Elisa Vaselli - HLPE 3531

Introduction:

Kicking is a fundamental asymmetrical skill required within soccer and is highly researched due to the popularity of the sport (Barfield, 1998). Different actions are required for different situations; for example, a powerful shot on goal will require a different approach to a short pass to another player. The ball can be kicked with different areas of the foot; the instep, the outside, and the inside. The in-step kick is one of the most powerful kicks used in sort, and is particularly utilised when a faster ball speed is required (Kellis & Katis, 2007). There are many variations of the in-step kick which is often used when passing the ball at medium and long distances, when shooting at goal, and when performing penalty kicks (Kellis & Katis, 2007). 

Understanding the biomechanical techniques during movement are important tools for many sport disciplines. In soccer, they are particularly useful for defining the characteristics of skills, improving mechanical effectiveness in execution, and identifying factors that influence successful performance (Barfield & Kirkendall, 2002). Knowledge and understanding of biomechanics can enhance learning and performance of sport-specific skills (Ismail et al., 2010; Amiri-Khorasani et al., 2010). Specifically, systems for the kinematic analysis of human movement provide precise measurement of values and parameters of athlete’s movements during performance of any sport technique. Understanding of the biomechanics of the kicking in soccer is important for monitoring progress and correcting performance of athletes to cater for any necessary technical performance concerns during training sessions (Meamarbashi and Hossaini, 2010). Therefore this blog will aim to break down the action of the in-step kick in soccer to quantitatively and qualitatively analyse and discuss the optimal biomechanics for the technique. There are 6 movement phases in achieving optimal biomechanics of the in-step in soccer (Barfield, 1998). As shown in figure 1, these include the angle of approach, force production during the foot plant, swing limb loading (backswing), hip flexion and knee extension during downwards motion, foot to ball contact, and deceleration of the kicking leg during the follow-through (Barfield, 1998).

Major Question:

This blog will aim to explore the major question “What are the optimal biomechanics of a soccer in-step kick?” In order to successfully analyse the optimal biomechanics required for the in-step kick within the sport of soccer, the action must be broken down into various movement patterns. After examining reputable sources which specifically analysed the in-step technique, 5 movement patterns were identified. These particular movement patterns will be individually analysed in terms of the specific biomechanics that are involved within each movement. 

The Answer:

There are 6 movement phases in achieving optimal biomechanics of the in-step in soccer. These include the angle of approach, force production during the foot plant, swing limb loading (backswing), hip flexion and knee extension during downwards motion, foot to ball contact, and deceleration of the kicking leg during the follow-through (Barfield, 1998).

Angle of approach:

During the approach phase, power within the kick is generated as the player increases momentum and the transfer on the length of the final step into the planting foot (Lees & Asai et al., 2010). Before the player closes in on the ball to progress through the in-step kick, an important aspect to consider is the angle of approach. Novice kickers would generally follow a straight-line approach however, according to Lees and Isokawa, a diagonal angle should be approached as it enables greater pelvic rotation, allowing it to move through a greater range of motion throughout the kick (Isokawa and Lees, 1998). The angle also enables the player to remain in contact with the ball for a longer period of time, increasing the possibility of a more accurate kick (Garrett & Kirkendall, 2000). Greater swing limb velocity is produced to enhance greater ball speed, ultimately to enable a greater direction of trajectory (Lees & Asai et al., 2010). As the angle increases, the leg must compensate through rotation to allow optimal ball contact (Isokawa & Lees, 1988). Therefore the optimum approach angle to achieve maximal swing velocity of the leg is 30 º, and peak ball velocity in relation to the direction of the ball path is 45º, as shown in figure 2 (Isokawa and Lees, 1988). Elite athletes, such as the one shown in figure 3 also tend to take longer strides as they approach the ball to allow for an optimal kick (Abo-Abdo, 1981). The approach component of the in-step kick initiates as the pelvis starts to rotate around the supporting leg (Barfield, 1998). The angular velocity of the thigh needs to remain positive while the trunks angular velocity under the knee flexion needs to remains negative (Barfield, 1998). While the athletes hip abducts while it remains externally rotated, the hip flexors contract concentrically. This motion causes the angular velocity of the thigh to reach its maximum and then linearly decline after contact is made with the ball (Isokawa & Lees, 1988). The knee begins to extend which causes the deceleration of the hip as a result of the motion-dependent movements of exertion from the shank (between the hip and the ankle), as well as the hip flexion movement (Nunome & Lake et al., 2006). There is an increase in shank angular velocity which continues until ball contact, due to the forward motion of the shank and the extension of the knee (Putman, 1983). Concentric contractions during the forward motion of the hip flexors and the knee extensors provide the largest torques (Putman, 1983). The ankle is also adducted and plantar-flexed during this forward swing in order to prepare the body for optimum ball contact (Barfield, 1998). At contact with the ball, a rigid foot and close contact with the ankle produces the most efficient results (Kellis & Katis, 2007).

