For many years, outdoor sports have taken place on natural grass fields. The use of natural grass entails high maintenance costs, increased wear and tear leading to destruction of the surface, vulnerability to varying weather conditions, challenges with drainage, and the necessity to rotate areas of the natural grass playing field as a method of maintenance and damage prevention. A lack of sunlight limits the use of natural grass in indoor facilities. In the late 1960s, the first generation of artificial turf was invented and was soon incorporated into an indoor stadium, the Houston Astrodome. This allowed for year-round play without the labor or resources necessary to maintain a grass field.
Since the initial use of this first-generation artificial turf, named "Astroturf," which consists of a short grass fiber carpet on top of padding over concrete, advances in technology have led to development of the most current third-generation artificial turf. This third-generation turf more closely mimics the properties of grass, containing longer synthetic fibers and an infill made from rubber pellets, sand, or a combination of the two.
The use of artificial turf has grown in popularity in recent years, with more than 3500 synthetic turf fields now in use across the United States. The Union of European Football Associations (UEFA) and Federation Internationale de Football Association (FIFA), the governing bodies for European and International soccer, respectively, have approved the use of artificial turf for competitions (1,7,17,21). Rugby union has approved the use of third-generation artificial turf (17), and it is in use in all levels of American football, from high school to the professional level (3,11,13).
Early studies comparing the incidence of injury on artificial turf versus natural grass demonstrated an increased risk of injury while playing on artificial turf. These studies examined the use of first-generation synthetic turf, characterized by short fibers, high stiffness, and high friction; and second-generation synthetic turf, which had longer fibers, sand filling, and a rubber base underneath the turf, but whose characteristics still differed significantly from natural grass (16). Current research, while limited by a lack of uniformity of both artificial and natural surfaces, has sought to compare injury incidence and patterns of injury between third-generation artificial turf and natural grass.
In a UEFA-funded study, Ekstrand et al. studied the risk of injury in elite soccer players in Sweden. In this prospective, cohort study, the investigators compared 10 soccer clubs that recently had third-generation artificial turf installed against nine teams that played their home games on grass fields. Over a 2-yr period, injury data were collected during match play and training. Using an intra- and intercohort analysis, the study examined the incidence of acute and overuse injuries on the different playing surfaces. They found no difference in overall injury incidence between surfaces within the artificial turf cohort. In the intercohort analysis, the players had a decrease in injury incidence during matches on artificial turf, while the injury rates during training were similar (7).
In two studies, Fuller compared the risk of injury between third-generation artificial turf and natural grass in NCAA men's and women's soccer. One study focused on match injuries, while the other examined training injuries. Using the NCAA injury surveillance system, injury data were collected over a two-season period, comparing injury rates between teams whose home games were played on third-generation artificial turf and teams whose home games were played on natural grass. As with Ekstrand's study, there was no significant difference between the overall incidence of match injuries between the two playing surfaces. During training, there also was no significant difference in injury incidence. The mean and median severity of injuries sustained on artificial turf and grass did not differ significantly. Incidences of mild to moderate injuries (4-28 d missed from training and/or competition) were significantly higher on artificial turf than on grass for men; for women, the incidence of mild injuries (4-7 d missed) was significantly lower on artificial turf. The incidence of severe injuries (greater than 28 d missed) did not differ between the surfaces (9,10).
In a study of U-17 (aged 16 yr or younger) female soccer players in Norway, Steffen et al. examined the risk of all injuries on third-generation artificial turf and natural grass. In this prospective cohort study, involving 2000 players from 119 teams, the investigators recorded injury data during match play and training for one season. As with the previously mentioned studies, they did not find a significant difference in overall injury incidence on either surface (16).
In a recent Japanese study, Aoki et al. compared the incidence of acute injuries and chronic pain complaints on grass and third-generation artificial turf in adolescent soccer players. Each cohort contained players that were between the ages of 12 and 17 yr and spent greater than 80% of their training time on either grass or artificial turf. The researchers found no significant difference in the incidence of acute injuries experienced on grass or artificial turf during training or competition. They also found no significant difference in the incidence of chronic pain complaints during training between the two surfaces (2).
Myers and Barnhill studied the difference in the injury incidence on the two playing surfaces in American football. Over a 5-yr period, they evaluated eight Texas high schools, documenting game-related injuries. They reported a greater number of injuries occurring on third-generation artificial turf compared with grass. This may be from the significantly higher incidence of minor injuries (injuries that resulted in zero time loss) that occurred on artificial turf compared with natural grass (13). In this study, counted injuries included any reported to or treated by an athletic trainer or physician, regardless of the amount of time lost. The previously mentioned soccer studies defined injury as the inability to take part in training or competition for at least 1 d (at least 1 wk in the Japanese study), omitting minor injuries that resulted in less than 1 d lost.
While many of the studies do not reveal a significant difference in overall injury incidence, they do note differing injury patterns occurring between grass and artificial turf.
