Traction on turf
With the introduction of synthetic turf in the 1970s, concern over injuries resulting from the condition of athletic-field playing surfaces became more intense. Researchers began recording the number of injuries occurring on synthetic- and natural-turf playing fields. Results of these studies were highly variable because playing-surface quality is affected by many factors. Natural-turf playing-surface quality depends on soil texture, soil density, soil-water content, turfgrass species, cutting height and level of wear.
In one study at the Pennsylvania State University, researchers examined 12 high-school football programs. They found that 21 percent of the recorded injuries in games and practices were either definitely or possibly related to the playing surface. A follow-up study showed that athletic-field managers could influence playing-surface quality through management practices that affect soil-water content, soil density and turf cover. Good maintenance practices and better soil and turfgrass conditions provided softer fields. Drier soil, high soil density and thin turf cover resulted in hard surfaces, while core-cultivated fields were noticeably softer than non-cultivated fields. Practice fields were harder than game fields, and high-wear areas were harder than low-wear areas.
Measuring playing-surface quality To make playing-surface and injury data useful, it is necessary to characterize the playing surface in question. Assessing playability and injury potential requires a quantitative method of evaluating playing surfaces that accounts for complex player-to-surface interactions. An athlete interacts with a playing surface in two general ways: falling on the surface and player-to-shoe-to-surface interactions. Surface hardness and traction affect these interactions respectively.
* Hardness. The ability of the surface to absorb impact energy created by a player is its hardness. Playing-surface hardness can affect both player performance and player safety and should not tend to either extreme. A soft field may create early fatigue in the player's leg muscles, while hard fields can be dangerous in impact situations.
A device that measures impact absorption is the Clegg impact tester. This has become a useful tool for evaluating the surface hardness of athletic-field playing surfaces. Field managers, technical representatives and consultants employ this device to demonstrate the usefulness of particular maintenance practices and construction designs.
In Europe, some researchers have suggested that managers use a system to evaluate soccer-field playability that relies on hardness and traction testing. The researchers developed the system using the Clegg impact tester to measure field hardness. To measure traction, they mounted cleats on a steel disk, weighted the disk with barbell weights and rotated it with a torque wrench. The researchers performed these tests on numerous fields prior to games and then questioned the athletes about field conditions. Although extremely broad, the European soccer system provides the first quantitative standard for athletic-field playability. However, these numbers say little about the safety of a field, and a more detailed understanding of surface traction is necessary before researchers can suggest even general safety standards.
* Traction. The effect a playing surface has on surface-to-shoe interaction is termed footing. An athletic-field surface should provide a level of footing that benefits the player's actions without causing excessive stress to joints or ligaments. The term refers to surface-to-shoe interactions of both smooth-soled and studded footwear. More specifically, the term friction applies to smooth-soled footwear while traction is relevant to footwear having studs, cleats or spikes to provide extra grip. Because of the danger of knee and ankle injuries to athletes wearing cleated shoes, surface traction should be a primary concern of athletes, shoe manufacturers and field managers. Researchers have developed various methods to measure the factors relating to traction. Differences among these methods include the shoe-sole surface material, vertical force-or loading weight-and whether the test method measures traction as the shoe sole rotates or with linear movement across the playing surface. The method that can best simulate the interaction of an athlete's foot in contact with the surface should provide the most meaningful measurement of traction.
At Penn State, we built a device that measures both rotational and linear traction using actual athletic footwear and realistic loading weights (see photo, above left). We named it Pennfoot. We measured traction under various conditions and found that traction values differed by turfgrass species, cutting height, turf density, soil-water content and shoe type. Following are our findings on each of the variables.
Turfgrass species We looked at the traction characteristics of four cool-season turfgrass species: Kentucky bluegrass, perennial ryegrass, red fescue and tall fescue. In this study, Kentucky bluegrass and tall fescue had greater traction values than red fescue and perennial ryegrass.
Although tall fescue had an average traction value similar to Kentucky bluegrass, traction on tall fescue was more variable than on any other species we tested. This may be due to the clumping growth habit of tall fescue, suggesting that maintaining stand density and uniformity for tall fescue maybe even more important to providing consistent traction than it is for other species. This is a notable problem with tall fescue because of the tendency of the plants to tiller in response to injury. Under heavy use, tall-fescue turf may become clumpy (see photos, below) and, possibly, more dangerous to athletes. Thus, tall fescue may not be the best choice for heavy-use fields. Rather, use it in softball outfields and general-use turf areas that receive less concentrated wear.
While tall fescue and Kentucky bluegrass had similar traction values over typical ranges of soil-water content, wet soil conditions affected Kentucky bluegrass less than other species we tested (See "Traction of turfgrass species," page C 18). This may, in part, be due to the rhizomatous growth habit of Kentucky bluegrass.
Perennial ryegrass and red fescue had significantly lower traction values than Kentucky bluegrass and tall fescue. However, relative to other species, perennial ryegrass establishes quickly, providing good early wear resistance. This compliments the traction and recuperative ability of Kentucky bluegrass. Thus, perennial ryegrass-Kentucky bluegrass mixes are popular with many athletic-field managers.
Bermudagrass is a favorite warm-season species of many field managers, especially those of sand-based fields. Bermudagrass possesses high recuperative ability, and its stolons and thatch add stability to the naturally unstable sand, creating a good base for cleat penetration. Zoysiagrass also has favorable traction qualities, as well as excellent wear resistance but recuperates from injury more slowly. Turfgrass breeders are working toward improving zoysiagrass in this respect.
Unfortunately, we still have a great deal to learn about the differences among turfgrass species. This is partly due to a lack of research but also because so many other factors affect traction. Therefore, it is difficult to determine which effects result from the turfgrass variety itself. Clearly, we need more traction research on turfgrass species and varieties, as well as the variability that exists from area to area on athletic fields.
