Traction Data Collection
For all results in this section it should be noted that this preliminary data is from the first two years of a long term study. These results will be updated as more data is collected.
Translational, rotational, static, and dynamic traction have been defined by Shorten and Himmelsbach (2002).
Translational traction refers to the traction that resists the shoe's sliding across the surface. For the athlete, high translational traction equates to the shoe gripping the surface and low translational traction means the shoe tends to slip.
Rotational traction refers to the traction that resists rotation of the shoe during pivoting movements. For the athlete, high rotational traction equates to a greater tendency for foot fixation during changes of direction and low rotational traction means the shoe tends to release from the surface more easily.
Static and dynamic traction represent slightly different aspects of the shoe-surface interaction. Static traction is the resistance to sliding or pivoting when there is no movement between the shoe and the surface. Static traction forces tend to resist the initiation of sliding or pivoting. Dynamic traction is the resistance that occurs during a sliding or pivoting motion. Dynamic traction forces tend to resist or decelerate pivoting motions.
In this study traction was measured using Pennfoot (McNitt et al., 1997). Pennfoot conforms to the proposed traction standard ASTM WK486 (American Society for Testing and Materials, 2000c) standard traction measurements and is shown in Fig. 19.
Description and Operation of Pennfoot
Pennfoot consists of a frame which supports a steel leg with a cast aluminum foot pinned on the lower end of the leg. We cast the simulated foot from a size 10 foot mold and the foot can be fitted with different athletic footwear (Fig. 20). Two holes located on top of the foot are used for connection with the leg. The first hole located toward the toe allows us to raise the heel off the ground and distribute the weight on the ball of the foot. We took all traction measurements in this study with the forefoot in contact with the surface and the heel of the foot raised off the ground.
Pennfoot allows us to measure rotational or translational traction. For translational traction the linear force is created by a single pulling piston that is connected to the heel of the foot (Fig. 21). The pressure applied to the piston is created with a motorized hydraulic pump and monitored with a pressure transducer connected to a computer. The pressure readings are converted to Newtons (N) by multiplying the effective area of the pulling piston by the amount of pressure required to maintain movement of the shoe. The rate of linear travel is approximately 0.5 m s -1 . Linear traction is thus measured as the amount of horizontal force (N) required to maintain translational movement at the given rate. It is customary to report a traction coefficient as the horizontal force divided by the vertical force. In the primary experiment all traction measurements were taken using a vertical force, or loading weight of 237 lbs. and a Nike Air Zoom high top shoe (Fig 22. original shoe). During 2004, we also tested a standard 7 post shoe (Fig 23. 7 shoe). In 2004, we tested select surfaces in both wet and dry conditions and we measured traction on all surfaces at a lower loading weight (119 lbs.).
When using Pennfoot to measure rotational traction, the rotating horizontal force is created by two pistons which are horizontally mounted on angle iron 38.1 cm above the ground as measured with the machine in position to take a measurement (Fig. 24). We connected a strike plate to the simulated leg for the pistons to push against. A lower collar around the simulated leg prevented it from tilting while the rotational force was applied. For a thorough description of design rationale and construction details of Pennfoot see McNitt et al. (1997).
The experimental design for the primary experiment was a completely random split-plot statistical design with three replications. The split was wear and no wear. Three Pennfoot measurements were taken for each subplot in this study. The means of the three linear and three rotational traction measurements were analyzed using analysis of variance and Fisher's least significant difference test at the 0.05 level. A LSD was not calculated when the F ratio was not significant at the 0.05 level.
Results and Discussion
The following is quoted directly from a study conducted by Shorten and Himmelsbach (2002).
Because of the link between foot fixation and knee injuries, resistance to rotation (rotational traction) between the shoe and the ground should be as low as possible providing adequate translational traction is maintained.
Other studies have postulated a link between higher resistance to rotation and injury rates. While many reports have shown that injury rates are 30 to 50% higher on (traditional) artificial turf (Cameron and Davis, 1973; Henschen et al., 1989; Skovron et al., 1990; Powell and Shootman, 1992; Zemper, 1989) others have failed to find significant differences in injury rates (Clarke and Miller, 1977; Culpepper and Morrison, 1987) One concern with all of these studies is that only surface effects were considered while it is the combination of both the shoe and the surface that is implicated in traction related injuries.