Note: To reduce the risk injury of extra tension in the knee and hip, a straighter angle approach is necessary (Gainor & Pitrowski et al., 1978).

Force production during the foot plant:

Prior to the back swing is the force production during the foot plant. The position of the foot plant is vital as it determines the balls path (Hay, 1996). According to Kellis and Katis, ground reaction force acting on the support foot during the instep kick should be measured at 1.93-2.67 and 0.5-1.24 times body weight at impact for the maximum vertical force and maximum horizontal frictional force, respectively (Kellis & Katis, 2007). The greater the step when approaching the ball, the more velocity produced through the kicking leg, ultimately generating more power (Lees & Asai et al., 2010). 

According to Abo-Abdo, the anterior-posterior position of the foot plant is what determines the trajectory or flight path of the ball (Abo-Abdo, 1981). The optimum anterior-posterior position of the foot plant should be perpendicular to the centre of the ball and 5-10 cm to the side, as shown in figure 4 (Abo-Abdo, 1981). This allows for a straight kick however, if the plant foot is too far behind the perpendicular line of the centre of the ball, the athlete will become off balance, ultimately affecting the velocity at initial contact with the ball and the point of contact (Barfield, 1998). This may also cause the ball to airborne unnecessarily. Therefore athlete must insure that they create a steady basis of support during the foot plant. This is done by keeping their centre of mass over their base of support in order to maintain balance throughout the movement, as shown in figure 5 (Blazevich, 2012). The optimal plant of the supporting foot allows for a greater production of ground reaction force and an increase in the velocity of the ball, as shown in figure 6 (Ben-Sira, 1980).

Swing limb loading (Backswing):

The next phase within the biomechanics of the kick is the swinging or cocking of the kicking leg in preparation for the downward motion towards the ball. The swing motion in kicking is a proximal to distal sequence that involves segmental motion of the lower limb (Levanon and Dapena, 1998). During this phase the kicker’s eyes must be focused on the ball, the opposite arm to the kicking leg must be raised and pointed in the kicking direction to counter-balance the rotating body, as shown in figure 7 (Chysowych, 1979). As the plant foot strikes the ground adjacent to the ball, the kicking leg extends, while the knee flexes (Barfield, 1998). This is so elastic energy can be stored as the swinging limb passively stretches to allow a greater transfer of force to the ball during the downward phase of the kick, as shown in figure 8 (Barfield, 1998). This backswing phase is characterised by extension and abduction of the acetabular femoral joint, flexion of the tibial, flexion of the tibiofemoral joint and plantar flexion of the talocrural joint. The hip adducts and rotates externally as the gluteus maximus and gluteus medius work concentrically to extend the hip (Nunome & Lake et al., 2006). The gluteus medius and gluteus minimus also work concentrically to abduct the hip (Nunome & Lake et al., 2006). To flex the right knee, a concentric contraction from the long head of the biceps femoris as well as a concentric contraction from the semitendinosus and semimembranosus, is seen (Barfield, 1998). The gastrocnemius also works concentrically to plantar flex the ankle (Barfield, 1998). The hamstring is responsible for hip extension and knee flexion, and to support this notion a 1990 study undertaken by Hoy suggest that because of hip and knee positioning during the backswing, the hamstrings exert much more torque around the knee rather than the hip (Hoy et al., 1990). A further 1996 study completed by Sorensen preserved eccentric contractions of the hip flexors and knee extensors during the backswing suggesting they play an important compensatory role in controlling the leg as it moves posteriorly along the sagittal plane (Sorren et al., 1996). Before the end of the swing phase when the hip is nearly fully extended and the knee flexed, the leg is slowed eccentrically by the hip flexors and knee extensors (Brown and Neumann, 2004). This is the phase of the kick where there is maximal eccentric activity in the knee extensors (Brown and Neumann, 2004).

