Sport injuries on synthetic vs. natural turf not as numerous anymore: newer studies
February 11, 2013 By Mike Jiggens
There remains a perception in several circles that synthetic turf
athletic fields continue to contribute to comparatively larger numbers
of sports injuries, but that’s not really the case anymore, those
attending the Sports Turf Association’s annual fall field day were told
in Vaughan, Ont.
“There’s that perception out there that you’re more likely to be injured on synthetic turf than on natural grass, but that doesn’t apply anymore to what we call third generation or infilled synthetic turf because it’s such a different product,” said Thomas Serensits, manager at the Center for Sports Surface Research at Pennsylvania State University.
A number of studies exist that suggest the contrary, he said, but they are outdated and do not include an extensive look at the vastly improved new generation of synthetic turf playing surfaces.
Older generations of synthetic turf that included no infill were problematic, resulting in several cases of lower-extremity injuries to athletes who played on those earlier fields, he added.
Newer studies coming out are taking into account a number of compounding issues associated with sports injuries. Concussions occurring from head-to-head contact, for example, are not necessarily a reflection of the type of playing surface underfoot, yet raw statistics might indicate a greater number of such injuries may occur on synthetic turf than on natural grass. Serensits said concussions resulting from head-to-head contact are just as likely to occur on natural turf.
The number of ankle and knee injuries have risen among high level athletes in recent years—many of which have occurred on synthetic turf fields—but new studies have suggested the increase is largely due to the type of shoe worn, especially by football players.
Serensits said the type of cleats worn have undergone aggressive designs in recent years, allowing players to run and cut quickly, “and they feel like they’re getting a good grab on the surface.”
But the shoes, by design, are actually becoming a hazard to the players who choose to wear them, he said.
“They’re putting themselves at a higher risk for injury than with more of the traditional cleat types.”
The newer type of shoe, which positions most of the cleats on the outside edge of the sole, are more conducive to knee and ankle injuries when worn by athletes, new studies suggest. Those who wear the shoe type can plant their foot and turn, but, as the cleat grabs, the built-up torque puts extreme pressure on the knee and ankle.
“The NFLâ€ˆis aware of the fact that these cleats are just too aggressive, and they’re going to start working with the shoe companies to ensure a safer shoe can be manufactured which still performs.”
Serensits said there are currently 14 different studies which compare injury rates on synthetic turf vs. that on natural playing surfaces. Nine of the studies are specific to soccer with most being conducted in Europe. They span the gap from children to professionals, and in both game and practice situations. Four of the studies are specific to football, with one dedicated to high school athletes, two to the college level and the other to the National Football League. The remaining study focuses on rugby.
“In football, we’re starting to see some conflicting results,” Serensits said.
One of the college-level studies found there was an overall lower risk of injury from play on synthetic turf fields, yet the NFL study reported knee and ankle injuries were more common when playing on synthetic turf.
The soccer studies found there was no discernible difference in injury rates or severity between the two types of playing surfaces.
“There were some differences in injury patterns,” Serensits noted.
Injuries caused by abrasion, for example, were more numerous on synthetic fields, according to the high school-level football study. But certain types of injuries were more common on natural turf fields when the playing surfaces were dry.
Serensits said the studies, in general, tend to lump all synthetic turf fields together and all natural grass fields together without variations taken into consideration.
“These studies really don’t make any account to differentiate between those two, so it can be a little tricky to understand the results.”
There isn’t as much variation among synthetic fields as there is with natural surfaces, but there are some differences, he said. About 10 to 20 per cent of all concussion injuries in football, for example, occur as a result of the head’s impact with the playing surface. One study suggested there was a higher concussion incidence on natural grass when the surface was dry and presumably harder.
Even though the football studies presented conflicting data, the soccer studies were more consistent, suggesting there was no real difference between the two types of playing surfaces and their correlation with sports injuries.
Serensits said there is new research underway in which cadaver legs are being used to measure the amounts of strain on different ligaments, including the anterior cruciate ligament (ACL). The study also allows for the testing of different athletic shoes and their association with injuries. He said the research will provide more realistic data, but it is still somewhat flawed because living subjects aren’t being studied.
