What are the optimal biomechanics for the instep kick in soccer?

The instep kick to striking a soccer ball is the foundation of shooting with power and precision. It is considered the most important kicking skill due to the loading and unloading of physical movements within a kinetic chain that transfer high amounts of power into the kick, which in turn forces the player to drive the ball forward. The instep kick is highly regarded by coaches to master as it forms the basis for learning other kicking techniques such as the chip, clearances, volleys, long passes, goal and corner kicks. A player’s skill outcome is dependent on many biomechanical factors which can have an impact on the overall objective in beating the goalkeeper to score a goal. The objective of this blog is to discuss and present the optimal biomechanics needed to perform the instep kick.

The Phases of Movement:

1 - Approach to the Ball:

Before the ball is even kicked, it is common to see players from the professional to amateur levels of soccer take an angled approach to kicking. Two important characteristics to consider in this movement phase is the angle of approach and running stride as they help determine overall foot to ball velocity. It has been proposed that an approach angle of 30-45° attains maximal ball to foot velocity as found from data that measured variables of 0°, 15°, 30°, 45°, 60°, 75° and 90° (Lees et al., 2010). These figures were achieved by results showing the maximal velocity of the shank (shin) was accomplished at 30° whilst maximal ball speed resulted from an approach angle of 45° (Lees et al., 2010). An approach angle within this range is optimal as it enables the supporting leg to tilt more, allowing the kicking foot to be placed further underneath the ball. This results in the player making better foot-to-ball contact. Furthermore, maximal ball speed coincided with the amount of steps taken in an approach. This was indicated through results showing maximal ball speeds of 30.8ms^-1 for running strides of 5-8 steps as opposed to 23.5ms^-1 for a stationary approach (Lees et al, 2010).

(Lees et al., 2010)

2 - Placement of the Supporting Leg and Foot:

Correct placement of the supporting leg is vital to sustaining the distribution of bodyweight and force production throughout the kicking motion. As the support leg makes contact with the ground, it produces a forward force which is exerted into the ground. This is subsequently met by a ground reaction force moving back up the leg which makes reference to Newton’s third law in every action producing an equal and opposite reaction (Blazevich, 2010). The support leg is flexed at around 26° at ground contact and extends to about 42° at striking of the ball (Lees et al, 2010). The supporting leg is continually flexed at the knee joint to absorb ground reaction force and in turn causes the forward motion of the body to decelerate. This acts as an offset for the body to stabilize and enables the lower limbs to produce and transfer power into kicking the ball.

(Blazevich, 2010; Kellis, Katis & Gissis, 2004; Lees et al., 2010)

3 - Rotation of the Hip:

Hip (pelvic) rotation of the instep kick provides precedence for the generation of maximal acceleration and the transferral of power through the extending knee into the shank and foot. The hip joint acts as a pivot point for the lower limbs and is comparable to the knee in regards to torque production.

Figure One

(Nunome et al., 2002)

Figure Two

(Reilly & Williams, 2003)

Table One: Hip Rotation

(Nunome et al., 2002)

Table Two: Knee Rotation

(Nunome et al., 2002)

The hip joint is able to produce more torque due to a sequence of proximal-to-distal motions that end at the foot. As torque is equal to force multiplied by distance (τ = Fd), the same amount of force can be applied at a greater distance with the addition of more torque and further external rotation of the hip. This enables the knee to be brought back further and allows for more acceleration time in tranferring power through the knee to the shank and foot.

(Blazevich, 2010; Nunome et al., 2002)

4 - Forward Movement of Thigh and Continued Knee Flexion:

The forward motion of the instep kick is initiated once the supporting foot has made contact with the ground and the as the hip starts to rotate around the supporting leg. This movement phase primarily produces force as the player aims to strike the ball from flexion of the hip, extension of the knee and planter flexion of the ankle. Force is produced as a result of the movement of muscles and tendons associated within the lower limbs. Concentric contractions occur at the hip flexors as it abducts whilst externally rotating to achieve maximal angular velocity. This rapid shortening of the muscles induces the tendons of the knee to stretch and rapidly recoil, thus prompting the transferral of force through extension as the ankle is kept planter flexed prior to kicking the ball.

(Blazevich, 2010; Hale, 2014; Hearn, 2013)

5 - Foot-to-Ball Contact:

Overall quality of the kick is determined by how well the foot impacts the ball at time of contact. A higher speed produced by the shaft and foot prior to impact will result in achieving a higher ball velocity with contact time lasting under 10 milliseconds. The impact of the ball for optimal technique should ideally be hit towards the distal end of the foot where involuntary planter flexion is caused. The total proportion of energy that remains after contact is quantified by the coefficient of restitution, with results ranging from 0.463 to 0.681 (Kellis & Katis, 2007). Overall restitution can be impacted by where the ball has been hit, rigidity of player limbs, footwear and quality of ball.