Cultural practices * Cutting height. Athletes often express the opinion that they experience greater traction on lower-cut turf. Some speculate that longer grass blades interfere with cleat penetration, leading to a reduction in traction. We found that maintaining turf at a cutting height of 1.5 inches resulted in consistently higher traction than turf maintained at 2.5 inches, regardless of species. However, we also measured traction on tall fescue before and after shaving off the leaf tissue and we found no difference in traction values relating to the presence or absence of leaf tissue. Thus, it appears that traction relates more to turf density than simply to height of cut. Turf maintained at a lower cutting height has higher density than turf maintained at a higher cutting height. Remember, however, that excessively short mowing weakens the stand and dramatically lowers turf density and traction. Thus, close mowing is not a guarantee of higher traction and turf must be high enough to sustain good turf health and density.
Step down your mowing height before the start of the playing season early enough for the stand to adjust its density at the new mowing height. For example, if you have been cutting turf at 2.5 inches, you will not increase traction by mowing at 1.5 inches just prior to a game. Further, the shock to the plants probably will increase the chances of plant injury and thinner turf. Obviously, you should avoid this because exposed soil and thin turf provide the worst traction of all.
A common practice for fields that receive little summer use is to incrementally raise mowing height as summer approaches. It is believed that this practice helps turf tolerate summer heat and drought. However, for football the turf must be stepped down in August, a period of little growth for cool-season turf. This adds an additional stress to the turf at the same time that play begins. Therefore, I recommend that football-field managers pick a mowing height and stick with it throughout the year.
* Soil and water. Soil quality affects traction several different ways. For instance, soil that is too wet provides little stability, but soil that is dry tends to inhibit cleat penetration. Thus, drainage properties greatly affect traction. Another example is sandy soil, which does not have the integrity of more finely textured soils. Anyone who has spent time at the beach already understands that the best traction is not in dry sand but on the moist sand at the water's edge, where the sand is firmest. By drying out their sand-based fields before a game, some field managers actually reduce traction and increase divot potential. This illustrates the need for turf managers to understand their soils.
Not only does soil affect traction directly (with penetrating cleats, for example), it also affects anchorage for turfgrass roots. An interesting example of how this can affect traction occurred on the practice fields of the San Francisco 49ers. Rich Genoff, the San Francisco 49ers' sports turf manager, relates that when these fields were Kentucky bluegrass, it was difficult to keep out annual bluegrass. Because these are sand-based fields, foot traffic easily dislodged the weakly rooted annual bluegrass, leaving exposed soil and poor field conditions.
The 49ers have since replaced the Kentucky bluegrass with bermuda-grass, and this seems to have solved the problem. Bermudagrass' dormant period allows Genoff to apply more aggressive weed-control measures against the annual bluegrass. Genoff keeps the bermudagrass mowed at 1 inch; any lower than this, he says, results in excessive turf damage.
Fortunately, soil-moisture levels favorable for growing turf are usually within a range that provides good traction as well. In fact, most management practices that encourage turf health have a positive effect on traction, either directly or indirectly. Proper irrigation and fertility management increase turf vigor and recuperative ability; aeration decreases soil compaction and aids water infiltration; overseeding promotes good stand uniformity and density. This leads to a point of major importance: Regardless of variety, healthy turf provides higher traction than a weak stand. Thus, good cultural practices and well-adapted turfgrass varieties should be the primary concerns of the athletic-field manager who wants to maintain good traction.
Creating a standard Our understanding of playing-surface quality is still in its infancy. We hope standardized tests that correlate well with athletes' subjective assessment of playing surfaces-as well as their actual performance-will emerge. It would then be possible to develop guidelines for proper construction and maintenance procedures to help the athletic-field manager provide a playing surface that maximizes player performance while minimizing the risk of injury.
Andrew McNitt is an instructor in the Department of Agronomy at the Pennsylvania State University (University Park, Pa.).
Athletic-field managers don't often devote a great deal of effort to maximizing traction. They realize it's important, of course, but they typically maintain turf for general health rather than traction. Rich Genoff, sports turf manager for the San Francisco 49ers, manages for overall turf health and gets no complaints about traction from athletes or coaches. Genoff's fields are bermudagrass, mowed at 1 inch.
George Toma, playing-field consultant for the Kansas City Royals, has had similar experiences. The recently installed turf at Kauffman Stadium (the Royals' home field) has drawn no criticism regarding traction from team members, even though the grounds crew does not perform any maintenance to specifically promote traction. Simply maintaining a dense, vigorous, well-rooted turf has provided all the traction the athletes require. The turf there is a mix of Kentucky bluegrass and perennial ryegrass.
In the past, athletic-shoe manufacturers designed shoes without much thought to specific turfgrass varieties or conditions. That is changing, however. The American Standards of Testing Materials (ASTM) committee on sports equipment has formed a task group to look at footwear traction.
The members of this task group represent a diverse range of interests. The group includes turfgrass researchers, artificial-turf manufacturers, NFL Player's Association representatives, ergonomic specialists and various athletic-footwear manufacturers. In September, members of this group met at the Joseph Valentine Turfgrass Research Center at Penn State to compare the traction of various shoes and to compare methods of measuring traction on natural turfgrass. Nike, Converse and Adidas all brought traction testing devices that they have developed.
The group hopes to develop a standard way to measure the traction on natural turfgrass surfaces. Armed with a convenient and reliable method to measure traction, the shoe manufacturers and turf researchers believe they can work together to increase both the safety and performance of cleated footwear and playing surfaces.
Andrew McNitt is an instructor in the Department of Agronomy at the Pennsylvania State University (University Park, Pa.).
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