Surface hardness is tested by the Gmax method, or the ratio of maximum negative acceleration on impact in units of gravities to the acceleration due to gravity. When a field becomes hard, there is a higher potential for head injuries. Gmax is the maximum point on a curve of G force.
“How many Gs can the surface absorb and how much is returned to the athlete?”
A paved parking lot, for example, will have a much higher Gmax than a pillow. The testing for surface hardness is accomplished by use of an ASTM (American Society for Testing and Materials) F355 device. The number 200 is considered the threshold.
“Anything above 200 Gs is when a life-threatening injury can be expected to occur.”
If a field is tested and its reading is higher than 200, some serious remediation steps must be taken to bring the field back into compliance with the threshold, Serensits said.
Every NFL field is tested, and games cannot be played on the field that week unless is passes the test. Gmax testing with a Clegg hammer is an important part of the test and is done 72 hours prior to a scheduled game. The field manager must go out to a number of prescribed spots on the field and use the hammer.
“If the field is out of compliance, they need to remediate that before the game can be played.”
Serensits said remediation on a synthetic turf field could include additional crumb rubber infill.
“Interestingly, with Gmax, there is no minimum standard, which tends not to be a problem, but, if the field is too soft, there is the potential for some ankle injuries or things like that and also potentially a decrease in performance.”
He compared that possibility to running on a beach as opposed to a harder surface.
Penn State is home to a large synthetic turf program, and different manufacturers installed small test plots where a battery of tests were conducted between 2003 and 2010 for Gmax readings and other data. Some of the test plots had wear while others had no wear. Half would be tested with simulated game-type exposure while the others wouldn’t.
A common question asked is, “If a field gets hard, why does it get hard?” Serensits said the assumption has always been that it gets compacted, especially if it’s a native soil field, but it’s not quite the same with synthetic fields. There is a “settling in” period after a synthetic field is installed, but the rubber is uniform in size and doesn’t become as much compacted as a natural field.
Paint buildup on a synthetic field can be a contributing factor toward surface hardness, he said, noting that if it’s not washed off on a regular basis, it can gum up the infill and harden the surface.
“That’s something we’re trying to communicate with the NFL’s field managers and anyone else who has to paint their fields a lot. It can be a problem over time with surface hardness.”
Research at Penn State suggests much of the surface hardness problem is the result of “walkoff rubber.”
Serensits said when a field is installed, there is a certain level of infill depth which, over time, becomes lesser and lesser. The fields lose little bits of rubber when it gets caught in shoes or is lost through surface grooming or snow removal. As the depth of the infill diminishes, the field becomes harder. Goalmouth areas for soccer and lacrosse become particularly hard because they face the most play and depths becomes lower.
It’s important to know from the manufacturer what the target infill depth is supposed to be. Data collected from NFL fields suggests each one needs to add rubber. Ideally, for a 2 1/2-inch system, infill depth should be 42 or 43 millimetres, Serensits said.
If additional rubber is needed for an isolated spot such as a goalmouth area, buckets should be used to spread the material and then it should be brushed in a little at a time until the desired depth is achieved, he recommended.
“You do want to try to use the same infill as was originally installed with the carpet,” Serensits advised, adding it will avoid sizing issues.
For larger areas, a topdresser may have to be used to broadcast the rubber.
“The bottom line is, you want to make sure you have some extra crumb rubber on hand at all times for fixing things like this.”
Another issue associated with synthetic fields is the incidence of staph infections among athletes. The assumption had always been that synthetic fields were the cause of such infections whereby athletes would fall to the turf, bacteria would set into the surface, and staph infections would result.
Most staph infections aren’t serious and can easily be treated, but more severe strains can lead to serious health issues, including death.
“The question becomes, is synthetic turf really harbouring this bacteria? Do we need to be spraying anti-microbial treatments on a regular basis?”
Penn State researchers conducted a study of both indoor and outdoor synthetic fields during a summer season at various locations throughout Pennsylvania, collecting fibre samples in Petri dishes.
“We tested for all bacteria and specifically for staph bacteria.”
No staph bacteria was found on any of the 20 fields tested, yet it was found on the hands and faces of randomly-sampled students in the campus hallways. It was also found on such equipment as blocking pads, stretching tables and weights, suggesting the tested athletes were being exposed to staph, but not from the turf. Because locker rooms tend to be humid and aren’t disinfected on a regular basis, the chances of one being exposed to staph are greater there than they are from the field itself.
