Our findings indicate that a selected group of professional players, representing a sample of professional soccer players in North America believe that there is an increased risk of injury, specifically non-contact injury, as a result of training and competing on FT compared to NG. Previous studies comparing the incidence of injury on FT and NG, found no differences in the risk of injury from training and competition on both surfaces [6–10, 17, 18]; however 94% of the players in this study felt that the risk of injury was greater on FT. Similarly, players strongly believed that they experienced greater muscle and joint soreness and longer recovery times after competition and training on FT. Three surface mechanical properties (i.e., surface stiffness, surface friction, and metabolic cost) were identified by the players as important factors in surface related injury. Furthermore players’ reported that the magnitudes of the three surface variables were greater on FT, and that these differences were the primary reason why they perceived injury rates, muscle and joint soreness and recovery times to be higher on FT. Along with these three factors players further believed that surface quality and climatic conditions could influence the risk of injury on FT and NG.
A pre-established bias towards synthetic surfaces could possibly explain the divergence of players’ perceptions. Player comments (see the ‘List of original player comments’ section) suggest that past personal injury experiences on FT can mar players’ attitudes toward the surface, and even affect the way they play on FT in the future. Players could have solidified their perceptions of FT based on previous negative experiences on earlier generation turfs that were shown to increase the risk of injury . However, it is unlikely that such experiences and opinions can fully explain why the majority of the players reported greater risk of injury on FT. Moreover players reported that surface type did not influence contact injuries. In an injury audit of 12 European Championships from 2006 to 2008 it was found that traumatic injuries due to player contact represented 54% of all injuries overall, and were more frequent among match injuries (63%) . In order to understand how surface type might affect these injuries future comparative studies should report the occurrence, and mechanisms of both non-contact and contact injuries.
In the NFLPA Surface Opinion Survey conducted in 2004 and 2008 it was found that 96% and 91% of all NFL players reported feeling more soreness and fatigue on artificial in-filled surfaces as opposed to grass [14, 15]. These results are similar to those found for the group of players in this study and it would be interesting to see if these findings extend to the entire MLS; and further, to other professional soccer leagues in North America and internationally. The findings of this study suggest that the full effects of training and competing on FT have not been captured in the current literature. This might be related to the definition of injury (i.e., using the time-loss definition, which according to the consensus statement on injury only records an event as an injury if a player cannot take full part in future training or match play). This method is not sensitive enough to capture self-reported problems, such as players experiencing soreness, as they would still participate in sessions or games. This notion is supported by Walden et al., who in a study of injuries in Swedish Elite Football suggested that subjective somatic complaints without objective signs of injury might not be captured by certain injury definitions . It is possible that these effects (i.e., greater muscle and joint soreness after games and training) could also explain why players perceive the risk of injury to be higher on FT. Future epidemiological studies using a time-loss definition should prospectively track players perceptions, specifically levels of muscle and joint soreness and recovery times, concurrently with injury incidence for players training and competing on both surfaces. Doing so will provide another level of comparison between NG and FT, and could potentially uncover novel information on the dynamics between soreness, recovery, and injury for professional players over a period of competition and training regardless of, or in light of surface type.
Surprisingly players identified without any prompt, the surface mechanical properties reported in the literature as risk factors modifying the risk of injury on both surfaces . Greater surface stiffness seems to be the primary reason why players in this study reported the risk of injury to be higher on FT, as it was reported with the greatest frequency. It is speculated that the stiffness properties of a surface influence the frequency of injury and that harder surfaces can increase the impact forces on the body, which in turn might have an influence on some chronic overuse injuries . Similar results have been reported in a study by Martinez et al., in which players reported that artificial turf had worse shock absorbency properties than NG . Similarly, 57% of all NFL players in 2008 believed that new artificial infilled surfaces should be made softer, and 92% reported that they could distinguish the difference between a softer or firmer artificial surface . The player comments also seem to support the postulation that high frictional forces between the foot and playing surface results in foot fixation and possibly injury . Evidence of higher physiological activation on FT found in the literature can also put the players comments into context . Similar findings were also reported by Andersson et al. who found that male elite players reported games on artificial turf as more physically demanding compared to natural grass . Fatigue has been associated with an increase of injury in soccer players , and therefore the reported perception of higher physiological activation on FT in this study, and others, could be a contributing mechanism to injury risk. Based on these findings more research needs to be undertaken to understand the player-surface relationship and how it possibly influences the risk of injury. In particular future studies should focus attention on how surface stiffness and injury risk are related on FT and NG, as it seems that independent groups of professional players (NFL American football players, soccer players) have the opinion that FT is “too hard”.
