A GMAX test measures impact attenuation – the ability of the playing surface to absorb the “shock”, or kinetic energy – from a collision, such as a player falling to the surface. The lower the GMAX rating, the more energy is absorbed by the surface. Alternatively, the higher the GMAX rating, the less energy is absorbed by the surface, meaning more energy is returned to the player.
According to the United States Consumer Products Safety Commission (UPSC), a GMAX rating greater than 200 can significantly increase the risk of serious injury. Suggesting that anything over 200 could yield injury and should be remediated or in some cases, replaced. Although the Synthetic Turf Council (STC), the leading industry voice, states that <165 is the ideal performance standard.
The ASTM F-1936 states that the test method ASTM F-355-01 has been used to test the impact attenuation of athletic fields for over 30 years. The development of the test can be traced back to the Ford and GM crash-dummy tests of the 1960’s, medical research papers from the 1960’s and 70’s, and a Northwestern University study in which an accelerometer was fixed to the football helmet of a middle line backer to measure impacts received during actual play. This study found the typical head-impact to be 40 feet-pound, which is equivalent to the impact generated by dropping a 20-pound missile from a height of 2 feet.
The ASTM F-355-01 and ASTM F-1936 GMAX test guidelines state that eight locations on the synthetic turf field are to be tested. The test method includes three drops of a flat-bottomed, 20-pound steel missile, to which an accelerometer is attached. The missile is dropped from 2’ above the surface through a vented guide tube. The first drop is to “condition” the field and the next two drops are averaged to yield the GMAX reading for that location. Prior to testing, the infill depth is measured at each drop location.
If you have a synthetic turf field, the STC recommends that you schedule a GMAX test annually to monitor your field’s rating. The Motz Group, as well as other manufacturers such as FieldTurf, AstroTurf, and Shaw Sports Turf offer GMAX testing. Visit STC Buyers’ Guide for additional information on vendors who offer GMAX testing. Having this test performed will provide you insight on your field’s impact attenuation and should results indicate, take action to address the matter. And if you are thinking about converting to synthetic turf, ensure your provider will warrant that your field will not become too hard over time.
For more information on GMAX testing and/or how to decipher the rating, please contact The Motz Group at 513.533.6452.