Serensits said researchers also wanted to know how long staph would survive on turf if someone who had it fell onto the surface. A high concentration of staph bacteria was mixed and put onto the turf where it was tracked over time. The staph was treated with anti-microbial sprays as well as Tide liquid detergent on both the indoor and outdoor fields, and the bacteria’s survivability was tested each hour. On the outdoor fields, researchers couldn’t accurately tell how well the treatments worked because the bacteria had died quickly.
“Within three hours, all the bacteria we had put out there was dead.”
Serensits said the treated fields proved to be an inhospitable environment for the bacteria, adding staph prefers to be on human skin where it likes the temperature and moisture. Synthetic turf typically has little or no moisture. Outdoor fields get hot in the summer and are subjected to several hours of UVâ€ˆlight each day.
Indoors, the bacteria was found to survive for several days, and the Tide detergent was just as effective in treating it as the anti-microbial sprays.
Perhaps the biggest issue associated with synthetic turf is its surface temperature, Serensits said. It’s not the black rubber infill which drives up surface temperatures because the first generation fields—often called Astroturf—had no infill yet got just as hot, he said. Instead, it’s a combination of the infill and the fibres which account for its high surface temperatures.
For the playing surface to get especially hot, a sunny, clear day is required.
“A sunny, clear day with a lower temperature will give you higher surface temperatures than a lot hotter day with high humidity and haze. You need that direct sunlight that isn’t filtered by any haze or humidity, and have no clouds.”
When temperature data is collected, a bright and sunny day will produce high temperatures on the synthetic playing surface, but, if a cloud passes by and blocks the sun, the temperature will drop significantly within a few minutes, Serensits said.
“It heats really quickly and drops really quickly.”
To test the theory that black crumb rubber might be the cause of a hot playing surface, various other colours of rubber were tested as well, and only a few degrees separated the different colours.
“What that means is the fibres are just as much a contributor to that surface heat buildup.”
Supporting the assertion that it’s not the black crumb rubber which greatly contributes to high surface temperatures is the fact that second-generation turf became extremely hot as well, and its infill was sand.
Natural grass, on the other hand, has its own air conditioning system built in with the plant transpiring and releasing water vapor around itself to keep it from getting too hot.
“We don’t have that luxury with synthetic turf.”
There are band-aid methods to lower the surface temperature of synthetic turf, but they are only short-term solutions and can trigger other issues. The fields can be irrigated, for example, to help lower the temperature, but the effect is short-lived and the playing surface can remain wet for long periods of time with the added moisture building up humidity levels above the surface.
Irrigation is OKâ€ˆin the short term, Serensits said, “but it’s not the answer.”
Currently, there is no solution to keeping surface temperatures of synthetic turf in line with those of natural grass. Serensits said the answer could lie with the construction of fibres, but chances are it will result in a spike in the cost of the product.
If there was a way to keep the top layer wet consistently, without compromising the field’s playability, that might work, he said.
Calcine clay was tried as a topdressing material because of its ability to hold water longer and keep temperatures down for three or four hours, but the problem is it breaks down and had turned to powder during testing of simulated field use. It was believed that, over time, the calcine clay would hamper drainage.
To avoid exposure to excessively hot surface temperatures, sports teams might consider using the fields either in the morning or early evening, Serensits suggested.
There has been some concern about exposure to harmful chemicals and volatile organic compounds which are used in tire production and can cause eye, nose, throat and skin irritation. Studies show no elevated health risks with indoor fields are associated with the inhalation of crumb rubber particles. With outdoor fields, studies show there is no difference in the air quality above the playing surface compared to that of the air above a parking lot or other areas away from the field.
Studies looking into the effects of skin contact found no increased health risk. The only potential problem is that some people may be allergic to latex, but, otherwise, there have been no documented risks in terms of skin contact with rubber infill.
Various other health-related tests were conducted such as pH levels in urine, the effects of ingestion and cancer risks. A group of high school football players, who played and practised on infilled synthetic turf, were tested for pH levels in their urine. No elevated levels of pH were detected, demonstrating that it wasn’t getting into their system.
