Major Question:
What are the optimal biomechanics for a Soccer free kick?

The Answer:
Soccer is known as the world game and millions of children and adults play this prestigious game all around the world (Kellis & Katsis, 2007). The soccer-style kick lasts for no longer than five seconds, depending on the length of the approach. The intensity of the kick depends on how far the kicker needs the ball to travel or how fast it has to go (Powers, S, and Howley, E 1997). A successful kick is usually defined in the literature either in terms of the velocity of the ball (which needs greater swing limb/ foot speed), or the accuracy of direction of kick, which relies on the position of the ‘plant’ (non-kicking) foot and hip position at impact (Phillips, S 1985).
In these two videos https://www.youtube.com/watch?v=dbHvekce0sI & https://www.youtube.com/watch?v=PzaseZTNveE. Both participants demonstrate how to perform a knuckleball free kick with many steps and considerations. By viewing both these videos and seeing the responses from people commenting and the like and dislike, i believe these two videos are really good for people who wish to learn how to perform the knuckleball free kick in a simple manner rather than understanding it from a biomechanics point of view. Both videos have received over millions of views and seem to be a good source for learners.   

(What are the biomechanical principles that influence a powerful kick in soccer and how can physical educators adopt this knowledge?, 2014), This diagram demonstrates a skeleton performing the free kick action in soccer. The views are side on and front on. This diagram is very useful in the way that it shows the actions of the body during each step of the free kick action. 







Approaching the ball
Soccer players either elite or beginners should use a run up that generates speed when approaching the ball. This is because a run up that generates speed allows the players to have more momentum when they go to kick the ball, thus resulting in their kicks being more powerful (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). A common error made when athletes try and perform the free kick is that they are approaching the ball directly behind and not on an angle, this is shown by studies to cause the leg to produce less swing-limb velocity which results in the ball having very little power and speed in the kick (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). By approaching the all on an angle as the diagonal approach will help the athlete produce a greater kicking velocity thus giving the ball more power and speed (Isokawa, & Lees, 1988; Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998).

Placing supporting leg next to ball
As with just a general pass in soccer, the supporting legs direction and positioning is key to accuracy of the pass (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). When the final stages of the approach is over the athlete will be looking to place their non kicking leg about 5 to 10cm away from the ball (Barfield, 1998; Hay; 1996). This will then lead into a kinetic chain for the body to generate enough force to strike the ball.   

Posture before kick
It is important that the athletes posture of their pelves is tilted when kicking the ball as this will help generate a more powerful kick (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). The trunk of the body should be leaning over the ball before the final stages of contact as this will help the athlete generate forward momentum when kicking although the trunk of the body should be leaning a bit back when the athlete is in the whined up of the kick, this allows the kicking leg to have a high extension which helps them generate more speed and power when striking the ball (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). 

The Swing towards contact
At this stage the athlete would have approached the ball and planted their supporting leg 5-10cm away from the ball, the kicking leg will begin to swing back. When the leg is swung back it is vital that the leg is extended and flexing the knee must be happening (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). The athlete must have their head over the ball whilst maintaining eye contact with it, this helps the athlete balance during the kick (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). Elastic energy will begin to store once the leg is swinging and then when extending it, it allows the athlete to transfer for force on to the ball when going to kick it (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). 

Arm positioning 
When performing the kick it is important that the athletes opposite arm to their kicking leg is elevated, this helps the athlete balance when kicking the ball (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998).

Final stages of swing before contact
At this stage a kinetic chain occurring from the athletes hips and then travels through the athletes thigh and down towards their lower leg (Blazevich, 2010). Whilst this kinetic chain is happening, in the athletes thigh the elastic energy and shared momentum release from the knee extensors, at this point the knee extensors contract strongly (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). 