Hip flexion and knee extension during downwards motion: 

Following the backswing phase is the force production component where the athlete swings their right leg to meet the ball during the foot plant. There is a linear relationship between foot velocity and resultant ball velocity (Barfield, 1998). It should be noted that the quick eccentric stretch of the hip flexors and the knee extensors during the back swing allows for a more powerful force production phase (Garrett & Kirkendall, 2000). Flexion at the hip flexors initiates this phase as the thigh swings forward and downward with a related forward rotation of the lower leg and ankle, as shown in figure 9 (Chysowych, 1979). According to Putnam, flexion at the hip is achieved primarily by a concentric contraction iliacus, psoas major and psoas minor and the rectus femoris (Putnam, 1983). As the forward thigh movement reduces, the kicking leg and foot begin to accelerate due to the combined effect of transfer of momentum, and release of stored elasticity in the knee extensors (Hay, 1996). In order to extend, the knee and the quadriceps (vastus lateralis) work concentrically (Barfield, 1998). This is influenced by extension of the knee, resulting in the knee extensors powerfully contracting to swing the leg and foot forward towards the ball, as shown in figure 10 (Barfield, 1998). As the knee passes over the ball, it is forcefully extended while the ankle is forcefully plantar flexed (Kellis & Katis, 207). This exposes the inside part of the medial dorsum which is propelled at the ball (Kellis & Katis, 207). Foot speed during this phase is governed by a combination of hip rotational torque, hip flexor strength and quadriceps strength (Garrett & Kirkendall, 2000).

The upper body of the athlete during this phase is also important. The non-kicking side arm abducts and horizontally extends before support foot contact, and then adducts and horizontally flexes to ball contact (Shan & Westerhoff, 2005). Furthermore, the shoulders are rotated such that they move out of phase with the rotation of the pelvis. This leads to a trunk twist also known as the ‘tension arc’ during the preparation phase of the kick and untwist (shortened arc) during the execution phase, as shown in figure 11 (Shan & Westerhoff, 2005).

 

Towards the end of the swing phase, prior to ball/foot contact, the hamstrings are maximally active to slow the leg eccentrically (Wahrenburg & Lindbeck et al., 1978). This is known as the ‘soccer paradox’, where the knee flexors are maximally active during knee extension and the knee extensors are maximally active during knee flexion (Wahrenburg & Lindbeck et al., 1978).

Note: Studies have shown that elite athletes kick the ball further with less muscle activity and more relaxation during the swing phase, but greater eccentric antagonistic muscle activity than novices (De Proft & Cabri, 1988). This idea supports the concept that skilled elite kickers have more efficient use of their motor systems and biomechanical control during the kicking action.

Foot to ball contact:

The foot contact phase is the instant moment the foot comes into contact with the ball. At this point the positions of the athlete’s feet are critical to the success of the kick (Powers & Howley, 1997). During this phase the support leg (left) is flexed to 26° at foot contact and through the duration of the kick, extends to being flexed to 42° (Lees & Asai et al., 2010).
Coefficient of restitution during this phase is dependent on the mechanical properties of the ball, shoe, ankle and foot upon the impact (Bull-Andersen, Dorge, Thomsen, 1999). The greater the restitution, the less energy lost during the collision of foot contact with the ball (Blazevich, 2010). This period of contact is vital in determining ball speed and trajectory. Striking the ball lower creates a higher trajectory, and striking the ball higher creates a lower trajectory (Blazevich, 2010). According the kellis and katis, striking the ball higher towards the dorsal side of the foot, generates the greatest ball speed (Kellis & Katis, 2007). Striking the ball closer to the toes, may leave an athlete vulnerable to a posterior ankle impingement injury (Gainor & Pitrowski et al., 1978). For the instep kick, a higher coefficient of restitution is required, and achieved as the player makes direct contact with the centre of the ball, as shown in figure 12 (Kellis & Katis, 2007). As contact is directed towards the centre of the ball, the ball is compressed and 15% of kinetic energy from the swinging leg is distributed on to the ball for an optimum outcome (Gainor & Pitrowski et al., 1978). The rest is dissipated by the eccentric activity of the hamstring muscle group to slow the limb down (Gainor & Pitrowski et al., 1978). As foot-ball contact is made, the kicking leg is modelled as a three link kinetic chain composed by the segmental forces of the thigh, shank and foot, where angular velocities are measured (Nunome, Asai, Ikegami & Sakurai, 2002). Forces are transferred as the reacting forces slow down the forward motion action. This causes the pelvis to rotate, allowing the kicking leg to being its forward motion (Lees & Asai et al., 2010). The forward motion is initiated by the rotation of the pelvis, and the thigh of the right leg which is brought forwards with the knee continuing to flex (Kellis & Katis, 2007). During impact on the right leg, the hip and knee are slightly flexed and the foot begins to move upwards and forwards (Barfield, 1998). The main joint movement during this phase is plantar flexion of the ankle as shown in figure 13. This is achieved through an eccentric contraction of the tibialis anterior. Where this takes place, contact should be closer to the metatarsals for an optimum outburst, as shown in figure 14 (Lees & Asai et al., 2010). According to Powers and Holly, foot contact lasts anywhere from 6 to 16 milliseconds depending on the inflation of the ball (Powers & Holly, 2004). The elasticity of the ball plays a vital role that contributes to effective foot-ball contact (Garrett & Kirkendall, 2000). Therefore the ball needs to have appropriate inflation to enable the ball to follow through correct restitution enabling maximum striking effectiveness to be increased through the limbs as they become more rigid through muscle activation (Garrett & Kirkendall, 2000). Towards the end of impact on the ball, the flexion of the knee continues to absorb the impact of landing and causes the slowing motion (Brown & Neumann, 2004).

