The Biomechanics Analysis of a Football

The full size ball is required to have an outer casing or cover which should be of leather or another approved material which does not prove dangerous to players. It should have a mass between 14 and 16 ounces (0.396 and 0.453 kg), a circumference between 27 and 28 inches (0.685 and 0.711 m), and an internal pressure of 0.6-1.1 atmospheres (60.6-111.1 kpa). The ball may be constructed in two main ways. The sewn ball is constructed of panels of leather or similar artificial material, while the moulded ball is made from rubber with cover panels bonded to the surface or from plastic with the cover panels painted on. The sewn ball may be treated to prevent the ingress of moisture, but some affect the way the ball will play. While its size only varies for junior play, its material and method of construction, and internal pressured may all vary. The effect of these variations on both performance and potential injury can be established. The performance characteristics of the ball describe the way it flies through the air, bounces on the ground and reacts to being kicked or headed. The three main performance determinants of a football ball are its mass, its surface roughness and its internal pressure. The mass of the ball is restricted by the rules of the game. Small variations in mass can occur due to the ingress of water through the means of the ball or by absorption through the material. Both of these are less important nowadays due to the developments in ball materials and construction methods. Nevertheless, a heavier ball will have a lower velocity when kicked, although it will retain more of this velocity during flight. Of more importance are the aerodynamic forces acting on the ball. The aerodynamic forces are drag (air resistance) and lift. An important characteristic determining how air resistance will affect the ball's flight is critical Reynold's number. This is a function of the diameter of the ball and its speed. If a ball has exceeded critical Reynold's number then there is a reduced air resistance on the ball and it will fly further for moderate to hard hit kicks this will be true and the ball will have a greater range than if critical Reynold's number had not been exceeded. A lift force acts if spin is put on to the ball. The lift force may not always act vertically to lift the ball but may act horizontally, depending on the direction of spin, to cause the ball to swing away from its intended direction of flight. This spin swing effect is knows as the 'Magnus effect' and is used tactically in corner kicks in football. The 'inswinger' and 'outswinger' is frequently used kicks from corner positions and the direction and amount of swing are determined by the direction and amount of spin put on the ball. In addition a curved ball flight is often used to get around the defensive wall from free kicks. Therefore a ball construction which allows good grip between foot and ball is important. The sewn method of construction is not only a practical method for manufacturing the ball but gives the opportunity for additional grip to be gained between the foot and balls apply spin.

The way in which the ball responds when bouncing from the ground depends on the internal pressure, ground characteristics and surface - ball frictional properties. The higher the internal pressure the better will be the bounce of the ball. However on soft turf surfaces, the condition of the surface is important. If the surface is too soft the surface dominates the behaviour of the ball. Only when a ball bounce on the ground, the ball tends to skid, slightly reducing its forward velocity. How much is lost depends on the frictional interaction between the ball and surface. If the surface is well lubricated by water the ball will skid more easily and lose less forward velocity. As the ball also loses vertical velocity when bouncing, the effect of a ball bounce is to lose considerable energy. It will slow down and bounce less high. The interaction of the ball surface and surface condition serves to alter this effect in any one case. Therefore the predictability of bouncing behaviours will affect the playing of the game. This has been noted when discussing surface above. The force imparted to the player during contact with the ball can lead to injury. Although repeated kicking of the ball will lead to overuse injuries of the toes and ankle, the main concern is usually in heading the ball. The possible injurious effect of heading the ball has been the subject of recent biomechanical investigations as a result of potential legal cases over the misuse the equipment for young players. This has furnished some useful data on ball characteristics.

Levendusky at al. (1988) investigated the impact characteristics of a stitched and moulded football ball and measured the force of impact using a force platform. They found that for velocities of impact of about 18 m/s the force of impact was about 6% higher in the stitch rather than the moulded ball. This finding has implications for the risk of injury of the players when heading a ball. Armstrong et al. (1988) continued this investigation by considering the effect of ball pressure and wetness on the impact force. They found that if a ball was wet it could increase the impact force by about 5% due to the extra weight as a result of water retention, and if a ball had a pressure increase from 6 to 12 psi there would be an increase in impact force of about 8%. There results clearly show the effect that poor combinations of conditions could have for the impact load sustained by the head during heading.

Burslem and Lees (1988) investigated the acceleration on the head during a moderate speed header (ball velocity about 7 m/s) and found that accelerations were about 60g, and rotational acceleration about 200rad/s. clearly there is more danger from the direct impact. In a mathematical stimulation of impact Townend (1988) estimated that the average acceleration of impact was about 25g, but increased with the reduction of mass of the player and the increase in mass of the ball, supporting the results of Armstrong et al. (1988). The conclusion that can be drawn here is that although heading is below the injury threshold, it is sufficiently close to it for care to be shown, particularly in dealing with young children in the development of the skill. The skill of heading can lead to greater head and neck rigidity thereby reducing the effect of the impact. This skill must be taught properly and carefully, and a reduced ball mass should be used for children.