Artificial turf has been associated with a higher incidence of ankle injuries. The incidence of ankle sprain was significantly higher on artificial turf in elite Swedish soccer players, and there was a trend towards a higher ankle ligament injury rate on artificial turf in U-17 female soccer players (7,16). Ankle injuries were the most commonly reported injury on artificial turf in male and female collegiate soccer players during training, with ankle ligament tears being the most common season-ending injury for men during training on artificial turf (10). In match play on artificial turf, there was a small, nonsignificant increase in ankle ligament injuries for men, whereas women experienced a significant reduction in ankle ligament injury (9).
Playing on a grass surface appears to be associated with a higher incidence of knee ligament injury. In high school football players, the grass surface produced a higher incidence of anterior cruciate ligament (ACL) injuries (13). Among male collegiate soccer players, the most common season-ending injury on grass in both training and matches is the ACL tear. This is in contrast to turf, where ankle injuries and hamstring tears account for the most season-ending injuries in training and match play, respectively. In female collegiate soccer players, ACL tears are the most common season-ending injury on both surfaces (9,10). In contrast, there was a trend toward a higher incidence of knee injuries on artificial turf in U-17 Norwegian female soccer players (16).
Muscle injuries also appear to have differing incidences on the two playing surfaces. In elite soccer players, fewer lower extremity strains occurred on artificial turf; however, artificial turf accounted for a higher incidence of muscle strains in American high school football players (7,13). While muscle and tendon injuries formed a much higher proportion of the training injury burden than they did for match injuries in collegiate soccer players, there did not appear to be a difference in the injury rate between the two playing surfaces (9,10).
There was a significantly higher incidence of concussion on artificial turf for male collegiate soccer players. However, player-to-player contact accounted for each one of these concussions. During match play, there were no concussions caused by player-to-surface contact on turf; on the other hand, 13% (5/38) of concussion injuries sustained on grass by men and 7% (6/81) by women occurred because of player-to-surface contact. During training, only one concussion (out of 14 total) on turf was caused by player-to-surface contact while 18% (4/22) of concussion injuries sustained on grass by men and 10% (3/30) by women occurred as a result of player-to-surface contact (9,10). In American high school football players, there was a significantly greater incidence of concussions seen during competition on natural grass (13).
In the only study to include minor injuries in its analysis, there was a higher incidence of surface/epidermal (abrasion, laceration, puncture wound) injuries on artificial turf in American high school football players (13). This finding is consistent with survey results from American professional football players and Italian amateur soccer players, who judge the risk of abrasion to be higher on artificial turf than it is on grass (11,21).
In the study of elite Swedish soccer players, there was no difference in the rate of overuse injuries between the teams whose home games were played on artificial turf versus those whose games were played on natural grass (7). In the Japanese study of adolescent soccer players, they found no difference in chronic pain complaints between artificial turf and grass. However, there was a significant trend noted in the number of hours of training that took place on artificial turf and the incidence of chronic pain complaints, with low back pain being the most common complaint (2).
The shoe-surface interface is a focus of much research regarding the mechanical properties of turf. Previous studies have indicated that excessive friction or rotational traction at the shoe-surface interface may precipitate ankle and knee injuries (20). A recent study by Villwock et al. examined torque and rotational stiffness between various cleat types and field surfaces. They found no relationship between cleat patterns and rotational stiffness, with the exception of a turf-style cleat, a shoe whose sole is composed of a high number of elastomeric studs of very short length, which produced the lowest peak torques. While there did not appear to be a difference among the different shoe types, the playing surface plays a significant role in peak torque and rotational stiffness, with artificial turf accounting for higher torque and rotational stiffness than grass (20).
A previous study also found higher peak torques and rotational stiffness on artificial turf. Two shoe-surface combinations produced the highest peak torque: the combination of a turf shoe with first-generation artificial turf and a grass shoe (one with fewer and longer cleats) with third-generation turf created the highest peak torques (12). Ford et al. demonstrated differences in foot loading on artificial turf and grass. Using an in-shoe pressure distribution-measuring insole, study participants completed a slalom course on both grass and artificial turf. Analysis of peak pressures demonstrated that the medial forefoot and lateral midfoot had a significantly higher relative load on natural grass when compared with synthetic turf. In contrast, artificial turf produced higher forces laterally, specifically over the central forefoot and lesser toes (8). This increased pressure of the medial foot on grass may contribute to the "cleat-catch" mechanism, where the cleat catches or digs into the natural grass, potentially transferring rotational and shearing forces proximally to the knee, placing the athlete at risk for an ACL injury. The increased lateral forces seen on turf may indicate increased foot inversion, placing the athlete at higher risk for overuse and acute injuries. This may explain the higher incidence of ankle injuries seen on artificial turf in the previously mentioned studies.
