaning backwards and increasing the angle of release for the ball will increase air time of the ball. Core stability through the run-up and swing of the goal kick, will determine the accuracy, spin, and force placed into the ball, strong core stability will keep the body upright and allow greater range of motion and thus transference of momentum through the pendulum swing into the ball. On the run-up of the goal kick the position of the body should be leaning forward to approach the ball with velocity, then the body as a whole will act as a pendulum through the non-kicking foot and the head will remain over the ball’s position to stay upright.

Contact point of ball and locking of the ankle - Locking the ankle during a powerful kick is essential in any powerful kick. Locking the muscles of the ankle when striking the ball lowers the area of contact on the ball, thus increasing the pressure placed onto a single point on the ball, this transfers energy generated during the kick effectively into a specific point. During a goal kick, the player will only lock their ankle at the point of contact with the ball as the dynamic motion of the leg is important to generate force, but the transfer of said force is accomplished by striking a singular point. The momentum of the leg will continue with the follow through after the kick is made, however the force will be far less due to newton's first and third laws, inertia is overcome by striking the stationary ball and the equal and opposite reaction from the ball changing its velocity from standstill to moving thus lowering the velocity of the leg, avoiding injury.

Example of the biomechanics in play from professional goalkeeper trainer 

During the approach, the player leans forwards and over the ball to create momentum and create a pendulum motion for the legs. Smaller balanced strides are taken to allow the player to swing their leg while keeping the momentum of the run-up. Keeping their head over the ball and keeping their body up-right will transfer the running momentum into one point.

The player’s arm swing is used for counterbalance, the momentum caused by the stride and the force from the non-kicking leg pushing force into the ground allowing for transfer of force into the kicking leg. The kicking leg’s knee and hip flexion increase the potential energy that can be used to strike the ball. The planter foot is to the side and slightly in front of the ball so that the ball can be struck using the “sweet spot” on the top of the foot. The internal rotation of the striking leg hip flexor allows the striking leg to follow through the ball more accurately and whilst conserving more momentum.

The counterbalance arm keeps the core tight and upright, allowing torque through the hip flexors to transfer energy into the lower leg and ankle, knee and hip extension transfers the pendulum momentum into a single point, and the ankle is locked to generate power into one point to overcome inertia. The upright position of the body allows the player to accurately strike the lower half of the middle of the ball to create a 35 degree angle with backspin to overcome gravitational pull and air resistance more effectively.

Equations to determine Time, Velocity and Distance

There are equations that can be used to estimate and calculate the velocity, distance, height and time of flight of the ball during the goal kick.

The time of flight can be calculated by either timing the time it takes for the ball to land after leaving the ground from the kick or using an equation:

The velocity of the ball can be calculated using technology or using and equation involving the horizontal and vertical velocity components:

The distance of the goal kick can be measured using technology or using equations involving information from velocity and time.

An example of finding the distance of a professional keeper’s kick using the d = v x t equation can be found by using the velocity of a keeper, for this example, one of Ederson’s (Manchester city keeper) kicks was used. The velocity of the kick was approximately 28.7m/s and the flight time of the ball was approximately 3.2 seconds. Using these two information we multiply 27.7m/s by 3.2 seconds to get approximately 88.64m distance on this goal kick.

Biomechanical analysis of amateur goal kick

 In this biomechanical analysis we will first be looking at and breaking down the execution of a goal kick done by an amateur player. This will involve dissecting specific movements and and actions that ultimately correlate to the amount of force that is outputted, as well as the specific movements of the ball. To analyse this goal kick multiple angles and measurements will be utilised to ultimately show the flaws and difference in comparison to a goal keeper of significantly higher level. 

The first stage of this analysis looks at this player's original position from the ball. As previously stated before in the stages mentioned, the most optimal angle for a player to start at is 45 degrees as research shows this generated the “maximum” output of power for these players. Looking at the amateur player's original position to the stationary ball, they use an angle of 69 degrees diagonally to the ball, being 24 degrees larger than the optimal. As the original starting point is significantly wider, it is much harder to make contact with the ball centrally. This can result in creating side or top spin onto the ball. By creating spin the ball is much more susceptible to wind which can ultimately create more lateral movement while it is in trajectory. This will significantly decrease maximum distance reached, as well as making it significantly harder to receive (Mehta, 1985).

