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FIELD SCIENCE A Gmax ratio >200 is the threshold value that the American Society of Testing and Materials(ASTM)has adopted.Surfaces caus- ing this rate of deceleration or above during an impact may cause a life threatening head and neck injury Bear in mind that in reality there may or may not be an injury or that an impact on a softer sur- face could cause one. Another criterion that is sometimes used in assessing impact severity is the Head Injury Criteria,HIC. This criterion is used a lot in assessing the safety of playground equipment. It takes into account GMAX CAN BE REDUCED THROUGH THE GROOMING OF SYNTHETIC FIELDS, REPLACING INFILL MATERIALS IN DIVOTS AND, FOR NATURAL TURF, RESEEDING, AERIFICATION AND IRRIGATION. the duration of the severest portion of the impact, as well as the Gmax. The HIC was developed from the Gadd Severity Index (SI) which analyzes the G data over the entire duration of the impact. Taking into account impact duration is important for head and neck injuries. Two locations may have the same Gmax, but the field having a shorter impact duration is harder i.e..the impact energy is returned to the head and neck more quickly. This results in a higher HIC and SI value. However, ASTM has adopted the Gmax value in CATS mtrm MU COTO determining the hardness of a playing field and not HIC and SI COMM a b mourn na ac =13 because the latter two pertain specifically to head injury and give no t , indication of how other body parts may be injured. � + s Peak deceleration,Gmax.is affected by surface hardness.A hard- t O _! '� ,40. t440. er surface causes a greater rate of deceleration. For example, if a 41-1. player struck a concrete surface they could decelerate(or reach zero TO velocity) almost instantaneously. Compared to how fast they had ,�,,, (fFl ,�R, been accelerating, Gmax would easily exceed 200. If the player "1-2; struck a softer surface they would decelerate more slowly because a the surface would be compressing below them. However,the surface u� doesn't compress indefinitely. It becomes stiffer as it is compressed. As a result. the player begins to decelerate more rapidly until they �� sa finally stop moving in the downward direction. This is the point of �� rs Gmax.Table 1 shows some examples of Gmax for different surfaces. COF➢[TIOFicc DROP Oh o DROP THREE Gmax will usually increase with high use compaction infill segre- OaOP-RO MTh gation, loss of thatch and groundcover, and soil dryness. Gmax can Figure 3.Illustration of drop sequence used during an be reduced through the grooming of synthetic fields, replacing infill F 193698 test sequence materials in divots and, for natural turf, reseeding, aefification and ® irrigation. 12 May 2006 www.greenmediaonline.com