By Taylor Williams and Dr. John Sorochan

Bermudagrass athletic fields are commonly located in areas with hot, dry summers and it is chosen due to its recuperative potential and drought tolerance. These playing surfaces are often used year round with little time for cultivation. In many cases, practice fields are not maintained to the same level compared to game fields though because they typically are not seen by many outsiders. Safety should always be the highest priority when managing athletic fields.

Practice fields are where athletes spend most of their time, which can lead to bodily harm if the field is not properly maintained. This leads to playing surfaces becoming hard and compacted, resulting in more frequent injury, as the ground is not able to provide the adequate cushion needed to protect an athlete during a fall.

A variety of testing methods are used to evaluate both natural grass and artificial athletic fields. Natural fields are constructed with sand rootzones or native soils, which often contain high clay levels. Sand-based rootzones are reliable because they allow for the surface to drain rapidly when compared to rootzones higher in clay. A sand-based rootzone allows rain to penetrate the surface and drain rapidly, instead of moving slowly through the tightly packed clay particles. Traffic simulation research has been carried out on both sand and clay rootzones at the Center for Athletic Field Safety at the University of Tennessee.

Protect yourself with testing

Changes in surface consistency include reductions in turfgrass cover and changes in surface hardness. Surface hardness for natural turf has been traditionally measured using the Clegg Impact Soil Tester (CIST), but the CIST has not been correlated to player safety. Alternatively, the F355-E Missile measures surface hardness and head impact criteria (HIC), and has been correlated to potential human head injuries. This testing device resembles a human head, as the missile has a curved bottom and weights approximately ten pounds. Other missiles used for testing have flat bottoms that do not resemble the impact a human would have when the head strikes the turf surface. The critical fall height (CFH) is identified by 1000 HIC, where 16% of the population would suffer a traumatic head injury.

The experimental area was established hybrid bermudagrass grown on two separate plots, sand and a high-clay soil. Eight simulated traffic events were applied weekly to the separate plots for a total of 5 weeks (40 traffic events). Each traffic event replicates the amount of traffic a football field receives in the highest wear areas (between the hash marks and inside the 40-yard lines). Performance tests were carried out once per week, at the end of each eight-game cycle. Percent green turfgrass cover was analyzed before and after each traffic simulation using digital image analysis. Soil volumetric water content was collected five times weekly to verify moisture fell in the ideal ranges for each soil type as identified by Dr. Kyley Dickson at the University of Tennessee. Three drop heights were used when testing with the F355-E Missile to simulate possible heights (1.3, 1.9, and 2.5 meters) from which an athlete’s head may fall during a practice or game.

A fall from 2.5 m on the native soil always had a HIC value above 1000. The sand rootzone was more predictable in terms of HIC as a result of the sand reaching a point where it could not be further compacted resulting in a firmer rootzone. After the bermudagrass canopy was destructed, revealing sand, surface hardness testing results became similar for the remaining weeks. The CFH for native rootzones shortens as the surface becomes compacted at a much faster rate than sand due to the ability of clay particles to compact, which results in a denser soil after traffic is applied to athletic fields.

Athletes must have a safe playing field to be able to perform at a high level for their entire career. An athletic field is at its best for only one game each season; before any games have been played on its surface. Throughout the length of a full football season, a field’s safety characteristics change in all types of rootzones. The data we are collecting will be useful to field managers, coaches, parents, and athletes to provide an estimate of when the turfgrass surface becomes unsafe during the playing season. This data will also give field managers the knowledge they need to limit play on their fields when they become unsafe. This will reduce the need to completely replace a natural grass field since the field is not being completely destroyed by the players during a season.

At this time, cultivation techniques can be performed to make the athletic field safe, and playable once again. Turf managers should periodically performance test during the season to ensure safe playing conditions at all times. Future research can be done to use this test procedure for other sports such as soccer or rugby.


Taylor Williams is a Master’s candidate in the Plant Sciences Department at the University of Tennessee in Knoxville; John Sorochan, PhD, is a Distinguished Professor at the University of Tennessee in the Plant Sciences Department and heads the Center for Athletic Field Safety in Knoxville.

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