Introduction

Football, better known as soccer in Australia, is a popular, professional sport and has been nicknamed ‘the world game', as more than two hundred countries around the world play it, making it the most popular sport in the world.  Although soccer has encountered many obstacles over the years in its attempt to achieve mainstream recognition and support in Australia, it is argued that once a sport has been established in a society it is difficult to dislodge (Kobe 1999). Biomechanics is the primary sport science focusing on movement technique. It provides conceptual and mathematical tools that are necessary for understanding how living things move and how kinesiology professionals might improve movement or make movement safer (Knudson, 2007). Biomechanics is often applied to soccer to define characteristics of skills, to gain an understanding of their mechanical effectiveness and to identify factors essential to optimal performance (Scurr & Hall, 2009). The in-step soccer kick is described as a complex motor movement consisting of six important stages; approach angle, plant foot forces, swing limb loading, flexion at the hip and extension at the knee, foot contact with the ball and follow through (Sterzing, 2010). The kicking procedure may also be divided up into five essential aspects; approach, support leg, kicking leg, foot to ball interaction, and ball flight. This biomechanics blog will look at, ‘how to achieve optimum force and ball speed for an accurate in-step soccer kick?’

Biomechanical process of an in-step soccer kick

Wickstrom (1975) has described the mature form of the kicking skill. It is characterised by placement of the supporting leg at the side and slightly behind the stationary ball. The kicking leg is first taken backwards and the leg flexes at the knee. The forward motion is initiated by rotating around the hip of the supporting leg and by bringing the kicking leg thigh forwards. The leg is still flexing at the knee at this stage. Once this initial action has taken place, the thigh begins to decelerate until it is essentially motionless at ball contact. During this deceleration, the shank vigorously extends about the knee to almost full extension at ball contact. The leg remains straight through ball contact and begins to flex during the follow through. The foot will often reach above the level of the hip during the follow through (Lees, 2003).

Biomechanical principles of the in-step kick

Kicking in soccer is influenced by the principles of range of motion, velocity, Newton’s laws of motion, angular kinetics, the Magnus effect, and segmental interaction. Velocity is one of the main principles of biomechanics identified in an in-step soccer kick. Velocity relates to the in-step kick in terms of how quickly and in what resultant direction the ball moves (Blazevich, 2010).

Newton’s three laws of motion are all present when performing the skill of an in-step soccer kick. The first law of motion is called the law of inertia. It states that ‘any object at rest, will tend to remain at rest, and any object in motion will tend to stay in motion, unless acted on by an unbalanced force’. In terms of the in-step soccer kick the unbalanced force is gravity, wind, air resistance, and in most cases the players foot. The player will use the muscles in their body to create a force to move their leg and kick the ball. If the ball is at rest it will continue to stay at rest, but once the ball has been kicked and is in motion, it will continue to stay in motion until gravity pulls it down (Tanglent, 2013). Newton’s second law of motion states ‘the acceleration of an object is proportional to the net force acting on it and inversely proportional to the mass of the object (F=ma) (Blazevich, 2010). Explaining this law further in terms of the in-step kick, the acceleration of the ball (a) is determined by the force applied (F) divided by the mass of the object that is being moved (m). If the ball has a lot of mass, it will require more force to accelerate. In terms of soccer it is important to understand this law of motion because if you want the ball moving fast, you must apply force, and if you want to ball to move just a little bit you must apply less force. Finally Newton’s third law of motion, 'for every action, there is an equal and opposite reaction'. This means that every time you kick the soccer ball, the ball kicks you back with the same amount of force.

Biomechanical Stages of an in-step soccer kick

Approach angle

Length, speed and angle of approach are the most important aspects of this preparatory movement, which has significant effect on soccer kick success. Kicking from an angled approach up to 45° may increase ball speed (Kellis & Katis, 2007). The approach path is curved and as a consequence the body is inclined towards the centre of rotation. The purpose of the curved step is to ensure the body produces and maintains lateral inclination as the kick is performed, this allows the kicking leg to get under the ball and make better contact with it (Lees, Asai, Andersen, Nunome, Sterzing, 2010).

