GMAX. It may sound like a nutritional supplement or the latest summer blockbuster. Sports turf managers know it’s the key to monitoring the safety of their fields. But do they really understand what GMAX is and how it works?

Parker Wood, with California-based Sports Turf Solutions, says managers’ knowledge about GMAX varies widely. “Even some of those who know about why the testing is done and why it’s important may not know the specifics of the procedure,” he observes. “The thing that I emphasize is that GMAX testing is fundamentally designed to ensure that a field is performing the way it should with respect to impact attenuation or shock absorption. That’s important primarily from the standpoint that, if the field is not performing well, the field can present a real risk of head injury or other types of injuries to players.”

T. Jay Wirth, with Pacific Sports Turf  in the Pacific Northwest, says only about 25 percent of sports turf managers have a good understanding of how GMAX works and why it’s important; another 25 percent have at least heard of it; and the remaining 50 percent have no idea what GMAX is.

According to Penn State’s Center for Sports Surface Research, GMAX testing measures how many G’s of force a field can absorb upon impact, and how many are returned to the athlete. A high GMAX test value means the field is absorbing less impact, and returning more force to the player (resulting in a potentially dangerous situation), than a low GMAX test value.

How the test works

“The equipment is relatively straight-forward,” says Wood. The American Society for Testing and Materials standard test consists of a guide tube that’s about 2.5 feet tall and a 20-pound, cylindrical weight that falls through the tube. An accelerometer mounted on the weight measures how rapidly the missile decelerates or stops, he explains.

That flat-faced “missile” is connected to a small device that records the velocity as the missile hits the surface, the G-forces that are experienced during decelerations, and more. When he visits a site for GMAX testing, Wood starts by laying out the ASTM-required 10 test points. He first takes preliminary measurements at each test point, looking at the height of the turf fibers and the depth of the infill material. Next, he performs three “drops” – letting the missile fall onto the surface – at each test point. To arrive at the final GMAX value for each test point, he averages the results from the second and third drops.

“That (final value) is the critical number in terms of the standard that the field is supposed to meet to determine whether it is in acceptable condition or not,” says Wood. The equipment does generate other “values” related to the impact – namely, figures for SI (severity index) and HIC (head injury criteria). However, the GMAX number is the one used to determine a field’s acceptability.

The ASTM test requires that every test point on a field have a reported value of less than 200 G’s, but field specifiers may set a lower value for their fields. “In other words, an architect designing a field can write into the specifications that all the test points on a field should be below 150 G’s, which would constitute a more stringent requirement that the manufacturer of the field would then be expected to satisfy,” he explains.

Which fields to test, and when?

Early generation synthetic fields constructed in the 1960s and 1970s often were dangerously hard; the numerous injuries that resulted soon led to the introduction of GMAX testing (which was originally pioneered by Australian researcher Dr. Baden Clegg for use in road construction) in the sports turf industry.

“It was important then because a lot of those fields were essentially just a carpet on a very hard base, and if they weren’t installed properly they could be seriously dangerous to the players,” says Wood.

While the safety and performance of modern synthetic surfaces have improved tremendously (beginning with the use of sand and rubber infill in the 1990s), GMAX testing remains critical. GMAX performance on a synthetic field can vary based on the amount and composition of infill material, how much the carpet has worn over time and whether the infill material has become contaminated, Wood explains.

While GMAX testing (including 90 to 95 percent of the testing he does) is focused primarily on synthetic fields, Wood says that some sports field managers also choose to test their natural turf fields. “You can do the testing on either,” he says.

There are challenges to both testing scenarios. Synthetic fields tend to be much more homogenous in terms of their performance across the field. “The factors that affect GMAX performance on a synthetic field include the amount of infill material, the composition of the infill material, how much the carpet itself has worn over time, whether the infill material has become contaminated,” he explains. “On a natural field, it’s pretty much a question of the type (composition) of the soil and the moisture content of the soil that will affect GMAX performance.”

Because there could be pockets with different soils or dry spots, etc., the GMAX test results on a natural field could have much more variability than on a synthetic field.

ASTM has been debating for some time the proper frequency of how often GMAX testing should be conducted. Wood adds: “I think the consensus is that testing should occur at least once a year. That doesn’t happen very often, but that seems to be the consensus of those involved with the development and publication of the test specifications.”