Torg et al. (1978) found that high school football players wearing shoes with shorter cleats had a lower injury rate than those using longer cleats, a difference attributable to differences in the shoes' rotational traction.
More recently, Lambson et al. (1999) studied the relationship between the rotational resistance of shoes and the incidence of anterior cruciate ligament tears among 3119 high school football players. Shoes with peripheral cleats were associated with a significantly higher injury rate, compared with other shoe types.
In summary, there is strong evidence that excessive resistance to rotation at the shoe-surface interface increases the risk of foot fixation and hence of lower extremity injuries. There is also ample evidence that this mechanism contributes to a higher rate of injury among football players playing on (traditional) artificial turf, although this issue remains controversial among parties with interests in artificial turf systems.
For these reasons, we chose to measure both rotational and translational (linear) traction.
Translational and rotational traction values for treatments are shown in Tables 9 and 10.
Table 9. Linear traction determined in 2003 by ASTM traction standard and 237 pounds of vertical force prior to and after grooming 1 .
Table 10. Rotational traction determined in 2003 by ASTM traction standard prior to and after grooming 1 .
During 2003, there were few meaningful traction differences between synthetic turf systems. Traditional Astroturf measured consistently higher in linear traction compared to the infill systems. This trend was not evident in the rotational traction results in 2003.
The 30 Oct 2003 data were collected shortly after grooming had taken place. Translational traction tended to increase after grooming whereas rotational traction tended to have no change or trend slightly lower. This data indicates that, immediately after grooming, an athlete will experience increased translational (linear) traction and either no change or a slight decrease in rotational traction, thus allowing football lineman more traction when pushing but affecting no change or a slight reduction in the rotational foot fixation that Shorten and Himmelsbach (2002) state has a direct affect on lower extremity injuries.
In 2004, there continued to be a trend of increased linear traction after grooming for the no wear treatments but the trend was less evident in the treatments receiving wear (Tables 11 and 12). It may be that as these systems age, grooming will have a diminished affect on linear traction.
Table 11. Linear and rotational traction determined in 2004 by ASTM traction standard and 237 pounds of vertical force prior to grooming¹.
Table 12. Linear and rotational traction determined in 2004 by ASTM traction standard and 237 pounds of vertical force after grooming¹.
During 2004, grooming resulted in a greater reduction in rotational traction compared to 2003 (Tables 11 and 12). This was true regardless of shoe type. The data in Tables 13 and 14 indicate that grooming resulted in a consistent increase in linear traction, measured using a nine post shoe, and a general but less consistent decrease in rotational traction.
Table 13. Linear and rotational traction determined in 2004 by ASTM traction standard and 237 pounds of vertical force prior to grooming¹ using a 7 post cleated shoe.
Table 14. Linear and rotational traction of plots received wear determined in 2004 by ASTM traction standard and 237 pounds of vertical force after grooming¹ using a 7 post cleated shoe.
When traction was measured on the various surfaces during wet conditions traction was generally reduced (Table 11). A lower loading weight resulted in higher traction coefficients for linear measurements. It is not customary to calculate rotational traction coefficients; however, the data in Table 15 was collected using a loading weight that was half that of the data in Table 11. When considered proportionally, the traction values reported in Table 15 follow the same trend as the linear traction values.
Table 15. Linear and rotational traction determined in 2004 by ASTM traction standard and 119 pounds of vertical force prior to grooming¹.
Comparing the data from 2004 to the data collected in 2003, linear traction decreased with age while rotational traction increased. With only two years of data it is impossible to predict if this trend will continue.
Traction measurements were collected on the natural turfgrass areas during 2004 and are shown in Table 16. Comparing traction values using the same loading weight and shoe type, the data indicates similar or lower linear traction and similar or higher rotational traction of the natural turfgrass compared to synthetic surfaces. Very little data was collected on the natural turfgrass as turf and soil conditions fluctuated. The natural turfgrass data is from one rating date and one soil moisture condition.
Table 16. Translational (static and dynamic) and rotational traction 1 of Kentucky bluegrass ( Poa pratentis , L.) in 2004 using tow shoes and two loading weights.