Many people rate the playing quality of an athletic field by its visual appearance. This aesthetic evaluation is generally an effective way of assessing a natural turf playing surface because a surface that lacks a uniform turf cover or is otherwise of low agronomic quality often presents poor playability. The appearance of the sports surface during the season is influenced by the ability of the soil and agronomic practices to support a vigorous, dense turfgrass that is persistent and able to quickly recover following wear. Thus, soil conditions leading to improved agronomic conditions also generally provide improved playing quality.
Along these same lines, athletic fields that are uneven or contain excessive microtopography variation provide lower playing surface quality than a smooth, uniform surface. Often visually apparent, measurement of surface evenness, defined as the standard deviation of closely spaced elevations, is one indication of athletic field quality where standards for different sports have been proposed. Spacing of the elevation measurements range from several inches for soccer, rugby and field hockey to 2 inches for lawn bowls and other low-cut turf. There is a direct connection between playing surface evenness and soil properties, since evenness is affected by subsidence, divot formation due to low soil strength or even erosion. Preservation of a true and even surface through topdressing or renovation is common.
The establishment of playing quality standards
Research over the last 40 years has produced a set of international standards to quantitatively evaluate the playing quality of athletic fields. Most of the playing quality tests were developed by actual player evaluation to set preferred and acceptable limits for each playing quality factor.
Another way to evaluate the playing quality of athletic fields is to compare the number of injuries received on various surfaces. However, the results from these tests are highly variable and present problems in formulating useful comparisons.
Currently there are common procedures that quantitatively measure the playing quality properties of athletic fields. Professional organizations like FIFA, the NFL and MLS are beginning to recognize these standards as a method of ensuring quality play and regulating safety within the sports turf industry. These playing quality requirements vary from sport to sport. For instance, in sports such as U.S. football, soccer and rugby, where the player exerts various vertical and lateral forces on the surface, surface traction is important so the players don’t lose their footing. In other sports, such as tennis, lawn bowling, baseball and cricket, the dominant playing quality factor is the interaction of the ball with the surface. With these latter sports, ball bounce and ball roll are the major factors in assessing surface quality, although interaction between the player and the surface are also considered. Consequently, playing quality falls into two groups: interactions between the player and the surface, and interactions between the ball and surface.
Table 1: Preferred and acceptable limits of surface evenness are specified for several sports played on turfgrass
Interactions between player and surface
Traction and friction: Both traction and friction between the player’s feet and the playing surface enable movement without the player slipping and falling. The standard definition of traction applies to footwear having cleats providing extra grip, whereas friction implies smooth-soled footwear.
If the traction is too low, slipping and falling will occur, and if it’s too high, there is a danger of the player’s feet jarring or locking to the surface and causing injury. Traction is principally influenced by the density, quality and wetness of the turfgrass ground cover. Soil factors influencing traction are texture and water content through their combined influence on soil shear strength. Consequently, cohesive soils having shear strengths dependent on soil moisture yield a high degree of traction when dry and little traction when wet. High sand content rootzones have shear strengths less influenced by moisture, unless quite dry. In this case, shear strengths become negligible and traction is extremely poor.
On a well-grassed sand rootzone, certain turfgrass roots are observed to increase the shear strength by a factor of two to three. The standard shear vane apparatus has been used for measuring the traction of turf surfaces. Alternatively, devices more specific to the action of studded footwear have been used for traction measurements. One such device uses a weighted and cleated plate dropped from a specified height and rotated on the surface. Measurement of torque records the shearing, or breaking point, of the surface in newton meters (N-m). Preferred and acceptable lower limits for various sports are given in Table 2.
Cool-season grasses typically have rotational traction (peak torque) values around 60 N-m if the athlete is wearing a standard 0.5-inch cleat. Bermudagrass peak torque is typically around 75 N-m. Hybrid systems (natural grass seeded into a synthetic base) have greater traction values if the synthetic fiber comes into contact with the athlete’s shoe. In our research, peak torque was 90 to 114 N-m at 0.75-inch cleat depth and exceeded 160 N-m once the cleat extended into the synthetic base at a 1.25-inch depth. Synthetic turf research at Michigan State University (MSU) reported peak torques in the range of 78 to 135 N-m, depending upon shoe type. Clearly, the length of the cleat has a significant bearing on traction values.
Table 2. Preferred and acceptable limits of torque using a studded disc apparatus.
There is currently no recommended upper limit for peak torque, since natural grass was used as the benchmark and that rarely exceeds 70 N-m. Research on cadaver legs suggests that the maximum torque an ankle can support is 75 N-m, and the MSU study suggests that a normal force of 1,000 newtons would generate a theoretically safe torque of 95 N-m. The Australian Football League recently made recommendations of a maximum limit of 50 N-m. What becomes the accepted standard is the U.S. is something that turfgrass research programs are working on.
High sand content rootzones can exhibit poor traction near the end of the season, particularly within areas of high wear and with turf cover less than 15 percent. Preventing the occurrence of low shear strength and traction on these sand-dominated rootzones is a major consideration. In some cases various reinforcement materials have been incorporated into the rootzone to provide additional strength. These materials include a combination of natural grass and geo-fabric material positioned just below the playing surface, rootzone inclusions of interlocking polypropylene mesh elements or individual fibers, polypropylene fibers sewn directly into the rootzone, and interlocking modules or trays. In addition to increasing rootzone strength, reinforcement materials may also reduce soil compaction and improve turfgrass wear tolerance by more widely distributing surface-applied loads.
