Shock Absorbency

PLAYER/SURFACE INTERACTION

The characteristics of a surface that a football player needs to play the game fall into several categories:

Shock Absorbency

The surface can feel “hard” or “soft”. A hard surface can lead to injuries to the body by causing the joints (particularly ankles, knees, hips and spinal column) to compress which results in damage to the cartilage between the bones in the joints. Furthermore falling on a hard surface can cause bruising to soft tissue like muscles and extreme cases can cause fractures to bones. A soft surface can cause fatigue to the player running on the surface. The ability of a surface to absorb the impact of a player running on the surface is called its Shock Absorbency and is measured using a device called the Berlin Athlete. The human body behaves like a spring when it makes contact with the surface. A spring when compressed absorbs a certain amount of energy.

This energy is released when the pressure on the spring is released. Similarly a human being walking on a surface absorbs some of the impact when his foot makes contact with the ground. However, once our human spring has been completely compressed any additional impacting force will feel like a physical shock. Walking on a surface our human spring can absorb most if not all of the shock. If we then jump on the surface it likely that we completely compress our spring and the extra force we apply by jumping rather than walking gives a physical shock to the body. If we jump from a sufficient height the shock can be so great as to do physical damage to our bodies.

The apparatus we use to measure incorporates these elements of the human spring and an impacting force. An anvil is placed on the surface to be tested, on top of the anvil is a spring that has the same spring coefficient as an “idealised” sports person and a weight is allowed to fall on the spring. The force received by the anvil is a function of the combination of the spring and the shock absorbing nature of the surface. The apparatus is first placed on a concrete slab and a value is obtained. The apparatus is then placed on the surface to be tested and the new value is obtained. The two values are compared and the reduction in the force received by the anvil due to the surface is recorded. Hence the values are expressed as a percentage of the force received when Force Reduction

The property we are measuring is called Shock Absorbency; the apparatus we use to measure Shock Absorbency is called the Berlin Athlete. The measure of Shock Absorbency using the Berlin Athlete is called Force Reduction and is expressed as a percentage. The higher the percentage the “softer” the surface i.e. the more shock absorbing is the surface. Natural turf in good conditions gives values of between 55-70%. Natural turf in “ideal” conditions produces values of between 60-70%.