Gmax is the numerical value used to indicate a playing surface’s hardness and therefore the likelihood of player injury upon impact. When a player falls, the impact is absorbed either by the surface, protective equipment worn by the player or the player’s body. The harder the surface, the greater the amount of the impact is absorbed by the player’s body. This rule applies to both natural and artificial turf systems. Natural grass systems are NOT inherently safer than synthetic turf systems. Gmax values may be utilized to determine the need for maintenance, remediation and/or replacement of the sports field surface.
The Triax and the Clegg Hammer are the primary Gmax machines available for testing sports fields. While there are numerous manufactures of both types and each manufacturer’s Gmax machine is slightly different, the function and operation is basically the same.
Understanding Gmax value starts with understanding the relevance of the numbers, Gmax math for lack of a better term. Example Gmax readings are illustrated below to provide a few points of reference. Most vendors will agree that a sports field should not be used if the average Gmax value for any tested location on the field is above 150; a value of 200 should result in the immediate closure of the sports field because it is unsafe for play and can cause life-threatening injuries.
Gmax values from a Triax machine:
• 60 – Muddy Grass
• 80 – Minimum acceptable Gmax
• 150 – Acceptable Maximum – Most Vendors
• 166 – Maximum Professional Football
• 175 – Acceptable Maximum – Certain Vendors
• 180 – Packed Clay
• 200 – OSHA – “Will Cause Death”
• 225 – Frozen Grass
For more information on Gmax testing, please email firstname.lastname@example.org.