The Penn Foot-Penn State is conducting research on traction in relation to athletic shoe type.
Hardness and resilience: These affect the playing quality of all sports surfaces. Surface hardness is easily perceived when comparing, say, a concrete surface to one covered by a soft foam pad. Resilience describes the amount of energy that a surface returns to the player on impact. A more resilient surface transfers less energy back to the player and results in player fatigue, whereas a less resilient surface may, upon player contact, cause injury.
On natural turf athletic fields, hardness and resilience are interrelated, since the surface as a whole does not deform, as would occur on a trampoline. Consequently, hardness is the typical measure of both surface attributes. Sports differ in hardness requirements. For soccer, tennis and field hockey, where considerable running is involved, harder and thus less resilient surfaces are preferred for faster play. Contact sports, like rugby and U.S. football, that exert strong impact forces between the player and the surface require lower hardness levels. Player safety is obviously of paramount importance for contact sports.
Ohio State University’s shear vane tester, fitted with cleats and 100 pounds of weight. Developed originally by the STRI.
Preferred and acceptable ranges of surface hardness have been published for various sports (see Table 3). Surface hardness in this case is measured with a Clegg impact absorption apparatus, where a 1-pound weight containing an accelerometer is dropped down a tube from a constant height. The maximum deceleration upon impact with the surface is measured, yielding greater deceleration values for harder surfaces. The unit of measurement is gravities. Synthetic fields are generally tested with the F355 apparatus, which has a similar mode of action to the Clegg hammer. The recommended upper limit for the F355 is 168 gravities (GMax).
The main soil factors influencing hardness are texture and moisture. In cohesive soils moisture plays a dominant role in controlling hardness due to the relation between soil moisture and strength. Hardness values of less than 20 gravities have been observed in cohesive soil when water content reaches about 30 percent. In high sand content rootzones, soil moisture has less effect on surface hardness unless the rootzone becomes extremely dry, resulting in lower unconfined strength and decreased hardness. Thus, proper moisture management is critical prior to sporting events demanding firm, stable surfaces.
Table 3: Ranges for Surface Hardness Measurements (gravities)
Turf cover and the degree of thatch also influence surface hardness, with a denser and thicker turf and thatch yielding reduced hardness levels. Loss of turf cover from excessive wear or a marginal soil environment for roots clearly results in a comparatively hard surface.
Interactions between the ball and the surface
There’s often a positive correlation between measurements of ball bounce and hardness of the playing surface. Consequently, factors influencing hardness also generally influence the extent of ball bounce. In sports where ball bounce is paramount, such as tennis and cricket, elevated clay contents in the surface layer are commonly used to provide a harder soil surface and increase ball bounce height.
Ball bounce resilience is measured by recording the vertical rebound of a ball when released from a fixed height and expressed as a percent of the release height. Rolling resistance is the force acting at the surface in the opposite direction of ball motion causing ball deceleration. Rolling resistance is important in all sports where ball roll is critical, as in soccer, field hockey, cricket and baseball. Rolling resistance is often referred to as speed, wherein less resistance yields a higher speed and vice versa.
Rolling the ball down a standard ramp and measuring the distance rolled is a usual measure of rolling resistance. Alternatively, some studies have used a series of infrared timing gates to record ball deceleration because windy weather conditions can affect ramp measurements. Rolling resistance is influenced by grass height, turf and thatch density and surface evenness.
Zach Willard tests the surface hardness with a Clegg impact hammer.
Agronomic reliability, playing quality and level of use
The principal goals of an athletic field are to be agronomically reliable and provide quality play for a particular level of use, degree of maintenance and climate. Agronomic reliability reflects the density, persistence, health and recovery from wear of the intended turfgrass species in the face of biotic and abiotic stress. Quality of play, while judged relative to measurable performance standards, is also tempered by player and spectator expectation. Thus, expectations are high for televised major sports played at the college and professional level, whereas sports hobbyists drawing few spectators tolerate lower quality playing conditions. It is here that an interesting dichotomy exists where unmodified native soils that hinder agronomic reliability and playing quality often exist for venues experiencing the highest use levels. Further, the budget for maintaining such sports facilities is generally low. Reliability and playability concerns are further compounded for sports, such as soccer and U.S. football, that are commonly played during periods of low evapotranspiration and high precipitation.
On the other hand, the use of sand-dominated rootzones with a soil profile favoring a proper air and water balance is a proven first step to satisfy agronomic reliability and quality play goals. Clearly, close attention is also given to use of the proper rootzone and gravel materials (with quality assurance), proper construction and drainage techniques, a sufficient maintenance budget, some reasonable control over level of use, and luck with the weather. Further, future developments in light technology are expected to refine and enhance the ability of a natural turf area to meet the specific needs of the sport in question.
Pam Sherratt is a sports turf specialist at Ohio State University and served on the STMA board of directors from 2010-2011. Dr. Ed McCoy is an associate professor at the School of Natural Resources, Ohio State University.