The debate about the sliding friendliness of synthetic turf will soon become more scientific with the introduction of a test apparatus and adoption of a testing protocol to establish the sliding characteristics of the turf. The new device will also help sports governing bodies in assessing new turf products and infill materials.
Ever since the introduction of synthetic turf, skin injuries have dominated the debate about whether it is the right or wrong surface for a particular sport. Arguments were usually accompanied by horrible stories and horrendous pictures showing skin abrasions or carpet burns. With the number of synthetic turf rugby fields growing and player welfare being a priority for World Rugby, the rugby governing body commissioned a PhD study to develop an understanding of skin interactions with synthetic turf surfaces and to develop a better test method and apparatus for assessing the likelihood of skin friction injuries on surfaces. All knowledge obtained should also allow World Rugby to improve regulations for quality surfaces accepted at World Rugby standards, to improve the perception of synthetic turf by athletes as well as reduce player skin injuries in the future. The three-year study was awarded in 2019 to testing institute Sports Labs and Cardiff University.
New testing rig
Earlier this year, Sports Labs revealed the testing apparatus they have developed. The new, six-metre long rig they built, has a 3D printed knee-shaped impactor that is wrapped in Lorica Soft. This synthetic leather provides similar frictional responses as in vivo skin. A linear induction motor rapidly accelerates the carriage to which the impactor is attached, up to the desired velocity of 5m/s by utilising electromagnets. Vertical impact velocities are generated by allowing the impactor to free-fall through a range of release heights to produce a maximum velocity of 3.85 m/s. “The design of the impactor, the angle with which it hits the surface and the speed with which this is being done, all resemble an rugby player falling on the turf. We have simulated an elite rugby player – these players are faster and heavier than amateur players, therefore, they are more likely to experience larger forces when interacting with the ground and be more susceptible to receiving a skin injury,” Cardiff University graduation candidate Max MacFarlane, who conducted the study, explains. “By assessing surfaces in the worst case scenario in terms of forces, this should make all players safer.” According to Marc Douglas of World Rugby, the new rig has all the potential to replace the Securisport test device that is currently being used. “The Securisport cannot really represent a player in motion as it doesn’t accurately simulate an authentic impact at realistic speed,” he says. “Furthermore, the repetitive circular sweeping action quickly influences surface condition which means the results are not representative of the specified turf system.”
Different carpets, different results
There are many aspects that play a role in how human skin responds to being dragged over a surface. To assess the risk for skin injuries on a particular synthetic turf, the apparatus establishes the abrasion zone, the coefficient of friction, the gradient of sliding velocity, the energy dissipated during the interaction and the change in temperature. “We use accelerometers, IR thermal camera and an imaging thresholding technique to quantify the abrasion zone. All contribute to the final result,” MacFarlane says. Results show that a skin injury is a mixture of carpet burn and abrasion. “First results indicate that the free pile height and stitch rate do have an influence. “The size of the affected skin area is much smaller for a 60mm carpet compared to a 50mm carpet. The same applies to carpets with 60-15 stitch rate compared to carpets with 60-12 stitch rate: the affected area is approximately 20% smaller on surfaces with a higher stitch rate.” However, Eric O’Donnell of Sports Labs points out it is unjust to claim that the shorter the free pile height, the bigger chance one has for getting a skin injury. “Interestingly enough, the shortest free pile heights showed a medium result while a surface with the medium free pile height posed the biggest chance of getting injured,” he says. The shape of the yarn itself didn’t matter. “We haven’t seen overly complex yarns yet. Most rugby carpets that we have tested use monofilament fibres. These fibres are favoured by the tufting companies, as they tend to make the surface more durable.” Sliding against the grain of the carpet showed a higher chance of an injury. In their study Skin tribology in sport (see sidebar), Max MacFarlane and Dr Peter Theobald explain that this is caused by the way the collagen fibres and Langer’s Lines, that form the skin, are organised.
