Sports field surface and injury risks: Grass vs. turf is just the beginning

Early research on the causes of sports injuries focused on identifying and evaluating athlete characteristics such as strength, flexibility, and fitness, emphasizing individual predispositions to injuries ^[1]. The next phase examined the relationship between training load and injury risk ^[3]. This shift paralleled a growing reliance on metrics from athlete tracking technologies – combinations of GPS, local positioning systems (LPS), inertial measurement units (IMU), and cardiovascular data – to monitor and manage athletes’ training loads. GPS technology has progressed rapidly, with sampling rates increasing from 1 Hz to 5 Hz and now 10 Hz and higher, along with more accurate optical tracking or LPS. These make the tracking systems more effective in capturing the dynamic movements of team sports ^[19].

Both intrinsic factors and training load elements contribute to injuries in a complex and interconnected manner ^[2]. However, “not everything that can be counted counts, and not everything that counts can be counted” remains relevant to injury research. 

The playing surface and its specific characteristics are among the next factors that we need to better understand in order to reduce injury risk.

Each surface presents a unique challenge affecting the player’s biomechanics, performance and risk of injury. Moreover, players adapt their movements based on the surface characteristics. 

Rotational traction is a measure of the force needed to rotate the foot against the surface, and can greatly influence the risk of lower limb injuries ^[18]. Pitches with high rotational traction might increase this risk, akin to a runner experiencing more strain while turning sharply on tacky trail surfaces. The player’s footwear may magnify or partially offset this risk, as the shoe can decrease or increase the rotational traction in shoe-surface interaction and corresponding injury risk. 

Another factor is surface hardness, which relates to how much “give” a surface has under pressure. 

Harder surfaces can lead to greater impact forces on the players’ musculoskeletal systems, including their joints, muscles, and tendons’ structural elements, thereby potentially elevating the risk for certain injuries ^[13]. Softer surfaces might reduce these impacts but could, in turn, induce other biomechanical and physiological responses ^[12].

Know your playing surfaces

The three main surface types in team sports are natural grass, artificial turf, and hybrid surfaces. ^[4]

Natural grass fields are the traditional playing surface for football. They provide a natural feel, good ball control, and cushioning for players. However, their quality can vary depending on plant health, maintenance, and weather conditions.

Artificial or synthetic turf is constructed from synthetic materials like plastic, along with infill materials such as rubber or sand, or the more recently introduced organic infills. It offers durability and can be used in various weather conditions, if maintained appropriately ^[15]. However, it may have a different playing feel compared to natural grass.

Hybrid surfaces combine natural grass with artificial fibers or backing to enhance durability and recovery by providing anchoring points for plant roots, reducing loss in grass coverage and stabilizing the surface under adverse weather conditions. These surfaces aim to provide the benefits of both natural grass and artificial turf.

Research on surface type and injury risk in football has been a topic of interest for sports medicine research. 

Some studies comparing natural grass and artificial turf suggest a slightly higher rate of injuries on artificial turf, particularly non-contact injuries such as muscle strains and ligament sprains. However, differences in injury rates, types, and burdens have not been consistent across studies. Factors such as player characteristics, footwear, and field quality influence these outcomes, causing discrepancies between studies. Recent findings have become more consistent, with higher injury severity and burden observed on artificial surfaces in rugby ^[14] and a higher rate of lower extremity injuries on synthetic turf in American football ^[9].

Within natural grass fields, the type of grass could play a role. Ryegrass is a cool season grass that may contribute to fewer non-contact anterior cruciate ligament (ACL) injuries than Bermuda grass, a warm season grass. This difference is attributed to reduced “trapping” of football boots due to less thatch and lateral root growth in ryegrass ^[11].

The quality and maintenance of natural grass fields factor in to injury prevention. Well maintained grass fields with proper drainage tend to be safer for players. By performing precision turf management, grounds staff can reduce the variability within a pitch, protecting players from unexpected changes in impact or grip at the foot-surface interface ^[17].

The composition and quality of infill materials in artificial turf can affect injury rates. Newer, more advanced artificial turf systems aim to mimic the playing characteristics of natural grass while reducing injury risk. Newer generations turf systems have imrpoved shock absorbing properties. Alternative infill materials, carpet materials, and turf maintenance practices are constantly being optimized to avoid excessive torque and gripping of the surface.

Contrary to the common misconception that artificial turfs require minimal maintenance, turf fibers need regular upkeep. This includes raking the fibers back to their vertical position and redistributing the rubber crumb to maintain the surface’s integrity. 

Additionally, cleaning the turf is essential for aesthetic, performance and health reasons. Players can suffer from severe staph infections due to skin abrasions on these surfaces, highlighting the importance of thorough and regular cleaning protocols.

