Injuries can be carrier killers for players. A serious traumatic injury could end untimely the player season. Moreover, it takes a while for the player to come back to its top physical and psychological level. Some of them never manage to do it.
These injuries are also financial burden on club. With the increase of player’s values, club cannot afford them to be badly injured, which can decrease their value and indisposed them for several games. That is the reason why player’s safety is becoming one of the most important aspects of the football clubs’ policy.
Playing surface & injury risk
It has been reported that 64% of the players who experienced an injury believed that was caused by the pitch. Up to 91% of them think that the type of playing surface can increase this risk. Elite players perceive the injury risk to be higher on artificial turf (AT) compared to natural grass (NG) particularly when ligament damages are considered.
The influence of the playing surfaces on the injury rate has been highly studied mostly through retrospective epidemiological studies. Despite the evolution of artificial turf system and the development of the 3rd and 4th generation technology, their impact on injury risk is still debating. This may be partly due to the variety of natural grass type they have been compared, to the geographic location of the pitch and the sport concerns.
Evolution of natural grass playing surface has also been accelerated in recent year mainly with the emergence of reinforced natural turf (so-called hybrid) technologies. These latter have been engineered to reinforce natural rootzone and provide resistance, stability and durability independent of environmental conditions. These reinforced natural turf pitches have been introduced into many top-level stadia and the difficulty lies in the fact that all these technologies differ to each other in the engineering process leading to drastic mechanical property differences on pitch.
Given the variety of the different playing surfaces including natural grass, artificial turf and reinforced natural turf, it is essential to characterize their mechanical properties and impact on player welfare. Even if some existing FIFA-standard tests define some safety limits, the range for impact hardness and shear resistance are not related to biomechanical or injury data yet. Then, the ability of the standards to define surface safety is limited, moreover if reinforce natural turf are taking into account.
Nevertheless, a clear consensus recognized that poor surface quality might predispose to more non-contact injury independently to the type of surface. Surface hardness and irregularity are the two main factors identified by players as cause of related-pitch-injury occurrence. The mechanical characteristics of sports surfaces related to athlete-surface interaction could be divided into (i) vertical behavior during impact and (ii) horizontal behavior (traction force) relating to the grip of shoes on the surface. Ideal surface allows players to move efficiently trough the stride, and should therefore present a combination of firmness (to give amount of support), cushioning (to damp the shock at impact), grip (to provide traction during push-off and turns) and rebound (to return energy from the soil during the push-up).
Even if no clear link has been showed between artificial turf and injury raise, players still complains. Therefore, some football championships such as Ligue 1 in France agreed to ban artificial turf on pitch in 2017.
ACL injury risk factor
Acute traumatic ligament sprains injury at the ankle and the knee is frequently cited as a significant risk to athletes in sports that involve sudden stops and direction changes such as American football, soccer or rugby. It represents a high proportion (up to 20%) of the total of the reported lower limb injuries. In particular, the very mediatized rupture of the anterior cruciate ligament (ACL) is known to be very incapacitating with at least a 4-months period away from the pitch. Even if the return to play rate after ACL reconstruction was very high, only two-thirds of the players competed at the highest-level 3 years later.
ACL injury is usually multifactorial combining player’s intrinsic and extrinsic factors, but it is interesting to highlight that for 85% of them it resulted from non-contact or indirect contact mechanism. In this case, 3 predominant playing situations have been identified leading to such trauma: (i) pressing following by re-gaining balance, (ii) re-gaining balance after kicking and (iii) landing after heading. These situations involve knee excessive valgus combined with internal rotation with the foot further away from the center of mass of the player. In addition, anterior cruciate ligament (ACL) injury is frequently cited as a significant risk to athletes on artificial turf mainly because of the changes in shoesurface interaction.
Thus, it could be interesting to compare the mechanical loading of knee joint during game situations performed on different surfaces with the assumption that higher knee inter-segmental load in valgus and internal rotation would lead to a higher risk of ACL injury.
A study was design to compare the impact of three different surfaces and their potential role in the ACL injury risk.
There is a paucity of on-field study assessing the impact of the soil on ground reaction forces under players’ loading due to the difficulty of outdoor experimental design. To the other hand there is a huge need to go further to understand the underlying causes of injury on the different surfaces in the real-world context.
So the first step was to adapt the experimental technique to the field environment. An experimental protocol was specially designed to analyze the influence of playing surface on force distributions during the landing phase of 3 movements, i.e. running, side-step cutting and jumping. To achieve this aim, motion analysis and ground reaction force of college rugby players were recorded during the same movement over the different surfaces.
A greenhouse (Figure 1) with three different surfaces was built in order to control surface environment and moisture. Surface was divided in 3 (15 x 2 meters long) tracks made in accordance with ISO standards (NF P90-112 & NF P90-113).
A natural grass track (NG) was composed of sand-based rootzone (Cargo-Green AG, Bassel, Switzerland), a third generation artificial turf track (AT) and a natural reinforced grass track (HY). The HY is composed of artificial rootzone of sand, cork and synthetic fibers in which natural lawn grows (AirFibr technology, Natural Grass, Paris, France). All tracks have been maintained following the same program.
Then, players’ ankle and knee joint strains related to the degree of impact absorption of the different surfaces were estimated by inverse dynamics taking into account body inertial parameters.
Our results showed that speed and cadence of running were similar among tracks contrary to the stress applied by the surface on players’ joints. Indeed, knee joint loads were higher (+6 to 21%) on AT compared to natural ones, as well as ankle joint loads (up to 16-26%). Furthermore, valgus and internal rotation of knee joint during sidestep cutting were significantly higher on AT compared with the natural counterparts (+43% and 36% respectively). Since the ACL injury is mainly involved in combination of high levels of knee valgus and internal rotation strains, natural grass surfaces and especially the natural reinforced grass (HY, AirFibr technology) appeared to be safer for players.
The main finding of this study is that playing surface has a significant effect on external knee moments during running, sidestep cutting and drop landing. As understanding the injury mechanisms is a key factor in injury prevention research, further investigations should focus on the development of detailed models of playing surface’s mechanical behavior
To an injury point of view, it is obvious that a lot of attention should be paid to the choice of playing surface technology and it maintenance across the season. As the important aspect of sport surfaces is to improve player’s athletic performance and safety, it is fundamental that club’s technical, medical staff and groundsman communicate with each other to tackle the player’s injuries issue.
Dr. Philippe Rouch is Professeur des Universités, Directeur de l’Institut de Biomécanique Humaine Georges Charpak; Xavier Drevelle is with Service de Chirurgie Orthopédique – Hôpital Avicenne – Université PXIII; and Patricia Thoreux works for both institutions, all in Paris, France.
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