Plant foot forces (pre-impact)

 

For kicking success the placement of the support foot behind and beside the ball is important. Although there is no general consensus regarding the placement of the ball beside the foot. It has been suggested that the foot should and 5-10 cm behind and 5-28 cm beside the ball.

Swing limb loading (pre-impact)

The kicking leg moves backwards, with the hip extending. The hip is slowly adducted and externally rotated. The hip action makes an important contribution in the early force-producing phase of the kick. Wang (1994) reported that for executing a perfect in-step kick a speedy and large final step approach is crucial for creating a condition that allows the players to increase the velocity of the kicking swing motion. The kicking leg should swing from back to forward as fast as possible.

Flexion of the hip and extension of the knee (pre-impact)

The powerful hip flexors initiate this next phase of the kick. The thigh is swung forward and downward with a concomitant forward rotation of the lower leg/foot. As the forward thigh movement slows, the leg/foot begins to accelerate because of the combined effect of the transfer of momentum and release of stored elastic energy in the knee extensors. The knee extensors then powerfully contract to swing the leg and foot forwards towards the ball (Barfield, 1998). Studies show that there is a relationship between the foot swing velocity and the resultant ball velocity. This implies that to achieve maximal performance, the energy generated before ball contact should not be reduced.

Foot contact with the ball (impact)

Foot-ball interaction during impact is complex as it only occurs for 10ms. There are four phases that are specific to foot to ball interaction.
Phase 1- Centre of ball gravity moves without ball movement.
Phase 2- Start of ball movement until ball velocity exceeds foot velocity .
Phase 3- Start of ball decompression with continuing decrease of foot velocity and further increase of centre of ball gravity velocity.
Phase 4- Foot loses ball contact while foot deceleration and ball acceleration stops (Sterzing, 2010).

A study found that the ball speed was maximised when the area of impact was near the centre of gravity of the foot (Basumatary, 1998).

Follow through

The follow-through serves two purposes; to keep the foot in contact with the ball for longer, and to guard against injury. A longer contact time will maximise the transfer of momentum to the ball and thus increase its speed (Barfield, 1998)

Ball Speed vs. Accuracy

Maximum ball speed has been widely accepted to be the main biomechanical indicator of kicking success, however, here are good reasons to reconsider this notion as kicking accuracy might be much more important, as it applies to most of all passing and kicking throughout the game regardless of technique and power applied. Accurate kicks are generally slower than powerful kicks. The accuracy of the kick, depends on how fast the player approaches the ball. It has been found that when players are instructed to perform an in-step kick at their own speed of approach, the slower kicks are the more accurate ones. In contrast, if players are instructed to kick the ball as powerful as possible, then the faster the run up speed the less accurate the kick. Accuracy of the kick is related to the point of contact between the ball and the foot. According to Asai, and Carre (2002) inaccuracy arises from the error in the force applied by the foot. The first error arises from the direction of the applied force and the second is due to the misplacement of the force. If the ball was being hit at the centre, it would follow a near straight trajectory and gain maximum possible velocity with minimal spin.

Although accuracy is seen as an important factor of a successful in-step soccer kick, ball speed or projection motion is also one of the main biomechanical indicators of kicking success. Ball speed is a result of; optimum transfer of energy between segments, approach speed, skill level, gender, and age. The faster the projectile speed, the further the object will go. If the ball is kicked through the air, the distance it travels before hitting the ground will be a function of horizontal velocity and flight time. Gravity and air resistance affects ball speed as it does to any object, by pulling it back to the earth’s surface.

The Magnus Effect

The Magnus effect is a lift force of tremendous importance to all athletes who want to bend the flight of the ball. As the spinning ball moves through the air, it spins a boundary layer of air that clings to its surface as it travels along. On one side of the ball the boundary layer of air collides with air passing by. The collision causes air to decelerate creating a high pressure area. On the opposing side, the boundary layer is moving in the same direction as the air passing by, so there is no collision and the air collectively moves faster. The pressure differential, high on one side and low on the other, creates a lift force that causes the ball to move in the direction of the pressure differential (Human Kinetics, 2013) (see image below).