Players who regularly train and compete on a specific surface may adapt to its characteristics, potentially reducing the risk of injuries associated with that surface’s properties. Conversely, stepping out and playing on an unfamiliar surface has frequently been associated with higher injury incidence and reduced performance ^[8].

Most studies have failed to consider specific surface characteristics, instead solely focusing on surface types in their analyses. Recent research has shown that some natural grass pitch surface characteristics change significantly over the course of a season ^[18]. Similarly, surface characteristics of artificial pitches can vary due to pitch maintenance strategies and wearing out. 

Researchers and practitioners also need to consider the variation between the different playing zones on a pitch. Risk zones for players include the area on either side of the touchline, as the assistant referee’s “lane” just outside the touchline can be different from just inside the pitch; or when there is a sudden change from natural grass to artificial turf on the side of the pitch. Even something as simple as shaded areas or where the grounds crew enter the pitch with machinery can result in a difference. 

Effects of surface characteristics on player performance

Rennie et al proposed a conceptual framework to investigate the impact of “pitch hardness” on injury incidence in football. Their approach involved a comprehensive literature review, encompassing studies from various football codes and other sports, to build a model that could explain the potential link between pitch conditions and injury risks. 

The authors found that current research does not conclusively prove that the hardness of natural turf directly increases injury risk. Most of these studies were comparisons, focusing on the differences in injury rates between artificial and natural turfs; and often relied on subjective assessments of pitch hardness. This methodological variance made it challenging to establish a clear causal relationship. The researchers also noted a potential seasonal bias in injury rates, observing higher incidences during the early season when pitches are typically harder. However, they acknowledged that other variables, such as the athletes’ fitness levels and training intensity, could also play a significant role in the injury patterns.

The study pointed out that limited studies suggest pitch hardness could influence players’ kinetic factors, like ground reaction forces. Variability in surface conditions could result in different energy demands and musculoskeletal load, potentially affecting injury risks. Harder pitches might lead to faster game speeds, possibly increasing injury rates due to greater playing intensity and resultant fatigue. They modeled how pitch hardness could influence players’ physiological and musculoskeletal demands, which might, in turn, affect their risk of injury. 

The upshot of their work is the need for more objective, longitudinal studies to deepen the understanding of this complex relationship.

Traditionally, coaching staff and players assess pitches with verbal descriptives like “dry and hard” or ’slippery.”  Various studies have examined player perceptions’ of the pitch in relation to injuries. 

Mears et al ^[10] found that over 90% of players believe that the type (natural vs. artificial) and condition of surfaces contribute to injuries. Key findings include the association of joint soreness and pain with artificial surfaces and the need to refine injury reporting systems to encompass a broader range of physical complaints. The player place surface characteristics like hardness and grip on par with the surface type. 

The study also highlights the variation in this perceived relationship across countries, suggesting that personal experience and the quality of playing surfaces significantly influence injury risk perception. This emphasizes the importance of considering both objective and subjective factors in future research to develop safer playing surfaces tailored to players’ needs.

A recent study conducted semi-structured interviews and focus groups with 103 professional football players ^[20], asking about the characteristics of a pitch that they deem important; which factors influence the condition of a pitch; how pitch characteristics influence the way they play the game; and whether these differences may be related to a player’s demographics, background or experience. 

Evaluating and quantifying surface characteristics

The First Law of the Game from the International Football Association Board (IFAB), dictates that “the field of play must be a wholly natural or, if competition rules permit, a wholly artificial playing surface except where competition rules permit an integrated combination of artificial and natural materials (hybrid system).” The field also has to be green, and any artifical surfaces must meet the requirements of the FIFA Quality Programme for Football Turf.

Natural and hybrid pitches are recommended but not bound to meet FIFA requirements on pitch quality standards. 

FIFA assesses natural and artificial pitches for shock absorption, vertical deformation, and energy restitution. To accommodate the need for quality in elite football, FIFA introduced quality labels to only accept the highest quality surfaces. Accredited test institutes establish pitch quality by evaluating stadium pitches to receive the necessary certification. Due to the time and expense involved nature, this type of testing is only a few times per year.

The gold standard devices for pitch certification are the Advanced Artificial Athlete (AAA) and the Rotational Resistance Apparatus (RTT). The AAA measures and assesses the vertical stiffness response of a surface, while the RTT measures the peak torque of the surface resisting a studded test foot shearing through the surface in a circular motion. 

Despite the efforts to quantify stadium surfaces, training pitches remain largely unexamined – and players typically spend more time on these pitches than on the match pitches. All football pitches exhibit substantial variability due to climate conditions, heavy use, and maintenance strategies. This points to the need for more frequent testing of both game and training pitches to inform practitioners and athletes about the actual state of the surface.

