"THE GOAL WAS SCORED A LITTLE BIT BY THE HAND OF GOD, ANOTHER BIT BY THE HEAD OF MARADONA." - DIEGO MARADONA
In soccer, heading is a permitted tactic and constitutes a fundamental skill of the game. With this in mind it is of the utmost importance that the correct biomechanical principles be applied to the skill, so as to both, avoid injury, and preform the skill at an optimal level.
ENERGY
Before attempting to understand specific biomechanical skill requirements, it is helpful to understand the numerous types of energies an athlete will harness when attempting a jumping header. An athlete must understand which specific energies they can manipulate and harness in order to perform their skill at on optimal level, giving them the most amount of energy possible, without exasperating their body for their remainder of the game. One of the primary energy stores that an athlete will utilise during this skill in kinetic energy. Kinetic energy is the energy that the body has because it is moving (Hay, 1993.). Kinetic energy is calculated with the mass of the athlete and the velocity at which they are moving. The given equation dictates the summation of kinetic energy:
Eᵏ = ½ mv²
Where Eᵏ = kinetic energy, m = the mass of the athlete’s body, and v = the velocity at which they are moving (Hay, 1993.).
If an athlete is able to produce a higher level of power, during their take-off phase, they will then move at a higher velocity, and thus have a great amount of kinetic energy for the rest of their skill (Blazevich, 2012.). When the athlete is undertaking the airborn phase of the jumping header they have potential energy. Potential energy occurs as the athlete is at the peak of their jump. The athlete has the capacity to do work due to their position relative to the earth’s surface (Hay, 1993.). Potential energy is determined by the following equation:
EP = Wh
Where EP = the potential energy, W = the weight of the athlete’s body, and h = their height above the ground (Hay, 1993.).
During the airborn phase of the jumping header, the kinetic energy of the athlete decreases and the potential energy increases during ascent, and the exact opposite occurs during descent. In order for the athlete to maintain a performance of this skill, at an optimal level, given the energy they will dispurse throughout an entire soccer game, is for them to produce a greater kinetic energy output for a smaller metabolic energy input. The more oxygen an athlete uses, the more metabolic energy they are producing (Blazevich, 2012.) This all tells us that if the athlete is given the correct biomechanical technique to improve efficiency they will, in fact, be able to increase kinetic and potential energy, while decreasing metabolic output. These energy factors are paramount to attaining peak performance of the soccer jumping header, particularly in a game situation where there are numerous other stresses on the body that may result in physical fatigue.
Eᵏ = ½ mv²
Where Eᵏ = kinetic energy, m = the mass of the athlete’s body, and v = the velocity at which they are moving (Hay, 1993.).
If an athlete is able to produce a higher level of power, during their take-off phase, they will then move at a higher velocity, and thus have a great amount of kinetic energy for the rest of their skill (Blazevich, 2012.). When the athlete is undertaking the airborn phase of the jumping header they have potential energy. Potential energy occurs as the athlete is at the peak of their jump. The athlete has the capacity to do work due to their position relative to the earth’s surface (Hay, 1993.). Potential energy is determined by the following equation:
EP = Wh
Where EP = the potential energy, W = the weight of the athlete’s body, and h = their height above the ground (Hay, 1993.).
During the airborn phase of the jumping header, the kinetic energy of the athlete decreases and the potential energy increases during ascent, and the exact opposite occurs during descent. In order for the athlete to maintain a performance of this skill, at an optimal level, given the energy they will dispurse throughout an entire soccer game, is for them to produce a greater kinetic energy output for a smaller metabolic energy input. The more oxygen an athlete uses, the more metabolic energy they are producing (Blazevich, 2012.) This all tells us that if the athlete is given the correct biomechanical technique to improve efficiency they will, in fact, be able to increase kinetic and potential energy, while decreasing metabolic output. These energy factors are paramount to attaining peak performance of the soccer jumping header, particularly in a game situation where there are numerous other stresses on the body that may result in physical fatigue.
