By Jessica Nagel

 What are the Biomechanics associated with Optimising Accuracy and Power in a Soccer Free Kick?

By the end of this blog you should be able to

Learn how understanding biomechanics can positively influence the soccer free kick technique.

Describe what biomechanical principles are involved in the breakdown of the soccer free kick.

Identify what biomechanical principles of the soccer kick are present in other sports.

Importance of understanding Biomechanics

Kinesiology is the study of human movement, where biomechanics is one of the many sub areas of kinesiology (Knudson, (2007), p.3). Biomechanics is the description of the human movement of living things and the study of the causes of human movement using the science of mechanics (Knudson, (2007), p.3). Trough understanding the biomechanical principles involved within a specific sporting skill, coaches and individuals can manipulate skill techniques in order to enhance physical performances. If the soccer free kick is broken down into phases of the kick, the biomechanical principles involved can be identified in order to understand how they influence the outcome of where the ball will go. There have been many famous soccer free kick takers over the years who have all developed their techniques to produce their desired outcome and ‘signature’ kicks. Some of these players include David Beckham, Cristiano Ronaldo, Gareth Bale and John Arne Riise (images 1,2,3 & 4). These images are some examples of the same biomechanical techniques that have been evolved slightly to emphasise more power or accuracy. 

Image 1: David Beckham (Chow, N., (2011))

Image 2: Cristiano Ronaldo (Marcus, J., (2008))

Image 3: Gareth Bale (Galvin, K., (2013))

Image 4 – John Arne Riise (Griffiths, L., (2011))

The Answer

What are the biomechanics associated with optimising accuracy and power in a soccer free kick?

Skill Description

The soccer free kick requires players to hit a stationary ball with the correct power and accuracy to either pass to a team mate or to score a goal. As the free kick only lasts up to ten seconds it relies on the ATP-PC system. Each player does however use their own individual run up, style and technique.

Approach 

From a biomechanical perspective, is a curved run up or a straight run up more conductive to maximising force summation?

Soccer players change their approach to the ball depending on the type of kick they wish to perform. 

If interested these links take you to two penalty free kick takers. Have a look at their run up approach to the ball. 

• Video link to Alan Shearer penalty kick
• Video link to Steven Gerrard penalty kicks

              

Images of the approach phase

A player’s body needs to accelerate for around two to four steps, generating optimal speed towards the ball. Players can accelerate in a forwards motion because of the backwards force they produce against the ground (Blazevich, A.J., (2012)). When an athlete runs they place one foot in front of the other, heel to toe, where two impulses are applied. The heel creates a forward impulse on the ground so the ground reaction force is backwards to the heel, this is a breaking impulse. The foot then moves onto the toe part of the foot which provides a backwards force onto the ground, accelerating the athlete forwards, called a propulsive impulse. The more force and faster acceleration that a player can produce, the more speed and force they can put onto the ball (Lees, A. et al. (2010)). 

As the soccer player is approaching the ball with a curved run up, their body would be leaning towards its centre of rotation, maintaining this lateral inclination as the kick is performed (Lees, A., Asai, T., Andersen, T. B., Nunome, H., & Sterzing, T. (2010)). This positioning allows the kicking foot to have a greater range of motion to swing the leg and get under the ball (Lees, A. et al. (2010)). Because human legs are the same length, a straight run up would encourage their striking foot to hit the ground in the swing phase and not contact the ball properly. The only way to have a straight run up and hit the ball properly, is to get up and over the ball (on the supporting foot's toes and bend knee over the ball). The last step of their approach should be greater than the previous ones, as it produces greater pelvic protraction and speed, this will then produce a greater speed on the ball (Lees, A. et al. (2010)). A curved approach provides a stable position for executing the kick with accuracy and consistency.