Foot contact 
With reference to Newton's first law, an object will stay at rest unless acted on by an opposing force. In relation to the free kick, the ball will not move unless the athlete kicks the ball  (Blazevich, 2010). Contact time between the foot and the ball is 10 milliseconds according to (Lees, Asai, Anderson, Nunome, & Sterzing, 2010; Lees, & Nolan, 1998). When the athlete makes contact with the ball, their hip and knee starts to flex which leads into the follow through. Approximately 15% of kinetic energy that has generated from the leg is then put onto the ball (Gainor, Pitrowski, & Puhl, 1978).   

Follow Through
With regards to the two youtube videos shown in this blog, it is important to note that the knuckle ball free kick does not have a follow through and just a regular free kick has a follow through. So depending on the athletes choice of free kick there may or may not be a follow through with the kicking leg. (Barfield, 1998) States that when the athlete kicks the ball it is important to transfer their weight forward and ensure that their leg follows through the kick. It is also stated that a correct follow through will help the foot keep in contact with the ball for longer during the kicking period which can reduce the risk of injury (Barfield, 1998).


How else can we use this information?

The information above discussing the biomechanics of a soccer free kick can also relate to a drop punt kick in the Australian Football League (AFL). The drop punt and the soccer free kick both share some similar movements and kinematic patterns. When performing the drop punt the athlete will run a few steps to gain speed and momentum so when they kick the ball it generates more power (Millar, 2004). Like in the soccer free kick, the athlete trying to perform a drop punt requires a supporting leg that is stable and flexed during the execution of the skill. This support leg also helps the athletes balance during the execution of the skill (Millar, 2004). When athletes who play AFL are running with the ball, their body is relatively upright and then when they go to kick the ball their trunk leans back on their non kicking side, this helps assist them with balance (Millar, 2004). At the stage where the soccer player has planted their supporting leg and their kicking leg swings back this movement is very similar to the stage in the drop punt where the athlete is about to kick the ball. The athlete would drop the ball with their opposite arm to their kicking leg, the opposite arm then begins to rise, this is similar to a soccer player when they go in to kick the ball (Millar, 2004). When the swinging leg comes in to kick the ball, their leg begins to rotate forward with their knee still flexed. Once the athlete makes contact with the ball their arm is raised to a very similar position to the soccer players arm when taking a free kick (Millar, 2004). As referenced above the foot and ball contact is about 10-20 milliseconds which is similar to the athlete kicking the soccer ball, it is important that the longer the foot contact on the ball, helps it transfer momentum into it (Barfield, 1998). This helps with the power and speed of the ball (Barfield, 1998). After the ball has been kicked, the follow through occurs. This is similar to soccer players, when the ball has been kicked it is important for the athlete to transfer their weight forward to avoid injury (Millar, 2004). Thus the soccer free kick and the AFL drop punt both share movement and kinematic patterns. 

(Kotton H, 2008) This image depicts a professional AFL player execute the AFL drop punt. As you can see the arm positioning is very similar to that of a soccer player performing a soccer free kick. Also the supporting leg and the kicking leg are both very similar to that of a soccer player kicking the ball. Lastly the follow through is present in this image as described in the information above.  


By John Papametis





References:

1. Kellis, E., & Katsis, A. (2007). Biomechanical characteristics and determinants of instep soccer kick. Journal of Sports Science and Medicine, 6, 154-165. 
2. Powers, S, and Howley, E (1997), Exercise Physiology. Theory and Applications in Fitness and Performance. WCB. McGraw-Hill: Boston.
3. Phillips, S (1985), Invariance between segments during a kicking motion. In Matsui, H, and Kobayashi, K (eds), Biomechanics. Human Kinetics: Illinois. pp 688-694.
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12. Hay, J (1996), Biomechanics of Sport Techniques. Prentice Hall: New Jersey
13. Millar, S. (2004). KINEMATICS OF DROP PUNT KICKING IN AUSTRALIAN RULES FOOTBALL – COMPARISON OF SKILLED AND LESS SKILLED KICKING. [Master’s thesis]. Melbourne: Victoria University.
14. Kotton, H. (2008). Accuracy is the goal. Accessed 13 June 2015, from http://mm.afl.com.au/portals/0/afl_docs/development/coaching/how_to_play/how_to_play_pt10_goal_accuracy.pdf