TAKE-OFF PHASE
Arms should begin behind the trunk in the take-off phase and then move towards the front of the body, as the body gains vertical momentum. This thrust forwards of the arms works with the body’s kinetic chain in creating momentum for the body. It should also be noted that the player, throughout the entire header, keeps his eyes on the ball. The player watches the ball, before and after impact, allowing him to maintain accuracy within the header. Not maintaining vision of the ball decreases accuracy and stability of the athlete’s body in the air. The jump should be initiated by the player swinging the arms up. The hips and knees should be flexed to approximately ninety degrees prior to take-off while the trunk should be flexed forward. Extending the elbows and then extending the legs and trunk. At the time of the jump, knees and hips should be fully extended and the arms pointing upwards to ensure a full range of motion within the joints.
Newton’s Second Law of physics is relevant to all styles of jumping. Newton’s Law states that: “ … acceleration equals force, divided by mass.” (Simonian, 1981, Pg. 187.) This means that an athlete of a lighter mass will achieve a higher take-off velocity. When attempting to achieve optimum vertical velocity within a jumping header, in accordance to Newton’s Second Law of physics, weight training for the legs should be undergone, as well as addressing a correct technique, such as the one discussed here.
To understand the forces that enable the take-off phase to come to full completion we must look at Newton’s Third Law of Physics. “For every action, there is an equal and opposite reaction.” (Holzner, 2011.) In the case of jumping, during the take-off phase, the ground exerts and equal and opposite reaction force, when the foot in placed upon it. During running, and jumping, we apply a force with both vertical and horizontal components. The ground acts in accordance to Newton’s Third Law, and produced an equal and opposite force to react with the athlete’s foot, and propel them upward. Newton’s Third Law of physics helps to explain why a two footed take-off is more beneficial to a one footed take-off. During a two footed take-off the athlete is able to apply more force to the ground, causing them to create a higher velocity for their body.
Newton’s Second Law of physics is relevant to all styles of jumping. Newton’s Law states that: “ … acceleration equals force, divided by mass.” (Simonian, 1981, Pg. 187.) This means that an athlete of a lighter mass will achieve a higher take-off velocity. When attempting to achieve optimum vertical velocity within a jumping header, in accordance to Newton’s Second Law of physics, weight training for the legs should be undergone, as well as addressing a correct technique, such as the one discussed here.
To understand the forces that enable the take-off phase to come to full completion we must look at Newton’s Third Law of Physics. “For every action, there is an equal and opposite reaction.” (Holzner, 2011.) In the case of jumping, during the take-off phase, the ground exerts and equal and opposite reaction force, when the foot in placed upon it. During running, and jumping, we apply a force with both vertical and horizontal components. The ground acts in accordance to Newton’s Third Law, and produced an equal and opposite force to react with the athlete’s foot, and propel them upward. Newton’s Third Law of physics helps to explain why a two footed take-off is more beneficial to a one footed take-off. During a two footed take-off the athlete is able to apply more force to the ground, causing them to create a higher velocity for their body.
AIR BORN PHASE
The initial movement of the trunk and hips on leaving the ground is joint extension, in order to produce a greater range of motion throughout the body. This will produce a higher velocity and level of power, which can, in turn, be imparted on to the ball. Trunk extension should be 1-15 degrees and hip extension should be approximately 30 degrees.
The knees should be flexed. In order to the jumping header to be performed at an optimal level, the athlete must reach full velocity during the air born phase. Within the jumping header the final velocity is a summation of several relative velocities within the body. Northrip et al, (1974) state that: “… a whole body motion is added to the velocity of a body part relative to the whole body in order to produce the final specific velocity desired.” (Pg 121.) This is to say that the final velocity obtained in the header is a direct result of the ankle plantar flexion, knee extension, and hip extension. The maximum height that the athlete will obtain within this skill is directly dependent on the summation of these three velocities (Northrip et al, 1974.)
While the athlete is air born the trunk and hips are extended to an optimal level, before impact, create a jack knife effect. This complete flexion of knees and hips should occur just before the peak of the air born phase. The arms should still be situated above the head. The jack-knife effect, displayed in the bellow picture, is an example of how an athlete can manipulate the center of mass of their body. The athlete can do this by bringing their legs up, under their body, as seen in the picture. When the athlete is about to fall back to earth, due to the pull of gravity, they quickly thrust their legs to full extension, causing the upper body to be thrust upwards (Blazevich, 2012.) This occurs just shy of impact, giving the athlete added velocity and power to impart upon the ball.