Decelerating of the kicking leg during the follow-through:

The final component of the in-step kick is the deceleration of the kicking leg during the follow-through phase. During this phase, the swinging leg needs to decelerate in order to contact the ground (Brown & Neumann, 2004). The follow-through phase of the kick serves two purposes. The first is to keep the foot in contact with the ball for as long as possible, as the longer the foot can keep contact with the ball, the greater momentum transferred to the ball which also increases its speed (Garrett & Kirkendall, 2000). The second is to protect the athlete’s body from injury, particularly the swinging limb (Barfield, 1998). The muscle and elastic forces that have been generated from other phases of the kick are dissipated during the follow through (Garrett & Kirkendall, 2000). There is also a decrease in the distance between kicking leg and opposite elbow (Lees & Asai et al., 2010). In addition to deceleration of a leg this component functions in order to increase transfer of kinetic energy (Lees & Asai et al., 2010). This movement is characterised by eccentric contractions from the hip extensors and abductors, and the knee flexors (Chysowych, 1979). All three of the gluteal muscles such as the gluteus maximus which inserts on the gluteal tuberosity of the femur, gluteus medius and gluteus minimus which insert on the greater trochanter of the femus, work eccentrically at the hip to abduct and medially rotate at the hip (Isokawa & Lees, 1988). The hamstrings eccentrically flex at the knee and the dorsiflexors change from concentric to eccentric contraction to slow the acceleration of the foot and prevent foot slap (Brown & Neumann, 2004). These eccentric contractions protect the body from injury by dissipating kinetic and elastic forces generate by the kicking leg (Lees & Asai et al., 2010). However, due to the abrupt slowing of a limb, injuries such as hamstring strains are a high risk during this component (Hay, 1996). Towards the end of the follow through phase as the kicking leg extends above the hip, the foot begins to relax and the hip extension brings the leg back down with the athlete ending the phase by landing on the kicking foot, as shown in figure 15 (Lees & Asai et al., 2010).

How else can we use this information?

The biomechanical components analysed within this blog is not limited to the movement patterns in soccer. Sports all around the world make use of the complex skill of kicking. These particular sports include, Australian Football League, Rugby League or Rugby Union, Gridiron Football and Gaelic Football. The movement patterns are also seen in Martial Arts, Taekwondo, and Sepak Takraw. However, within each sport, a variety of kicking styles are appropriately developed to best suite different ball types, games and the part that kicking plays in the game. There are also a wide range of biomechanical principals used during this blog such as base of support, transfer of kinetic energy, direction of trajectory, angle of approach, momentum, velocity etc. (Blazevich, 2012). These components are commonly required and applied in many other sports to provide develop performance. For an example, gymnastics is heavily focused on base of support as it relies essentially on the value of balance (Blazevich, 2012).

Furthermore, additional research is required into various situations of kicking in soccer. This is because most studies identify and assess the movement process of the performer in static situations, while the ball is stationary. However, in a competitive situation the ball is more than likely moving or potentially in mid-air, and therefore the body will react differently to various situations. 

References

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Barfield, W. R., Kirkendall, D. T., & Yu, B. (2002). Kinematic instep kicking differences between elite female and male soccer players. Journal of sports science & medicine, 1(3), 72.

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