Hot-spot demonstrating optimal area for foot-to-ball contact on most common boot designs.

(Dorge et al, 2002; Kellis & Katis, 2007; Freekickerz, 2013) 

6 - The Follow Through:

The follow-through of the instep kick is essential to maintaining longer contact with the ball and guards against possible injury. Attaining a longer foot-to-ball contact time will ensure for the maximal transfer of energy into the ball, resulting in increased levels of velocity as the kicking leg remains extended whilst following through and latter beginning to flex. Peak angular velocity of the shank can occur at either foot-to-ball contact or just after. As displayed in table three, the shank will angularly decelerate and reach negative angular acceleration as the ball is released (stage four). This justifies the coaching cue in kicking through the ball due to its effective transferral of energy into the kick and bodily protection against injury as elastic and kinetic forces are gradually dissipated. 

Table Three

(Reilly & Williams, 2003)

(Nunome et al., 2006; Reilly & Williams, 2003)

Example demonstrating the full instep kick motion from multiple camera angles.
(George Cummins, 2012)

7 - Accuracy:

Current research attributes the accuracy of the instep kick back to the angle of approach. Basumatary (1998) found that an approach angle of 45° achieved most accuracy whilst other variables of 30° and 60° also exhibited high-quality kicking accuracy. This is supported by research conducted by Scurr and Hall (2009) who recorded an angled approach of 45-60° producing greater pelvic rotation. Increased rotation of the pelvis opens the hips prior to ball-to-foot contact thus providing the player with a greater range of motion. A greater range of motion therefore enables the player to remain in contact for a longer of period of time as they strike the ball, allowing for greater accuracy.

(Basumatary, 1998; Scurr & Hall, 2009)

8 - Centre of Mass:

Centre of mass is essential to maintaining stability throughout the kicking sequence. The horizontal extension of the non-kicking arm and the keeping of the head and kicking knee in line over the ball are examples of how players manipulate their bodies to maintain stability during skill execution. Stability is achieved as higher degrees of freedom and joint motions are attained by the player. A player’s centre of mass benefits from this as displacements within the body are quickly manipulated and corrected during the movement. This can be compared to kicking with the non-preferred foot as players display more rigid movement and less independent control over body segments. A player has a higher chance of maintaining stability through lowering their centre of mass during the support phase of kicking. This is demonstrated through the body decelerating whilst the supporting leg maintains flexion at the knee joint.

(Blazevich, 2010; Zago et al., 2014)

9 - Position of the arm and trunk:

The upper body demonstrates supportive characteristics to the overall quality of the kicking technique through maintaining balance and helping release tension from the lower body. The arm of the non-kicking side abducts and extends horizontally before placement of the supporting leg. The arm then adducts and flexes horizontally going into ball contact, prompting the shoulders to rotate out of phase with the pelvis. This causes the trunk to twist during preparation for and untwist through the kicking motion. Tension is caused from the kicking leg up to the non-kicking arm as it abducts and adducts where it is subsequently released during the forward motion of the kick and shortening of muscles. A player who has more control over this release of bodily tension facilitates superior conditions to make their kick more powerful through the generation of explosive muscle contractions.

An example that highlights the tension created across a player's body, stemming up from the kicking leg to the non-kicking arm.
(Getty Images, 2015)

(Lees et al., 2010; Shan & Westerhoff, 2005)

The Answer:

It is clear that the performances of independent movement phases provide support and influence a player’s kinetic chain throughout the instep kick. For optimal performance to be achieved, a player should aim to develop a more fluid kinetic chain to allow for the maximization of power and accuracy. Fluidity stems from the maintenance of stability, higher degrees of freedom and joint mobility. 

A player should therefore pay attention to  movement in how their own body manipulates, corrects and sustains its centre of mass. Greater control over the supporting movement phases (non-kicking arm, trunk, supporting leg) assist the body to stabilize against ground reaction forces which facilitate forward movement and provide better conditions for force production in higher quality muscle contractions within the lower limbs. 

The deceleration that prompts forward motion as a result of centre of mass and support leg stabilization set the foundation for more mobile hip rotation. Further external rotation of the hip coincides with taking an angled kicking approach that assists in opening up the hips. Being allowed to bring the knee back further provides the player with a greater range of motion that increases acceleration and muscle contraction time during force production. This in turn also brings the tendons of the knee back further prior to rapidly recoiling. 