While the athlete is air born the trunk and hips are extended to an optimal level, before impact, create a jack knife effect. This complete flexion of knees and hips should occur just before the peak of the air born phase. The arms should still be situated above the head. The jack-knife effect, displayed in the bellow picture, is an example of how an athlete can manipulate the center of mass of their body. The athlete can do this by bringing their legs up, under their body, as seen in the picture. When the athlete is about to fall back to earth, due to the pull of gravity, they quickly thrust their legs to full extension, causing the upper body to be thrust upwards (Blazevich, 2012.) This occurs just shy of impact, giving the athlete added velocity and power to impart upon the ball.
IMPACT
At impact, the player should be ideally, just shy of the peak of their jump. This allows them to maintain some vertical velocity, which can be imparted on to the ball. Before impact the arms should move forcefully into horizontal extensions to impart force onto the ball, and to help keep the body stable in the air. The neck remains stationary, and used with the trunk as one, solid force. The trunk should extend approximately 30 degrees over the hips. When the head strikes the ball momentum is transferred to the ball, and the head is slowed. Given that the head is anchored to the neck and the head weighs several times as much as the ball, the loss of speed within the head is approximately 10% of the speed given to the ball (Wesson, 2002.) The athlete may use some trunk rotation or side-flexion, depending on where they are aiming the ball (Wesson, 2002.) Ideally the ball will be struck in the middle of the forehead, as this provided the largest amount of space. This will allow for a greater level of accuracy and power.
The moment of impact is where we can most clearly identify Newton’s First Law of physics within this particular skill. Newton’s First Law states: “An object will remain at rest or continue to move with constant velocity as long as the new force equals zero.” (Blazevich, 2012, Pg 44.) In the case of the soccer header we can view it as this; as the ball flies through the air, it has inertia proportional to its mass, and would continue in a straight line for eternity, should it not be reacted upon by either gravity or air resistance. The ball, once struck by the player’s head, is then given a new direction, and most likely a new speed. As the player’s head meets the ball, an equal force is acting upon the head as the two objects collide (Simonian, 1981.)
In cases where the head is struck by an unseen ball, or the player does not have sufficient time to prepare themselves for the ball, it is possible that the ball’s loss of momentum can be transferred to the head. In a severe case of an 80 kilometre per hour kick, the head can be moved an inch within one hundredth of a second. Results like these, while not pleasant for the player, can typically remain safe. A ball that travels faster than 80 kilometre per hour kick and catches a player unawares can result in unconsciousness (Wesson, 2002.). This emphasises the need for the application of correct biomechanical principles, when performing this skill.
The moment of impact is where we can most clearly identify Newton’s First Law of physics within this particular skill. Newton’s First Law states: “An object will remain at rest or continue to move with constant velocity as long as the new force equals zero.” (Blazevich, 2012, Pg 44.) In the case of the soccer header we can view it as this; as the ball flies through the air, it has inertia proportional to its mass, and would continue in a straight line for eternity, should it not be reacted upon by either gravity or air resistance. The ball, once struck by the player’s head, is then given a new direction, and most likely a new speed. As the player’s head meets the ball, an equal force is acting upon the head as the two objects collide (Simonian, 1981.)
In cases where the head is struck by an unseen ball, or the player does not have sufficient time to prepare themselves for the ball, it is possible that the ball’s loss of momentum can be transferred to the head. In a severe case of an 80 kilometre per hour kick, the head can be moved an inch within one hundredth of a second. Results like these, while not pleasant for the player, can typically remain safe. A ball that travels faster than 80 kilometre per hour kick and catches a player unawares can result in unconsciousness (Wesson, 2002.). This emphasises the need for the application of correct biomechanical principles, when performing this skill.
