Can We Reduce Non-Contact Lower Extremity Injuries via Field Selection?
The US Women’s Soccer team has made headlines and led the way in the fight for more equitable pay and playing conditions by reaching a collective bargaining agreement in 2017. The fight, however, continues as US Women’s Soccer are repeatedly set to play matches on fields that are outright dangerous and not suitable for play. The US Women’s Soccer Team also had their final 3 scheduled matches on artificial turf, compared to none for the US Men’s Soccer Team . Artificial turf is perceived by professional players to have different characteristics than that of natural grass .
While the main discussion points brought to the forefront by the USWNT are equal, consistent, and safe playing conditions , in this blog post we will examine the literature on injury incidence rate on artificial turf vs. natural grass and work towards the question of preventing non-contact lower extremity injuries.
The Physics of the Soccer Pitch and Lower Extremity Injury
The physics behind the soccer or any other sport pitch can be complex. Confounding factors in injury studies comparing various fields may include type of sport played (and thereby varying movement patterns), competitive setting (practice or real match), type of surface, condition and roughness of the pitch, player footwear, player position (e.g. defender, forward etc.), pitch moisture level, team resources and expertise in assessing pitch condition, as well problematic movement patterns.
An Example Lower Extremity Injury Mechanism
While the causal relationship is still not exactly clear, valgus knee is a characteristic widely accepted as related with lower extremity injury . ACL loading is higher during quadriceps muscle application combined with an internal, rather than an external rotation . However, quadriceps loading combined with external rotation leading to ACL impingement. cannot be ruled out as a ACL injury mechanism . Finally, Shimokochi et. al. note that “weight-bearing, decelarating activities” increased ACL loading.
As an anecdotal illustration suppose higher friction causes the planted foot to have a higher chance to be fixed. Now that the foot is fixed, a misalignment between the direction the foot is planted and the direction the knee and hips are flexing or rotating can lead to high stress, potentially leading to ACL strains or tears.
Introduction to Artificial Turf and Natural Grass
Studies have used physics to describe why first generation artificial turf (1960s) is more dangerous than natural grass . Friction, cutting, stopping and turning forces are a primary point of discussion in comparative field studies. Wannop et. al. perform a comprehensive review of decades of literature on the cleat-surface interface . The simplest model of friction is the static, linear one. While it does not describe the complex cleat-surface relationship, the basic premise is important to consider. The friction force is a function of the product of at least two key variables – the coefficient of friction between the player’s cleat (or shoe) and the surface, and the normal force (which is equal to the force of gravity). However, three dimensional effects (e.g. torsional friction), varying player cleat surface profiles and multiple substrate layers beneath the turf and other variables can add layers of complexity on top of a linear friction model.
Figure 1: Linear friction and model and the complexity of the artificial turf (schematic created by EuMotus is for illustration purposes only and not representative of all types of pitches)
It is also an observed phenomenon that artificial turf playing fields can be up to 50 degrees F higher than natural grass and generally 35 to 55 F warmer than grass fields . It can therefore be surmised that increased player discomfort (e.g. increased overheating, dehydration) could be another confounding factor in the question of injury rates in artificial vs natural grass pitches. There is evidence that artificial turf mechanical properties are a function of temperature , leading to the implication that artificial turf surfaces may be better suited for certain temperature ranges and conditions relative to others (e.g. cold vs. hot climates).
Moving away from physics theory, let’s investigate the literature on lower extremity injury incidence rate and if one type of field increases or reduces risk.
(Recent) Literature Review: Lower Extremity Injury Incidence on Various Fields & Limitations
(Lack of a) Conclusion
It should be noted that the below review is far away from comprehensive or performed with scientific rigor. The results point in differing and sometimes contradicting directions. A clear conclusion, and even further proof of causality, in which type of fields may pose greater or lesser incidence of lower extremity injury is not apparent. Some selected studies may point to an increase or decrease of specific injury incidence associated with a particular field. However, on aggregate there appears to be no significant difference between modern artificial turf and natural grass injury rate.
Limitations and Comments
Below is a non-comprehensive list of limitations and comments on this literature review:
- Literature on artificial turf considered focused on the latest, third-generation artificial turf.
- Throughout all studies, correlation does not necessarily imply causation.
- Most studies grouped together both contact and non-contact ACL injury.
- Some studies grouped together acute/traumatic or overuse injuries, while others separated these.
- Different confidence intervals and p-values were defined as significant throughout studies for accepting or rejecting hypotheses. While some accepted a 95% confidence interval (or p=0.05 value), others selected more rigorous 99% confidence interval or p=0.01 value. P-value is used to indicate the probability of a false positive result. There has been recent discussion in the scientific community on imposing more rigorous hypothesis testing criteria, which would limit the amount of false positives in hypothesis rejection .
