A reader recently asked us what is considered a safe degree of hardness for an athletic field. In addition, he wanted to know how hardness is measured. So we asked Dr. Andrew McNitt, a Penn State researcher who has worked extensively on the subject of field hardness. McNitt supplied the following information.

Surface hardness refers to the ability of a surface to absorb impact energy. Playing surface hardness affects both player performance and player safety. A soft field may create early fatigue in leg muscles, while fields that are hard may be dangerous when players fall.

The standard way to measure the playing surface hardness of a natural turfgrass field is with the Clegg impact tester. This device is simply a hollow tube through which a weight is dropped onto the surface. A device inside the weight measures how quickly the weight stops upon impact. The faster the weight comes to a stop, the harder the surface. Surface hardness is measured in Gmax: the higher the Gmax value, the harder the surface.

The Clegg impact soil tester has been used by researchers to relate field surfaces to soccer ball bounce, player response and performance, including injury potential. In an attempt to relate surface hardness to athlete performance and safety, British researchers correlated athletic field surface hardness measurements with athletes' perceptions of surface hardness. Based on results of hardness values, these researchers suggested a preferred upper limit of 80 Gmax with a 0.5 kilogram weight (Clegg testers use various sized weights, depending on use).

In the United States, the most commonly used weight is 2.25 kg (about 5 pounds). When dropped from a height of 18 inches, this weight has an impact velocity that could be described as similar to an athlete falling on their elbow. This weight and height (5 pounds at 18 inches) is the standard method to measure the cushioning properties of natural playing surfaces as specified by the American Society for Testing and Materials ( The test method is number F 1702.

A standard for artificial turf does exist. This standard ASTM F355 Method A is similar to the Clegg, but uses a 15 pound weight. The ASTM upper limit is 200 Gmax; above that, they suggest repairing or replacing the surface. This number was generated from the auto industry and data regarding a human head impacting a dashboard.

Should the ASTM standard be adopted for natural turf? Perhaps. A problem is that an F355 unit costs upwards of $15,000, while a Clegg impact tester can be purchased for a little less than $2,000. Neither testing method has been researched enough to thoroughly correlate injuries with field surface hardness. To do so would require a large scale study involving of a sort not yet attempted.

Unfortunately, there are no set standards as to how hard is too hard. Of course, we can surmise that a very hard field will result in increased injuries. You can reduce surface hardness by maintaining adequate soil moisture through irrigation; decreasing soil density by hollow-tine aerating; and increasing turfgrass cover by excercising control over how much wear you'll allow on the field and implementing an overall high-quality maintenance program. Sometimes easier said than done, but that's what it takes.

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