IMPACT - COEFFICIENT OF RESTITUTION
It is important to understand that when striking a soccer ball with a head the ball will not retain the same level of energy it had before it was struck. This is called the coefficient of restitution. The coefficient of restitution describes the energy lost from a ball when its path is interrupted by another object, such as a head. The coefficient of restitution for a soccer ball is 0.76, meaning that the ball will retain 76% of its energy after collision with the head (Hay 1993.) In order to overcome the coefficient of restitution, and make the ball travel further and faster after impact, a number of things can be done. The most beneficial way of overcoming this energy loss is to increase the speed of the athlete’s body as it moves towards the ball. This allows the athlete to impart more power on to the ball. In many sports it is possible to attempt to overcome the coefficient of restitution within a ball by decreasing the mass of the ball, however, FIFA dictate that a ball must weigh no less than 410 grams at the start of a match and 450 grams at the end of a match (FIFA, 2012/2013.) This law makes decreasing the weight of the ball impossible within a game.
The below chart dictates the various coefficient of restitution levels in numerous sporting balls.
The below chart dictates the various coefficient of restitution levels in numerous sporting balls.
LANDING
When landing, the player should attempt to land on both feet in order to decrease the force of the impact, thus decreasing the risk of injury.
HOW ELSE CAN WE USE THIS INFORMATION?
This information can be used in any sport where the momentum of the body has a direct effect on the performance of a skill. Skills such as the soccer throw in, the basketball jump shot, and a gymnastics forward roll all rely on the body’s kinetic chain to create momentum, a high level of velocity, and the athlete’s ability to manipulate their center of gravity.
REFERENCES
Blazevich, A, (2012.) Sports Biomechanics, The Basics: Optimising Human Performance, Second Edition, England: Bloomsbury.
FIFA, (2012/2013) Laws of the Game 2012/2013, Switzerland: FIFA.
Hay, J.G, (1993.) The Biomechanics of Sports Techniques, Fourth Edition, USA: Prentice Hall.
Holzner, S, (2011.) Physics for Dummies, USA, Indiana: Wiley Publishing Inc.
Infoplease, (2007.) Diego Maradona, Pearson Education, http://www.infoplease.com/biography/var/diegomaradona.html [Viewed 21/04/2013]
Northrip, J, Logan, G, McKinney, W, (1974.) Introduction to Biomechanic Analysis of Sport, Second Edition, USA, Iowa: Wm. C. Brown Company Publishers.
Reilly, T, Lees, A, Davids, K, Murphy, W.L, (1988.) Science and Football, England: E. & F. N. Spon Ltd.
Simonian, C, (1981.) Fundamentals of Sports Biomechanics, USA: Prentice Hall.
Wesson, J, (2002.) The Science of Soccer, England: Institutes of Physics Publishing.
Images
Coefficient of restitution chart: http://www.physics.hku.hk/~phys0607/lectures/chap05.html [Viewed 18/04/2013]
The remainder of images were taken by the author of this blog.
Athlete: Leo Angove Photographer: Eilis Toth
FIFA, (2012/2013) Laws of the Game 2012/2013, Switzerland: FIFA.
Hay, J.G, (1993.) The Biomechanics of Sports Techniques, Fourth Edition, USA: Prentice Hall.
Holzner, S, (2011.) Physics for Dummies, USA, Indiana: Wiley Publishing Inc.
Infoplease, (2007.) Diego Maradona, Pearson Education, http://www.infoplease.com/biography/var/diegomaradona.html [Viewed 21/04/2013]
Northrip, J, Logan, G, McKinney, W, (1974.) Introduction to Biomechanic Analysis of Sport, Second Edition, USA, Iowa: Wm. C. Brown Company Publishers.
Reilly, T, Lees, A, Davids, K, Murphy, W.L, (1988.) Science and Football, England: E. & F. N. Spon Ltd.
Simonian, C, (1981.) Fundamentals of Sports Biomechanics, USA: Prentice Hall.
Wesson, J, (2002.) The Science of Soccer, England: Institutes of Physics Publishing.
Images
Coefficient of restitution chart: http://www.physics.hku.hk/~phys0607/lectures/chap05.html [Viewed 18/04/2013]
The remainder of images were taken by the author of this blog.
Athlete: Leo Angove Photographer: Eilis Toth