Remarkable results from non-filled
Sports Labs has now moved to testing as many carpets as possible to build up a database. This also includes synthetic turf used in other sports. Widening this cope has yielded a remarkable result. The abrasion zone on non-filled carpets turned out to be the smallest of all surfaces tested. “Compared to the 907.9 mm2 on a 60mm or 1314.6 mm2 on a 50mm carpet, the 70.2 mm2 abrasion zone on a non-filled was significantly smaller,” O’Donnell continues. However, in terms of COF and Energy dissipated, both are significantly higher on non-filled surfaces than on surfaces that use infill. “The larger results from accelerometer data would suggest that the simulated player would be at a higher risk of injury on a non-filled, compared to a filled surface.” It is evident that the hardness, length and shape of the infill particles in third-generation synthetic turf surfaces play a role in whether or not a falling or sliding player gets injured on the surface. This result has surprised Marc Douglas of World Rugby. “We suspected the fibre to be the issue and that crumb rubber would be the lubricant because it rolls. This seems not necessarily to be the case,” he admits. Douglas points out that non-filled turf has not been approved for rugby yet. “We need to have an amount of traction available to players when undertaking scrums and mauls. Also, the surface needs to be stable enough to allow players to do these actions effectively and with minimal risk. This means that the synthetic turf needs to be able to contain the infill and to prevent it from slipping underfoot,” he says, explaining why World Rugby still advises using synthetic turf carpets with 60mm pile height for rugby. “One thing this study has made very clear is that maintenance is absolutely the key to a well-performing surface. We have always maintained that maintenance is required to extend the life expectancy of the surface and to improve the comfort and safety features of the turf. This testing procedure clearly shows that a well-maintained surface is easier and more comfortable to slide on.” O’Donnell adds: “We noticed that the infill would be much more compact after having subjected samples to the Lisport-XL test. As it was less likely to spread or displace on impact, this had an adverse effect on the impactor sliding over the surface.”
The route forward
With all tests performed in the climatised environment of the lab, MacFarlane now wants to establish the performance of the carpet in a hot environment. “The idea is to subject the surface to heat from special incubation lights, before testing their performance in relationship to the skin to establish whether the surface will perform differently in other, warmer parts of the world.”
On the other hand, World Rugby is busy working out a system to classify testing results. “The idea is to use a rating system like we do to describe the likelihood of infill splash from a tested synthetic turf surface,” Douglas says. For the time being, World Rugby will only pursue a pass/fail kind of result. “This allows the industry to revise which products it wants to carry forward.” He points out that the ambition is to include the test in the upcoming test manual. “FIFA and World Rugby intend to update the Handbook of Requirements in 2023 and we both have the ambition to have this test included.” However, including the test in the upcoming manual might be a challenge. “This device can only be used in a lab where we would like to have something that can be taken out to the field for an optional test or as a test to identify if there is an issue with an installed field.” It’s a challenge that has already been picked up by another testing institute. Therefore, one shouldn’t be surprised that establishing the likelihood of a skin injury on a particular synthetic turf surface will soon become mandatory. With a testing apparatus and protocol about to be ready for use, the debate on whether a skin injury is an abrasion or a turf burn will soon be one that will have a final result.
From the study, it has become evident that the human skin is very tough and able to handle a lot. For it to burn, it will need to experience a temperature of up to 60⁰ Celsius for a period of one second minimum. In their study Skin tribology in sport, Max MacFarlane and Dr Peter Theobald explain “that the skin consists of various layers with each layer having unique mechanical properties that contribute to the combined highly non-linear response. Its structural response is dictated by the elastin components, making skin relatively soft. As strain levels increase, the skin rapidly stiffens. The skin friction is governed by adhesion and deformation and the skin coefficient of friction (CoF) is influenced by factors including physiology, the contacting surface, and environmental conditions. Skin hydration, thickness, hair and composition of the surface hydrolipid film all have an influence on the final result.”