The stabilization of movement and higher amount of acceleration time and force production allows the player to transfer the generated power into the shank and foot more effectively as the knee extends. This ensures that there is maximum velocity prior to foot-to-ball contact. Maximum velocity will be maintained by hitting the ball towards the distal end of the foot and following through with the kick to maximize contact time for the transferral of force into the ball. As a result, this will help generate a higher proportion of energy that remains within the ball after contact. 

How else can we use this information?

The presented biomechanical research and information can easily be transferred to the development of complex skills in other sports. Coaches of different sports can utilize this knowledge to develop a player’s centre of mass for more efficient maneuverability or skill execution. Examples of this are programs that aim to facilitate a player’s ability to evade opposition players during a rugby match or elicit upper body stability whilst performing a basketball shot. Coaches can also facilitate player skill ability through targeting the fluidity of movement within their kinetic chains. This is particularly applicable to the development of younger athletes where a coach may use modified equipment that suits the needs of learner and allows them to fully explore skill performance and movement connectivity. Adult athletes can also benefit from this by training to increase their range of motion and movement variability which can assist in performing stronger and more efficient performance.

(Blazevich, 2010)

References:

Basumatary, S. (1998). Biomechanical analysis of the instep kick in soccer (Doctoral dissertation). Retrieved from http://vuir.vu.edu.au/15563/1/Basumatary_1998compressed.pdf 

Blazevich, A. (2010). Sports Biomechanics, The Basics: Optimising Human Performance. London: A&C Black, pp.44-45, 63-67, 117-118, 199-205.

Chris Hale. (2014, Nov 13). Biomechanics of the Instep Kick [Video file]. Retrieved from https://www.youtube.com/watch?v=PQjoSNaFf5c

Dörge, H. C., Andersen, T. B., SØrensen, H., & Simonsen, E. B. (2002). Biomechanical differences in soccer kicking with the preferred and the non-preferred leg. Journal of Sports Sciences, 20(4), 293-299.

Freekickerz. (2013, May 14). How to Shoot a Soccer Ball with Power [Video file]. Retrieved from https://www.youtube.com/watch?v=X_bYbJi4agw

George Cummins. (2012, March 12). Biomechanics - Analysis of a Football Free Kick [Video file]. Retrieved from https://www.youtube.com/watch?v=aJjv8tWbj4c 

Getty Images. (2015). Ryan Taylor of Newcastle. [image]. Retrieved from http://www.gettyimages.com.au/detail/news-photo/ryan-taylor-of-newcastle-takes-a-free-kick-during-the-news-photo/464789996

Hearn, A. (2013). The Biomechanics of Kicking in Soccer. Retrieved from http://www.scienceinsoccer.com/2013/06/the-biomechanics-of-kicking-in-soccer.html 

Kellis, E., Katis, A., & Gissis, I. (2004). Knee biomechanics of the support leg in soccer kicks from three angles of approach. Medicine and Science in Sports and Exercise, 36(6), 1017-28.

Kellis, E., & Katis, A. (2007). Biomechanical Characteristics and Determinants of Instep Soccer Kick. Journal of Sports Science & Medicine, 6(2), 154–165.

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.

Nunome, H., Asai, T., Ikegami, Y., & Sakurai, S. (2002). Three-dimensional kinetic analysis of side-foot and instep soccer kicks. Medicine and Science in Sports and Exercise, 34(12), 2028-36.

Nunome, H., Lake, M., Georgakis, A., & Stergioulas, L. K. (2006). Impact phase kinematics of instep kicking in soccer. Journal of Sports Sciences, 24(1), 11-22.

Reilly, T., & Williams, M. (2003). Science and Soccer. New York: Routledge, pp.109-112.

Scurr, J., & Hall, B. (2009). The Effects of Approach Angle on Penalty Kicking Accuracy and Kick Kinematics with Recreational Soccer Players. Journal of Sports Science & Medicine, 8(2), 230–234.

Shan, G., & Westerhoff, P. (2005). Full‐body kinematic characteristics of the maximal instep Soccer kick by male soccer players and parameters related to kick quality. Sports Biomechanics, 4(1), 59-72.

Zago, M., Motta, A. F., Mapelli, A., Annoni, I., Galvani, C., & Sforza, C. (2014). Effect of Leg Dominance on The Center-of-Mass Kinematics During an Inside-of-the-Foot Kick in Amateur Soccer Players. Journal of Human Kinetics,42, 51–61.