JSP

Steven R. Sopher

The design and construction of sports surfaces plays an important role in playability, performance, injury reduction, as well as overall impact management and shock mitigation. Expanded Polyolefin Particle foams are being used to fulfill this role. These include Expanded Polypropylene (EPP) and Expanded Polyethylene (EPE) foams. The properties of EPP and EPE Foams allow for designs which take advantage of the isotropic nature of particle (bead) foams, the highly efficient energy management properties, and the ability to offer fall protection by managing energy absorption and mitigating impact with a combination of compression, flex and tension performance.

The ability to shape mold the material allows for the most efficient three-dimensional and multi-axis design for energy management. It also allows further performance optimization through changes in geometry and changes in density.

As evidence mounts for both the short and long term significance of concussion and the resulting risk of severe brain trauma, the search intensifies for improved methods of concussion prevention and impact mitigation for sports padding. Although the role of sporting equipment – specifically pads and helmets – dominate the industry conversation, one opportunity to reduce risk that is generally overlooked is the role of the sports playing surface, and more specifically the opportunity to minimize head impact and concussion risk by improving the sports surface underlayment.

In a recent American Journal of Sports Medicine study of a nationally representative high school population, 15.5 percent of concussions across multiple sports occurred as a result of contact with the playing surface. An additional 6 percent of concussions were caused by secondary head to turf impact after a player to player impact. This means that 1 in 5 concussions are caused by a head to surface impact. The CDC estimates that between 1.6 and 3.8 million concussions occur in sports and recreation every year. Therefore, contact with the playing surface may account for between 350,000 and 817,000 concussions per year in the United States.

The relationship between an artificial turf field and concussion comes down to the ability of the surface to attenuate impact forces. With a harder surface, less of the impact forces can be absorbed by the material and instead are absorbed by the athlete’s head. In synthetic tuft systems, the utilization of different component types can affect impact attenuation properties, with some component selections offering greater impact attenuation than others.

When comparing and benchmarking the interaction properties of natural grass in terms of running, turning, sliding, friction, and ball behavior are well documented, the performance standards for artificial turf and the underlayment impact padding has developed accordingly. The degree of safety of a natural grass surface has been researched and tested, and is well documented, but performance still varies widely from location to location. However, the safety criteria of artificial turf surfaces (using International Rugby as an example) is expressed in terms of Head Injury Criterion (HIC) and is in fact, roughly one half that of a natural grass surface.

"Expanded polyolefin particle foam offer the optimal combination of lightweight, low cost, with good impact properties and good rebound properties, with the ability to operate across a wide temperature range and isotropic."

The degree of safety of an artificial turf surface underlayment pad is consequential to a surface compliant with the performance properties for a given sport. However, with newer underlayment padding and impact mitigation technologies, optimal surface performance and safety no longer have to be mutually exclusive of one another. The real challenge is to find a way to design a playing surface to reduce the risk of head or other injuries to the greatest extent possible. This can be done by testing the shock attenuation properties of a sports surface to estimate the probability that an impact on the surface will cause an injury. The two test methods and references used to describe the degree of safety of a surface are Head Impact Criterion (HIC) and Maximum G-force (Gmax). Research demonstrates that the HIC is the most accurate measure of head injury potential of a sports surface as it takes into consideration not only the peak force of an impact, but the duration of the impact as well.

Understanding the HIC of a surface is to accurately predict the likelihood of a moderate, severe and catastrophic head injury. Empirically determined relationships between HIC scores and the probability of head injury are widely used as a way of estimating injury risk. An HIC score of 1,000 represents the “safe” limit of human tolerance, above which the risk of fatal head injury is non-zero. However less severe head injuries with long term effects can be incurred at much lower HIC scores. In Figure 1 shown below, each curve estimates a specific level of trauma for a given HIC score. These curves show that at a HIC score of 500 there is a 79 percent probability of a minor injury, a skull trauma without loss of consciousness, or that the athlete will incur a minor concussion. At the same HIC value the risk of a major injury (skull fracture, extended period of unconsciousness) is 13 percent. The risk of a 500 HIC score producing a critical or fatal head injury is very low, but the probability of experiencing this head impact and not being injured at all is only 21 percent. This information is based on the expanded Prasad-Mertz Curves published by NHTSA in 1985. Each curve estimates the probability that an impact with a given HIC score will result in a specified level of head trauma.