In a previous study of first-generation artificial turf, which consisted of a short-fiber carpet on top of 5/8 of an inch of padding over concrete, the artificial turf produced significantly higher forces than outdoor grass, measured with a computerized sphere dropped from a distance. These forces exceeded the experimental threshold for a single impact to cause significant brain injury (14). A more recent study comparing third-generation artificial turf to grass at 72°F and 32°F showed that the turf had better impact attenuation than frozen grass, but produced more force than did grass at 72°F. In comparison with the previous study, however, the forces seen on the third-generation artificial turf did not exceed the threshold for severe head injury. Of note, the investigators measured impacts at five different sites on the artificial turf field. There were significant differences in impact attenuation between the sites, with the more heavily trafficked areas associated with the highest forces. This was presumably caused by compacting of the rubber infill base (15). While artificial turf displayed less impact attenuation than grass at temperatures above freezing in the studies, the previously mentioned studies on injury incidence reported a higher incidence of concussions with player-to-surface contact on grass than on artificial turf. The studies did not document weather conditions at the time of the concussions, and the temperature and field condition of the grass may have contributed to the observed concussion incidence.
Game Play and Player Perceptions
In a study examining the influence of different playing surfaces on game play, Andersson et al. studied time motion analyses and technical analyses of elite male and female soccer players on artificial turf and natural grass. Of the movement patterns studied, including walking, running, sprinting, and tackling, only slide tackle frequency differed between the surfaces, occurring significantly less on artificial turf than on grass. There were a significantly higher number of passes per team and passes in the midfield zone seen on artificial turf than on grass, and the fraction of successful long low passes tended to be lower on turf (1).
Male soccer players who regularly played on grass had a negative perception toward artificial turf, citing that playing on artificial turf was physically harder than on natural grass and that it was more difficult to run without the ball. They also noted increased difficulty with precision passing, ball control, and taking a shot on artificial turf. Those players who regularly played on artificial turf also reported negative impressions of artificial turf and noted that playing on natural grass was easier both physically and technically. The female players reported a neutral impression of artificial turf and did not feel that it influenced the game in general. However, they did note that it was easier to run with the ball and pass the ball while playing on artificial turf (1). In a study by Zanetti, who polled more than 1600 amateur soccer players in Italy, the players preferred artificial turf to grass with regard to multiple factors affecting the play of the game. The only exceptions were that the risk of abrasion on artificial turf was higher than on grass and that the players disliked playing on artificial turf in hot conditions (21).
In the previously mentioned survey of elite soccer players, male players noted an increase in difficulty while playing on an artificial surface. Di Michele et al. studied the physiologic response to running on different surfaces. Male soccer players performed a multistage running test on a treadmill, natural grass, and third-generation synthetic turf. The investigators recorded blood lactate concentration and heart rate. Synthetic turf produced higher values in blood lactate concentration and heart rate at all speeds. This indicates a higher physiologic activation on synthetic turf compared with natural grass and may explain the subjective differences noted between the surfaces in the player survey. The authors postulated that the higher compliance of the synthetic turf, caused by the cushioning properties of the infill, was responsible for the higher physiologic effort (6).
The use of artificial turf has been implicated in recent Methicillin-resistant Staphylococcus aureus (MRSA) outbreaks among American football players. In a study of professional football players following an in-season MRSA outbreak, the researchers found an indirect relationship between the use of artificial turf and MRSA skin abscesses. While they isolated no MRSA from swabs taken from the artificial turf, all abscesses developed at sites of turf burns (11). In a study of a MRSA outbreak among players on a college football team, those who experienced abrasions as a result of playing on artificial turf were seven times more likely to have a MRSA infection than those who did not suffer an abrasion. They did not find an association between the type and timing of turf burn care, suggesting that avoiding such abrasions entirely would be the best way to prevent infection (3).
With the growing popularity of synthetic turf, recent publicity has been paid to the risk of toxic chemicals contained in artificial turf infill (5). Many fields use recycled automobile tires to provide the rubber infill. Vulcanized rubber tire is a material with many highly toxic additives and compounds. There is concern for the health effects on athletes actively playing on this type of field, because of the exposure of possible constituents released from the rubber infill. Polycyclic aromatic hydrocarbons (PAH) and zinc have been found in high concentrations in these rubber pellets, although the concentration of PAH has been found to decrease with the age of the field (4,22). Studies on the bioaccessibility of these harmful chemicals have shown low uptake of PAH and zinc, comparable with uptake from other environmental sources and/or diet (19,22). A recent field monitoring study by the Environmental Protection Agency found that concentrations of materials that make up the tire infill were below levels considered harmful (18).
As the popularity of artificial turf surfaces increases, it is imperative that team physicians, athletic trainers, and coaches understand the unique properties of and differences between artificial turf and grass in order to better care for and train their athletes. Current research shows that overall injury incidence does not differ between the two surfaces; however, ankle injuries, abrasions, and concussions resulting from player-to-player contact occur more often on artificial turf, with a higher incidence of knee injuries taking place on grass. Muscle strains and chronic pain complaints, while not statistically significant, occur more often on artificial turf. MRSA outbreaks are a cause for great concern in the care of the athlete, and their relationship to abrasions experienced on artificial turf indicates that more research is needed into the prevention of these infections. Additional issues, including surface-specific injury prevention measures and the role of artificial turf in heat illness, remain unresolved and should be included in future research.
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