Although it is widely accepted that without proper maintenance the performance and physical characteristics of FT decline, it is unknown if a decline in surface quality can affect the risk of injury. The fact that players in this study reported surface quality as being important in affecting the risk of injury for both NG and FT suggests that there could be a link between these two variables. It follows that proper maintenance of FT and NG is important to players, and that perhaps the relationship between the risk of non-contact injury and the quality of FT should be explored in future studies.
Climate and weather conditions can have a significant influence on the playing conditions of NG, and this has been shown to impact sport related injury . Considering this, it is perplexing that players in this study believed weather to have a greater influence in affecting injury on FT. Although FT retains a significant amount of heat in hot weather, wet weather was reported in the greatest frequency for affecting injury on FT. A possible explanation for this finding is that wet weather accelerates the movement of the ball, thereby the speed of play, more so on FT than NG forcing players to work harder and imposing increased strain on the body to compete in such an environment. This postulation is supported by the opinions of the expert group of players and coaches interviewed in a study by Martinez et al., who found ball roll to be more rapid on artificial surfaces . Further evidence into how speed of play could affect the risk of injury on FT can be found in the original player comments section.
It has been suggested that familiarization with FT is important to consider when measuring players impressions of artificial and natural grounds . In a study by Nedelec et al. the absence of negative perceptions of FT by a group of young male professional soccer players was explained to be in part due, to the familiarization of FT for this group of players . In contrast, in the present study it was found that players who had a history of playing on FT and players who currently trained on FT expressed preferences for NG, and had negative impressions of FT. The divergence of our findings and those of Nedelec et al. could be due to the age of the players employed in each study (average age 17.7 v 24.5 years for the study by Nedelec et al. and the current study respectively). Although we did not observe any significant differences in players’ opinions across the 3 age cohorts, it may be possible that a difference exists in how younger and older professional players perceive FT and NG. Future studies should aim to elucidate how familiarization to FT and age may impact players perceptions of FT.
Practitioners working with male professional soccer players could use the findings of this study to help them manage their players after exposure to FT and to make appropriate decisions for future sessions knowing that players may possibly have longer recovery times and experience greater soreness. This could be especially true when dealing with players who have a history of muscular-tendon injury, past injuries to weight bearing joints, degenerative changes in weight bearing joints, or a history of recurring injury, although this conjecture is unsubstantiated and would need to be explored. Lastly another practical application could be in making appropriate decisions surrounding surface type exposure for various activities when returning a player back from injury in different steps of the rehabilitation process.
There are a number of limitations of this study, which should be noted. The sample size was small and therefore the results may not be representative of the opinions of all MLS players and the much broader, and more diverse, international professional male soccer population as a whole. Furthermore participants were not selected randomly, which could also affect the generalizability of the findings. Recall of information could have been inaccurate due to the study design, which was cross-sectional and retrospective. Players’ responses could have been biased due to previous negative experiences on artificial surfaces, in addition to the cultural stigma surrounding artificial surfaces in soccer. Lastly, this study looked at professional male soccer players and therefore the results may not be characteristic of the opinions of sub-elite or amateur male players and elite, sub-elite or amateur female players. In a study by Zanetti it was found that Italian male amateur soccer players preferred playing on 3rd generation artificial turf rather than natural grounds , and Andersson et al. found elite female players reported a neutral position towards artificial grounds. Further research needs to be conducted on these populations .