Plant the Foot 

Where should the foot be placed in order to maintain balance and segmental interaction?

soccer player planting their
 foot before ball contact

The placement of the non-kicking foot is essential for the preparation and outcome of the kick. Once the soccer player gets to the ball, they need to plant their non-kicking foot next to it (roughly 10cm away). When the player plants their foot they need to apply the largest force possible for the longest time possible (Lees, A. et al. (2010)). There is a ground reaction (backwards) force that that provides an equal and opposite reaction to the foot striking the ground, referring to Newton’s third law (Blazevich, A.J., (2012)). This force prevents the foot from sinking into the ground. A breaking impulse is applied when the planted foot hits the ground, heel first, which is usually greater the further the foot lands in front of the body (Blazevich, A.J., (2012)). This impulse slows the body so the swing phase can be carried out. The breaking impulse provides a pivot point that the rest of the body can rotate on, especially with a curved run up as the leg swings around the body. The foot needs to face the direction of where the player wants the ball to go which will determine the trajectory and flight path of the ball. The support leg is slightly flexed at contact with the ball. It remains flexed for longer than necessary because it is absorbing the impact of the landing as well as acting to slow down the forward motion (Lees, A. et al. (2010)).

Players need to be able to balance on one foot while they plant their foot and initiate the swing phase. Balance is achieved when the body’s centre of mass lies within its base of support, however the soccer free kick requires centre of mass to move outside the base of support. This does not make the player unbalanced because the player’s arm (on their non-kicking leg side) is abducted, extending out and up as the ball is struck (Lees, A. et al. (2010)). This arm movement counteracts the lean. The shoulders rotate, twisting the trunk during the preparation phase and untwists during the execution phase (Lees, A. et al. (2010)).  

Swing Phase

How can speed be built up in the leg swing in order to be imparted on the ball?

                             soccer player in the swing phase of the kick 

After the foot is planted the ‘kicking leg’ needs to swing through to make contact with the ball. The body is still inclined towards the non-kicking side and is tilted slightly backwards (Lees, A. et al. (2010)). This allows the pelvis on the kicking leg side to rise, providing a greater kicking leg extension and speed of foot (Lees, A. et al. (2010)). After the pelvis has risen, the leg extends behind the body and the knee flexes. Here, elastic energy is stored as the kicking leg stretches allowing for greater transfer of force to the ball during the downward phase of the kick (Lees, A. et al. (2010)). The thigh comes through first with a forward and downwards movement where the lower leg continues the lever. The thigh movement is then slowed eccentrically by the hip flexors and knee extensors which causes the lower leg to accelerate, this transfer of momentum releases stored elastic energy (Lees, A. et al. (2010)). It is this phase of the kick that produces maximal eccentric activity in the knee extensors (Lees, A. et al. (2010)). At the point of impact, where the foot makes contact with the ball, 15% of the kinetic energy of the swinging limb is transferred to the ball (Lees, A. et al. (2010)). The rest of the energy is spread to the hamstring to slow down the limb. The risk is that because of the large forces involved in the kicking action, the hamstring is likely to produce injury to the hamstring (Lees, A. et al. (2010)). The foot is plantar flexed (pointing down) during the action. The foot speed is governed by a combination of hip rotational torque, hip flexor strength and quadriceps strength (Lees, A. et al. (2010)). As the player is swinging their leg through, their eyes are focused on the ball to make sure their foot hits the centre of the ball.

Push like movement patterns are movements where joints are linked segments that extend or flex simultaneously (Blazevich, A.J., (2012)). They can be used to improve force production and accuracy but it can’t move at high speeds. These movements are usually straight lined. Throw like movement patterns however, are movements where joints are linked segments that can extend or flex in sequential order (Blazevich, A.J., (2012)). The proximal joints increase their velocity first and the more distal segments increase their velocity later. When soccer players kick, they bring their leg backwards rapidly and then using the muscles around their hip they accelerate the thigh segment forwards before the lower leg and foot swing through, resulting in high foot and ball velocity. By accelerating the proximal segments of their leg (thigh) and them stopping them, a transfer of momentum runs along the leg that results in a high velocity of the end point (the foot) (Lees, A. et al. (2010)). Elastic potential is stored in the patellar tendon at the knee so when the tendon is stretched and then released, it extends back at a very high speed of the knee and foot.

Follow Through

What follow through of the leg position would optimise the kicking technique when it comes to power, spin and accuracy?