Continuous monitoring of key pitch characteristics could provide more insights in player-surface interactions and support data informed decision making in daily operations. However, the AAA and RRT are impractical for daily use. New technology, though, is improving the ability to quickly quantify key surface characteristics that are critical in player-surface biomechanics; to understand the interplay between environmental conditions and pitch management strategies; and assess the cumulative impact on the musculoskeletal and physiological demands placed on athletes.

Pitch planning for performance practitioners

Implementing a comprehensive approach to integrating surface characteristics into training load and injury risk management is undeniably a long term project. Just as screening and training load research required multiple seasons of data accumulation to draw meaningful conclusions and refine methodologies, understanding the impact of playing surface characteristics on performance and injury risk demands a sustained, longitudinal effort. 

This starts with regular, precise measurements of surface characteristics of different pitches and conditions, combined with performance data and injury incidence. Practitioners and researchers can also glean biomechanical insights from movement analysis studies on pitches with different surface characteristics and with the players wearing various footwear types. Knowledge about movement adaptations can provide a nuanced understanding of how different pitches affect players and specific musculoskeletal structures. 

Next, insights in player-surface interactions can be integrated into multifactorial injury causation research studies. These can then lead to incorporating surface characteristics into injury risk management systems. 

Finally, the performance staff and grounds crew staff should collaborate to tailor maintenance and management of playing surfaces, make informed footwear selections, and plan appropriate training and rehabilitation sessions, all with an end goal of enhancing athlete safety and performance.

Training load and pitch periodization

Surface hardness significantly impacts both musculoskeletal and cardiovascular load. Athletes can achieve maximal sprint speeds on harder pitches, but practitioners must carefully manage their training volume and intensity to prevent excessive musculoskeletal strain. Softer surfaces are better for high volume training sessions, reducing impact stress and allowing for a higher cardiovascular training load with a comparatively lower musculoskeletal load. 

A periodized pitch approach can maximize benefits while reducing injury risk. During the preseason, gradually increase the frequency, volume, and intensity of training on harder surfaces to promote tissue adaptation. However, during congested competition periods, reduce the frequency of training on harder surfaces to manage residual fatigue and minimize injury potential.

Training aims and pitch periodization for team tactical training

Aligning training surfaces with game day conditions is a key component of tactical preparation. Understanding surface characteristics, like hardness and rotational traction, is essential, as they directly influence ball behavior and player movement. 

To successfully prepare for a match on a pitch known for its hardness and moderate traction, the coach must identify a training pitch with similar characteristics. This allows players to acclimate to the expected ball speed and adjust their movements accordingly. 

The coach must strategically periodize training surfaces, recognizing the increased musculoskeletal load associated with consistent training on firm pitches. Mitigation strategies might include scheduling a session on a softer surface on MD-2 or MD-3 to manage peak stress. Tactical training on a pitch with anticipated match characteristics would focus on drills specific to the anticipated game pace, including ball control, passing accuracy, and set pieces. Scrimmages simulating match conditions (e.g., area per player, pitch dimensions) allow for technical and tactical refinement in a contextually relevant environment. 

Crucially, the coach actively integrates player feedback to fine tune tactical guidance and address surface specific challenges. This holistic approach maximizes physical preparation while ensuring the team is technically, tactically, and mentally ready for the unique demands of the game day pitch.

Rehabilitation focused on rotational traction

Increased rotational traction heightens the risk of rotational stress on the knee. Minimizing rotational stress on the knee is critical during the initial stages of ACL rehabilitation to promote optimal healing. Therefore, rehabilitation should commence on surfaces with lower rotational traction. Exercises emphasize easy movements such as light jogging, balance training, and controlled directional changes. This environment reduces the risk of the foot becoming fixed during rotation, minimizing stress on the healing ACL. 

Throughout the rehabilitation, the focus is on progressively restoring knee stability and building movement confidence. Consequently, as strength and stability improve, the athlete gradually transitions to surfaces with higher rotational traction. This prepares them for the demands of competition. The program includes dynamic agility drills and sport specific movements to ensure readiness for increased stress without risk of reinjury. Reactive and uncontrolled changes of direction are initially introduced on the lower rotational traction pitch and only progress to higher rotational traction once the athlete safely completes controlled changes on the lower traction surface. 

It is important to monitor closely the athlete’s response to varying traction levels throughout the process. Adjust the program in response to the player’s comfort, performance, and direct feedback, ensuring a safe and progressive return to baseline performance.

Footwear selection

Selecting appropriate footwear can enhance a player’s experience on different pitch conditions, supporting performance and reducing injury risk. For pitches with high rotational traction, boots with short, round studs decrease rotational traction, thus lowering the risk of foot fixation without compromising performance. This consideration is important for players trying to minimize the risk from high rotational forces, for example, due to a previous ACL injury or while rehabilitating for a different injury. 

Additionally, on firm pitches, where hardness increases musculoskeletal impact, insoles that provide extra shock absorption and comfort could be beneficial.