The Answer

After analysing the biomechanical factors that come into the equation when performing an in-step soccer kick, the findings point out that it is very difficult even at a professional level for an athlete to achieve optimum force and ball speed to perform an accurate soccer kick. There are many factors that effect both ball speed and accuracy; the individual can influence some, while other factors are determinant of features of the environment, such as gravity, and air resistance. According to James Watkins, a professor of Sports Science, the velocity of a kicked ball is 30 metres per second, or 67mph. The force of the ball can be determined by the impulse-momentum form of Newton's second law of motion, while holds that the force equals the weight of the ball times its velocity divided by the time of foot contact (Watkins, 2007).

Although ball speed has been widely accepted to be the main biomechanical indicator of kicking success, there are good reasons to reconsider this notion of kicking accuracy as it may be more important, as it applies to most of all passing and kicking throughout the game. Depending on the nature of the game, errors can occur for athletes trying to achieve maximum ball speed, and accuracy. The Magnus effect, momentum and velocity are all factors that impact the speed and accuracy. There are also several other factors like fatigue, age, gender, and limb preference, that cause biomechanical changes that impact on the principles. For example, fatigue involves the development of less than the expected amount of force as a consequence of muscle activation that is associated with sustained exercise and is reflected in a decline in performance. Fatigue causes a decline in leg power, and was concluded that together with the force capacity results, fatigue disturbed the effective action of the segmental interaction during the final phase of the kick, which led to a decreased ball speed, and ball/foot speed ratios (Kellis & Katis, 2007) 

How else can this information be used?

The biomechanical principles that where used to determine, how to achieve optimum force and ball speed for an accurate in-step soccer kick’, can be used throughout a variety of sports. Many studies have been done within the game of soccer, as it is a controversial sport that is played in over 200 countries, making it the world’s most popular sport. In general, sports teams all over the world try and develop their skills and abilities to become the best they can. Biomechanics is used to assist with this as coaches are able to analyse athlete movement to improve technique and reduce the risk of injury. Some of the biomechanical principles talked about in this blog would be used in a variety of other sports including; football, baseball, tennis, and cricket.

In an educational context, physical education teachers use biomechanics, as it is the primary sport science method focusing on movement technique. It is logical for teachers to use it as it assists them in helping their students move safely and effectively. As a physical education teacher planning a soccer unit, biomechanical principles would be used throughout each lesson in evaluating critical features of the skills the students are performing and diagnosing student’s performance.

Reference List

Asai, T, Carre, M, Asatsuka, T & Haake, S (2002) The curve kick of a football I: impact with the foot. Sports Engineering 5, 183-192

Barfield, B (1998), The biomechanics of kicking in soccer. Clinics in Sports Medicine. 17(4): 711-728.

Basumatary, S (1998) Biomechanical analysis in-step kick in soccer, Masters Thesis Victorian University.

Blazevich, A (2010) Sports Biomechanics: The Basics Optimizing Human Performance, Bloomsbury, 2nd edition, London

Human Kinetics (2013) Magnus Effect, accessed 17th April 2013, retrieved from <http://www.humankinetics.com/excerpts/excerpts/magnus-effect->

Kellis, E & Katis, A (2007) Biomechanical characteristics and determinants of instep soccer kick, Journal of Sports Science and Medicine, pg 154-165

Knudson, D (2007) Fundamentals of Biomechanics, California State University at Chico, Springer Science

Kobe, D (1999) Soccer in Australia- What’s going wrong? Journal of Sport Marketing. 3(1), 67-75

Lees, A (2003) Science and Soccer: Biomechanics applied to soccer skills, Routledge, New York

Lees, A,. Asai, T. Andersen, T. Nunome, H & Sterzing. T (2010) The Biomechanics of kicking in Soccer: A review, Journal of Sports Sciences, 28(8): 805-817

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 and Medicine, University of Portsmouth, United Kingdom

Sterzing, T (2010) Kicking in Soccer, XXVIII International Symposium of Biomechanics in Sports, Institute of Sports Science, Germany.

Tanglent (2013) Physics of Soccer, accessed 18th April 2013, retrieved from <https://thescienceclassroom.wikispaces.com/Physics+of+Soccer#Physics%20of%20Soccer-Momentum>

Watkins, J (2007) An Introduction to Biomechanics of Sport and Exercise, Churchhill Livingstine, United Kingdom