What does the hardness of the football field have to do with concussions?
According to a recent post in USA Football's "From the Field" blog, field density plays a sizable factor in head injuries. In fact, Penn State's Center for Sports Surface Research reported that 10 percent of concussions come from how hard the ground -- or the artificial turf -- is on a football field.
A properly maintained playing surface can help reduce head injury risk. Whether natural or synthetic turf, field management practices directly affect field hardness and, in turn, risk of head injury.
As a result, monitoring field hardness is key. In fact, the NFL now requires field managers to measure surface hardness before every game.
Surface hardness is measured by dropping a weight -- often referred to as a missile -- from a fixed height onto the playing surface. The missile contains an accelerometer that measures how fast the missile stops once it hits the surface. A numerical value (referred to as Gmax) is then generated.
A high Gmax value indicates the missile stopped quickly. Think of this as dropping the missile onto concrete. If the missile was dropped onto a pillow, it would take a longer time for the missile to stop and the softer surface would produce a lower Gmax value.
The NFL field testing program requires playing surface hardness of both natural and synthetic turf fields to be measured with the Clegg Impact Tester. Fields must be tested in multiple locations prior to every game and must be below 100 Gmax.
The Center has a separate device for measuring the hardness of artificial surfaces.
Another device used to measure surface hardness on synthetic turf is the F355 device. Named after the American Society for Testing and Materials (ASTM) standard that details the testing method, the concept behind this device and the Clegg Impact Tester is very similar.
However, the two devices use missiles of different weights and are not dropped from the same height. As a result, the Gmax values are not interchangeable. While the NFL uses an upper limit of 100 Gmax with the Clegg Impact Tester, ASTM F1936 sets an upper limit of 200 Gmax with the F355 device.
The center, which suggests testing fields once a year, has a process for making the fields softer for landings. For natural grass, aerate it just like a golf courses.
The best way to alleviate soil compaction is through mechanical aeration. Aeration involves using a machine that removes small soil cores. Aerating creates more airspace in the soil and should be done several times each offseason.
Fields often are aerated after the final game of the year and then again in early spring. Highly compacted field can be aerated more often, but it is important to avoid aerating during hot, dry weather as well as immediately before and during the season.
Adding compost in conjunction with aeration can further reduce surface hardness in addition to creating better conditions for turf growth. Information about applying compost can be found in this previous From the Field column and this report.
For artificial turf, add more recycled rubber.
When infill levels drop below the field manufacturer’s range, additional rubber should be added. This means you should have additional rubber on hand and the rubber should be the same as what is currently in the field.
You may only need to add rubber to a small area such as a lacrosse goal mouth if it is a multisport field. Or the entire field may require additional rubber.
Additional information on "walk-off" crumb rubber and adding rubber to your field can be found here. If your field is painted on a regular basis, paint can build up over time and result in elevated surface hardness values.
-- Bill Bradley, contributing editor