                                               Soccer player in the follow through phase of the kicking technique

When taking a free kick, soccer players need to strike the centre of the back of the ball. Soccer players can choose to strike the ball to influence power or spin/curl, however this is not determined by where they make contact with the ball but by where the follow through of their leg ends up. The follow through keeps the foot in contact with the ball for longer which maximises the transfer of momentum to the ball and increases its speed (Blazevich, A.J., (2012)). 

Power strike

When kicking the ball with power, soccer players need to strike the middle of the back of the ball. Their follow through leg should end up straight in front of them (Refer to image on right >>).


When kicking the ball with spin/curl the follow through of the leg will come through slightly across the body or slightly away from the body (refer to images just below). 

Outside curl strike

Inside curl strike

Accuracy is obtained by kicking through the ball so the follow through of the leg ends up where you are aiming for the ball to go. The follow through helps with injury prevention by gradually reducing the kinetic and elastic forces of the leg generated in the swinging phase after the ball has been struck (Lees, A. et al. (2010)). If this did not happen and the leg was suddenly stopped then the leg would be at risk of hamstring strain or injury (Lees, A. et al. (2010)).

How else can we use this Information

The biomechanics used in the soccer free kick can be transferred into other kicking sports like Australian Rules football, rugby league and union, gaelic football and grid-iron football. Although each football code uses the same principles to the kick, they have developed a variety of kicking styles and game rules as well as the role that kicking plays within their game.Impulses are produced in running through ground reaction forces. When an athlete is running they place their foot on the ground heel to toe. When the heel hits the ground it provides a breaking impulse, the further the foot is in front of the body, the more of a breaking impulse it will produce. Therefore in running the heel is placed slightly in front of the body as they want to continue moving forward at pace, not wanting to slow down. When the foot then moves onto the toe, it produces a propulsive impulse where a force is applied acting to accelerate the athlete. Balance is used in other sports, especially in gymnastics where gymnasts have to understand how their body reacts to bases of support and centre of mass and gravity. If spin can be acted on the soccer ball then it can also be produced in other ball sports like tennis, cricket and bowling. Tennis has the same objection as the soccer ball as the racquet hits the ball in the same spot each time but it is the follow through of the racquet that determines its spin. The cricket ball and bowling ball gets its spin from the wrist and hand action of the athlete. Force summation is used in other sports as each action requires a build up of larger muscle movements to smaller muscle movements where energy can be transferred. 

References

Blazevich, A.J., (2012), Sports Biomechanics The Basics Optimising Human Performance (2^nd ed), London: A&C Black Publishers Ltd

Chow, N., (Photographer). (2011). Top 10 famous athlete signature moves The moves that make them icons [photo]. MSN Sports. Retrieved April 22, 2013. from http://sport.malaysia.msn.com/photogallery.aspx?cp-documentid=5161696&page=9

Galvin, K., (Photographer). (2013). The secret behind Bale’s free-kick prowess that can be traced back to baseball a century ago [Photo]. Mail Online. Retrieved April 22, 2013. from http://www.dailymail.co.uk/sport/football/article-2279588/Gareth-Bales-secret-free-kick-method-revealed.html

Griffiths, L., (Photographer). (2011). Liverpool FC: Top 10 Left-Backs of All Time [Photo]. Bleacher Report: Liverpool. Retrieved April 22, 2013. from http://bleacherreport.com/articles/807721-liverpool-fcs-top-10-left-backs-ever/page/10

Knudson, D., (2007), Fundamentals of Biomechanics (2^nd ed), New York, NY: Springer Scrience + Business Media

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. Retrieved from http://search.proquest.com/docview/578483747?accountid=10910

Marcus, J., (Photographer). (2008). ‘I Wouldn’t Sell Them a Virus’ [Photo]. New York Times Soccer Blog. Retrieved April 22, 2013. from http://goal.blogs.nytimes.com/2008/12/18/i-wouldnt-sell-them-a-virus/

Sports Injury Bulletin, n.d., Biomechanics of Soccer: The soccer-style kick – a slow-motion commentary on one of the most common sporting actions in the world, http://www.sportsinjurybulletin.com/archive/biomechanics-soccer.htm