In the second stage the goal kick, the run-up to the ball will be analysed. When a goalkeeper is looking to generate more power in their kick to ultimately reach more distance, it is normal for a keeper to take 4 to 5 steps in the approach to kicking the ball. It is also noted that too many steps can create too much speed therefore causing the player to lose control, which therefore is not optimal in taking a goal kick (Mick D'arcy, 2000). In video of the amateur player, it is counted that they approach the ball with 5 steps which can ultimately be concluded as optimal for their run-up.  

The second to last stage of the goal kick is the contact made with the ball. Optimally before making direct contact with the ball, the player's non-kicking foot should be placed directly next to the ball with the middle of their foot in line next to it.  For the contact of the ball the contact should occur within the bottom half of the soccer ball to ensure that the ball gains significant height. The contact made on the foot should occur with the striking foot being angled down with contact being made on the top of the instep with the first metatarsal, being the hardest point on the foot (Lees et al, 2010). The video of the amateur players shows that the non-kicking foot is placed next to the ball while the kicking foot is bent downwards at a 130 degree angle from the calf and shin. While this part of this stage is optimal, the side angle shows the non-kicking foot being placed too far in front of the ball. Significant issues can arise with this flaw as this means there is less time for the kicking leg to reach its maximum velocity,  therefore causing it to make contact at a time where it is still accelerating but not at the optimal velocity (Lees & Nolan, 1998).

In the final stage of a goal kick, a players kicking leg should reach full extension ensuring all the momentum that is created is utilised, while the player’s body should follow through in the direction of the ball to maintain accuracy and correct utilisation of energy that is created (Kellis & Katis, 2007). The video of the amateur player shows the ball moving slightly to the right while they follow through with their body to the left side of their original placing foot. This follow through can cause force and energy to be lost, as it is not being put directly through the ball. 

The distance of 3 goal kicks were recorded by the amateur player, reaching distances of 44.78 metres, 50.71 metres and 54.82 The air time of these goal kicks in the same order were 2.31 seconds, 2.58 seconds and 2.72 seconds. Using these recorded results the velocities for each of these goal kicks were 19.39 m/s, 19.65 m/s and 20.15 m/s. Comparing this to the professional player’s goal kick, there is approximately a 8 m/s difference in the amateur player's longest goal kick to the pro’s kick. This then resulted in a significant difference in distance being 34 metres longer than the amateur’s goal kick.

Practical findings

In the biomechanics of a soccer goal all of the lower body muscles are utilised to achieve power in kicking the ball. This involves major muscles such as the quadriceps, hamstrings, gluteus maximus and calf muscles. Research shows that soccer contains multiple explosive actions that require a high level of strength to perform, it is stated that players with higher levels of strength can have better performances, therefore justifying the use of strength training. This level of training also increases endurance, which increases the time that players will fatigue (Turner & Stewart, 2014). There are multiple exercises that can be performed that will increase the strength of the lower body muscles, these exercises include squats and lunges which focus on all the major muscles such as the quadriceps, hamstrings and glutes. Another method of training essential to building explosiveness on the field is plyometric training. Studies show that plyometric training is essential for all players to undergo, results stated that after a 6 week program both muscle power and endurance was increased after utilising this method of training (Wang & Zhang, 2016). As kicking is an explosive action, this type of training is especially relevant to goalkeepers looking to improve their goal kicking ability to maximise both power and distance in their kicks.

Biomechanics used in other actions in sports

The discussed biomechanical concepts for a goal kick can be used in other action such as an AFL punt which has the differently shaped ball kicked from hand held height upwards through the goalposts. The actions can even be used in different actions within English football such as shooting the ball at a goal, the kinematics remain the same, besides small differences such as approach angles, follow through techniques, body positioning such as leaning forward and keeping head over the ball. 

The key concepts that may apply to AFL and other kicking sports include locking of the ankle to transfer force, use of a pendulum swing from the hip to foot, creating torque through hip dynamics and knee flexion and extension and the follow through action to improve accuracy and transfer momentum and force into the ball. In AFL these actions are used to do a long pass to get a mark or to score a goal, which are actions used throughout the game to retain possession, attack and score. Rotational force is also important when hand passing an AFL ball as torque is generated using the hand and elbow instead of the leg, and the different techniques to push the ball out of the hand also use upper body dynamics such as rotation of the shoulders and flexion/extension of the biceps.

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