- Iacovelli brings up a potential source in bias of under-reporting of injuries if not deemed significant by players or athletic personnel .
- Injuries recorded are typically time loss. Injuries such as turf burn, may not be recorded when comparing artificial turf to natural grass. Ekstrand found 0.4% turf burn injuries reported and noted that FIFA requires annual surface friction tests and speculates that modern artificial turf surfaces may not be a problem .
Studies With Mixed Signals
In male European soccer leagues, researchers once again did not find a significant incidence of injury risk when comparing play on natural grass vs. turf fields. However, the researchers did find with a 95% confidence interval that there was a higher incidence of ankle sprains on turf than on grass. The authors noted caution, as the sample size may not have been large enough.
Soligard et. al. found no significant difference in acute injuries in Norwegian youth female and male soccer teams playing on artificial turf relative to natural grass . However, the researchers observed a lower risk of ankle injuries with a higher risk of back and spine injury for play on artificial turf relative to natural grass.
In a study of young (15.4 year old mean age) female soccer players, Steffen et. al. found that the risk of acute injuries did not differ significantly between artificial turf and grass. However the research team found that during matches, there was a higher incidence rate of serious injuries on artificial turf than on natural grass (95% confidence interval) . Furthermore, there was a correlation between a signficant number of ankle sprains and artificial turf (95% confidence interval).
Ekstrand et. al. examined 15 male and 5 female teams on the latest (third-generation) UEFA approved artificial turf and did not determine increased incidence of injury relative to natural grass pitches for either females or males. With at 95%+ confidence interval the authors found that men were more likely to pick up an ankle sprain, and less likely to pick up a quad muscle strain during matches .
Studies Showing No Difference of Increased Injury Risk on Artificial Turf
Strutzenberger et. al. found that there was not an increased incidence of injury for female collegiate level soccer players on artificial turf. Ground contact times did not differ between natural grass and artificial turf surfaces. The researchers commented on potential for knee injury risk reduction artificial turf due to decreased valgus and decreased internal rotation .
Fuller et. al. did not find significant differences between the probability or consequence of injury during training sessions on new generation turf compared to natural grass for US collegiate women or men . Fuller et. al. also did not find significant differences for the same hypothesis during matches.
Studies Which Find That Turf Has Higher Incidence Rate of Injuries
Iacovelli found a correlation between incidence of injuries on turf during game day relative to natural grass in collegiate football (note, not soccer) players at The University of Iowa .
Dragoo and Braun do an excellent job reviewing field types and their association with injury incidence . Here are selected findings and references pertaining to artificial turf vs. natural grass debate:
- Interestingly, Andresen et. al. find that there is a significant difference between wet/slippery natural grass @ 1.7 injuries/game and natural grass fields in good condition @ 3.3 injuries/game .
- In Australian Football, natural grass fields with significantly less evaporation and with high rainfall significantly reduced the incidence of ACL injury . While the causal mechanism is not proven, the authors hypothesize a “softening of the ground” as a potential causal mechanism, which in turn reduces the cleat-ground friction.
- Different types of grass can be linked with fewer or more ACL injury. In particular, Orchard et. al. found that rye grass is linked with fewer non-contact ACL injury than bermuda grass . It was hypothesized that there was increased traction in the thicker Bermuda grass ‘thatch’ layer.
Coaches & Trainers: How to reduce the risk of non-contact lower extremity injury
Ok, so if we can’t know for sure which type of field is safer, what can we do to reduce the risk of lower extremity injuries due to field conditions?
- Local knowledge is key – consult with other teams and trainers in the area on field conditions.
- Inspect the local pitch conditions with ample time before the game or practice. If the USWNT is subjected to playing in potentially hazardous field conditions, chances are that other professional and certainly non-professional teams and players in collegiate and youth formations are also exposed to hazardous fields.
- Take action. If a field is unsafe, report it to the field’s managing authority. Perform a risk assessment of playing conditions, and the associated probabilities and consequences of player injury. Investigate alternative playing pitches. Both natural grass and artificial turf fields can wear out. Both can be repaired or replaced.
What else can we do to reduce the risk of lower extremity injury? Outside of field conditions, you can systematically screen athletes for problematic movement patterns using movement analysis software. Trainers and PTs can institute corrective and strengthening exercises to reduce the incidence of problematic movement patterns.
Considering implementing movement analysis software for problematic movement pattern screening? Get in touch!
ACL injury typically originates from problematic movement patterns, which may be characterized by valgus knee – rapidly decelerating (on a weight bearing foot), jump landing, perhaps while suddenly changing direction . Athlete biomechanics and problematic movement patterns can be assessed, trained and improved, helping reduce the risk of lower extremity injury. Read more at our post on preventing non-contact ACL injury.
 NYTimes. New Turf Fight Has U.S. Soccer and Women’s Team at Odds Again. 9/21/17
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