Wirth recommends evaluating GMAX immediately after a field is installed, both to ensure that the initial specs were met and to establish a baseline. He then recommends annual testing to monitor and track the field’s condition. “Fields will get harder over time,” he explains. “When you start getting close to whatever number that you’ve chosen not to exceed, then you may need to start testing more frequently in order to really monitor those numbers. You may be testing two or three times a year, because you don’t want to exceed that 200 number and have somebody get hurt.”

In such a scenario, the Occupational Safety & Health Administration (OSHA) would become involved, says Wirth. “If it’s over 200, you better shut it down.”

Maintaining infill depth according to the specifications of field manufacturers both helps to create safer playing conditions for athletes, and extends the life of the field itself because the infill helps to protect more of the grass fibers, says Wirth.

Testing versus “feel”

Testing is important because you can’t judge its safety by feel.

“What a field feels like is different from how it actually absorbs shock,” emphasizes Wirth. “People will tell me that their field feels soft, and that it feels great. But I explain have to explain that what GMAX is testing is the absorption of the impact. The rubber is only, in some cases, 1 inch thick. Once you’re through that, the energy is coming right back up.”

Natural turf fields typically are better able to absorb energy, even if the field itself feels hard and is in poor condition, he adds. “We’ve done tests on bare, baked dirt. When you hit that, the energy continues on down past that 2 inches that’s hard at the top,” Wirth says. “So the field may feel harder, but it’s actually softer on impact.”

Technical properties such as “vertical deformation” and “energy restitution” can influence how a field feels to those playing on it. “In the last few years I have had several situations where someone designed a field and they established a minimum GMAX requirement,” Wood says. “They were doing that with the assumption that, if they were getting GMAX values below 100, the field would be too ‘soft’ and could result in the possibility of other injuries, or in poor performance.”

In reality, Wood emphasizes, it’s possible to have a field with GMAX performance well below 100 and yet is still very firm and playable because the other factors have been properly addressed.

The bottom line is that a field’s “feel” doesn’t necessarily correspond to its shock absorption properties. More specifically, a field may feel very firm but do an excellent job of attenuating shock, and therefore have a low GMAX value. “The way I try to explain it is, if you walk out on a field and it feels soft – very cushy, and spongy – it will almost certainly have low GMAX numbers. But if we go out and test a field that has low GMAX numbers, there is no guarantee that the field will field will feel soft or spongy,” he explains.

Shock pads can help. “If the right materials are used, and installed properly, then the GMAX numbers can be very low, but the field has excellent playing characteristics,” Wood summarizes.

Per ASTM standards, GMAX testing is done in 10 locations on a field. T. Jay Wirth with Pacific Sports Turf recommends having GMAX testing done immediately after having a field installed, and at least once per year after that.

Play it safe

While concussion safety is one reason for GMAX testing, Wood says it’s really more focused on determining the risk of skull fractures and broken bones: “I try to emphasize to people that concussions can occur even under very low impact; there are lots and lots of things that go into determining whether or not a person will sustain a concussion when they fall,” he says. What can be done in the event of a high GMAX test result? “It’s all situational, but certainly the very first suggestion is increased maintenance,” says Wood. Adding more infill can have a direct impact on GMAX numbers, he explains.

“Addressing areas where the infill may have become contaminated may also have a positive effect on GMAX performance and try to forestall the need to replace the field,” Wood adds.

Wirth says he’s found that many field owners lack education on the importance of maintaining the infill depth. Maintaining proper depth not only helps create safer playing conditions, it extends the life of the field because the infill helps to protect grass fibers. “You also need to groom it, clean it, get any metal off it – your normal maintenance,” he adds.

It might cost $1,000 per year to add rubber to a field, but that could extend the life of the field by two years, which makes it a very wise investment, says Wirth.

Similarly, the cost of GMAX testing (which can range from $500 up to $1,300, depending on the number of drops needed) is a relatively small investment to be able monitor – and document – the safety of a field.

Pacific Sports Turf charges $600 for the 10-drop ASTM protocol, and another $150 if the field owner wants the 20-drop NIAAA (National Interscholastic Athletic Administrators Association) protocol.

For those considering GMAX testing, one factor to weigh is liability, notes Wood. “God forbid something happens and someone is injured, but, if you are able to show that you have been proactive in terms of monitoring the condition of the field and doing other maintenance-related activities that are designed to keep it safe and playable,” he says, “you’re in a much better position.”