No elevated health risks were found among children who may have ingested any of the infill. Based on one gram, there were no health concerns, and heavy metal contact complied with typical toy standards.
“Unless someone’s really chowing down on it, there are no real concerns,” Serensits said.
Compared with other common human activities, there were no elevated cancer risks associated with exposure to synthetic turf.
“There’s really nothing that sticks out to point that you have an increased cancer risk from playing on synthetic turf.”
With regard to the turf fibres themselves, there was a scare which occurred a couple of years ago in New Jersey when high levels of lead were discovered in some of the fibres on a field. But it was learned the field in question was a first generation, non-infilled surface. The incident, however, precipitated studies into the current generation of synthetic fields which contain little to no lead. It’s been deemed the absence of lead in today’s fibres make the surfaces safe to play on and to install. Today’s manufacturers, in fact, are diligently trying to remove any trace of lead from the fibres.
“The presence of chemicals alone is not cause for concern,” Serensits said. “Many of the levels found were comparable to background levels.”
Questions being asked about potential environmental concerns include:
• Are there heavy metals leaching from the crumb rubber into water sources?
• What’s the danger to aquatic life?
• Are there organic compounds being off-gassed?
• Is there particulate matter in the air?
To help answer these questions, researchers looked at different kinds of crumb rubber and simulated precipitation in the lab to replicate worst-case scenarios. Field studies were also conducted.
“What they found was the metal of most concern was zinc, and that was in truck tires only.”
Car tire rubber was tested as well as other sources.
Although the zinc levels from truck tire sources were a little higher than what is regarded as typical, they were still in compliance with accepted standards.
Arguably, the greatest environmental concern about synthetic turf is its eventual disposal once its lifespan has been realized.
“The issue with disposal is the one we’re really going to see now,” Serensits said.
The earlier generations of synthetic turf have reached the end of their life expectancies, but there remains no large-scale plan in place for recycling the turf.
“We really don’t want to put them in landfills, but that’s, unfortunately, where the majority of them are going right now. In terms of environmental concerns, hopefully this is something that will be addressed sooner than later.”
Aside from the health and environmental concerns associated with synthetic turf, there is the matter of performance, and studies continue to be done to assess this all-important issue. One such study involves traction on the fields.
“Traction is that balance between safety and performance. We want to maximize a player’s actions without causing excessive stress to the joints or ligaments.”
Traction is measured in two ways: linear traction, or pushing off the back leg, which is related to performance and change in direction; and rotational traction which examines the movement of an athlete’s foot. Does the foot stay in place or does the cleat release as he’s twisting?
“Any sort of resistance is not a good thing,” Serensits said, adding a point has yet to be reached where safe levels or thresholds can by measured.
Tests are being conducted to see how fibre breaks down over time. Serensits said a sophisticated computer program allows for the equivalent of 15 years’ use to be simulated on synthetic turf over the course of only three or four days.
A survey of 1,600 amateur soccer players in Italy largely found the synthetic field to be their preferred playing surface, outscoring natural turf fields in eight of nine categories. Natural turf won out in only one survey category—that it is less likely to contribute to skin abrasion. Serensits, however, said the survey might leave some of the opinions open to interpretation as many of the athletes surveyed would otherwise play on hard pan natural fields covered with little or no grass.
A survey of the NFLâ€ˆPlayers Association in 2010 found that 82 per cent of the league figured synthetic turf fields are more likely to contribute to injuries, and 69 per cent preferred to play on natural fields.
“Anytime you see natural grass fields being compared to synthetic turf, you’ve got to keep in the back of your mind, ‘What is the condition of that natural grass?’”
Natural grass playing surfaces can range in quality from high-end fields used sparingly during the course of a season to amateur-level fields subject to smaller maintenance budgets which undergo a lot of play and are worn easily.
Serensits said sports turf managers who are contemplating the installation of synthetic fields must realize that they are not maintenance-free.
“Sometimes people get these fields and think, ‘I don’t need to do anything to them. There’s no maintenance required. We put it in and we’re done.’ That’s not the case by any stretch of the imagination.”
Sports turf managers must also maintain a balance between safety and field performance, Serensits said, noting a somewhat soft surface is important for player safety, “but you don’t want to be playing on a field of pillows because you won’t be able to play the game.”
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