The Real Home-Field Advantage Vol. 2

Yannick Lambrichts

5 May 2022

The crowd stands waiting at the gates to enter their favourite sanctuary, the cameras are set up and the spectacle’s stage looks, well… pitch perfect. To ensure the ideal playing surface, aesthetics alone just isn’t enough. Grounds management staff have nurtured and prepared the pitch for days to get it ready for another sportive heyday.

The modern grounds manager uses objective data to decide on how to manipulate the pitch’s agronomic and playability characteristics. To cover every part of the pitch, multiple locations are tested. This enables the grounds team to perform precision management and achieve better consistency. This is not that evident when working with a living product which is affected by usage, maintenance and the local micro-climate of a stadium or training ground. In this second part of The Real Home-Field Advantage, we will discuss how spatial within-field variability affects players’ performance and safety.

As most pitches look very consistent due to the clean mowing cut and tight lining, a player is not able to visually spot differences in the mechanical properties of the surface. Grip and compaction can alternate all over the pitch which is shown to significantly increase injury risk and deteriorates performances. Just before a player lands his/her foot, lower limb muscles synchronize. They produce optimal tension to maintain stability during the foot strike and behave as a single mechanical spring that stores and releases the potential elastic energy. This pretension is referred to as “leg stiffness” and is neuro-muscularly determined by the expected foot-surface interaction (previous steps or visual cue). However, as elite playing surfaces do not provide visual cues when the underlying mechanical properties have altered, the brain needs to rely on the tactile information of the previous steps which can cause the player’s leg stiffness and kinematics to be out of synch.

Impact
In the previous blog, we discussed how vertical impact forces and returned energy can be used optimally to produce speed efficiently using the elastic properties of the musculoskeletal system. However, when leg stiffness and the surface’s mechanical characteristics aren’t in synch, the ground reaction force is not used optimally. When facing a suddenly stiffer surface, leg stiffness will exceed the needed muscle tension, thus structural elements of the musculoskeletal system will need to process the excessive shock. If this exceeds tissue load capacity, the tissue will be damaged. On the contrary, when facing a suddenly more compliant surface, leg stiffness will not produce sufficient tension and foot contact time will increase, slowing down the player who is now required to produce additional force to reaccelerate the gait.

Grip
During a change of direction, the foot is placed to have sufficient, yet no excessive grip to decelerate and push back off. The player’s foot placement (kinematics) and leg stiffness (kinetics) are based on the expected mechanical behaviour of the surface. If there’s a sudden change in available traction or the amount of deformation on foot contact, the forces exerted on the surface can make the cleats get trapped or initiate a slipping event when insufficient grip is provided. Both events have a significant effect on the player’s performance and injury risk. Just like before, the body expects a different amount of force and torque coming back from the surface and is put at risk of injury or resulting in inefficient movements.

Artificial surrounds
Unfortunately, it often happens when players step out on low quality (artificial) surround and its playability differs too much from the actual playing surface, they are suddenly exposed to excessive or insufficient firmness and/or grip. The movement pattern and exerted forces need to be tailored to the new surface, if not performance deterioration or injury risk is imminent. These risks are mitigated when players have a visual cue of the changing surface as they can adapt their movements and exerted forces accordingly. However, when looking at the ball or during backward running, they lack the visual cue. Maintaining the surround of the pitch to have similar playability characteristics is key to ensuring a safe and performant surface.

Spatial variability parameters
The brain is capable to adapt leg stiffness accordingly to the surface you’re running on after a single foot strike cycle. This means that transitions and one-step differences in the surface are key when assessing the safety and playability of your pitch. Straw et al. (2018) found most injuries that happened in areas with significantly high or low turfgrass quality, soil moisture, and surface hardness occurred along edges of hot and cold spots. These cold and hot spots are measured areas which differ significantly from the rest of the pitch (CI 90%). Additionally, the bigger the difference at these edges, the higher the risk. By testing the surface frequently, grounds staff can perform precision turf management and manipulate each zone accordingly to achieve better pitch consistency. In conclusion, this results to a safer and more performant surface.