On Sunday 6 March, several of us watched in shock as Brøndby's Kevin Mensah suffered a nasty twist to his knee in the Superliga match against Silkeborg.
The accident turned out to be an injury to the anterior cruciate ligament. The same injury in the same knee that Mensah has previously had surgery for - a nightmare injury for a professional footballer.
It later emerged that Mensah and the rest of the Brøndby team were playing in football boots that were not designed for the surface played on in Silkeborg - artificial turf.
Could Mensah's injury really have been avoided if he and his teammates had played in the 'right' football boots?
I work at the Technical University of Denmark, where I am specifically researching the contact mechanism between shoes and surfaces, with the ultimate aim of (hopefully) minimising the number of sports injuries and work accidents.
The question of Mensah's injury and the not insignificant difference between football boots - and how to choose the right one - I dive into here.
Football boots designed for different surfaces
At first glance, there is not much difference between the football boots in the pictures below.
The brand and the model are the same. They weigh the same (205 grams). However, there is one key difference between the two football boots - the button system under the sole.
Under the sole, you can see that the appearance, number and position of the buds are different. So is their function.
Two football boots of the same make and model - but designed for very different surfaces. (Photo: Mathias Strømfeldt)
The boot on the left is a normal football boot, a so-called FG ('firm ground') designed for ordinary grass football pitches.
The football boot on the right, on the other hand, is an AG (artificial grass) boot, i.e. a football boot made specifically for artificial grass.
Small difference, big impact
Although the difference between the two boots may not seem great, the impact is huge.
Artificial turf boots are specifically designed to rotate more easily against the ground. In concrete terms, this means that the boot does not 'lock' onto the ground.
For example, if you're tackled hard by an opponent, the boot can let go of the ground more easily, preventing a twisted ankle or knee - and a potentially serious injury.
A mechanical test of football boots and surfaces across brands and models found that football boots for ordinary grass have on average 40 percent better ability to resist rotation against the grass surface than boots designed for artificial turf.
This means that it takes 40 percent more force to rotate an ordinary football boot against the ground, compared to a model for artificial turf.
This means that an ordinary football boot can in principle deliver a 40 percent greater rotational force from the ground back - up the leg.
Many footballers choose wrong
We know from pictures from the match on Sunday 6 March that the majority (including several Silkeborg players) played in normal football boots designed for ordinary grass - even though it is no longer news that both Farum and Silkeborg play on artificial turf.
The same observations apply when reviewing images from matches in Farum and Silkeborg: the majority of players on the pitch are simply playing in normal grass boots on artificial turf.
This suggests that many professional footballers are not aware of the importance of choosing the right football boots to reduce the risk of fatal injuries that could, in the worst case scenario, ruin their careers.
The higher the friction, the more injuries
Artificial turf pitches are much more durable than ordinary grass pitches. This means that the grass does not break down in the same way as a normal lawn does when the load becomes too great.
This also means that you can stand extra firm on an artificial turf pitch.
We know that, across sports, more sports injuries occur when friction, the resistance to sliding against the surface, is high.
This knowledge comes from a large Canadian study that followed 555 high school athletes for three years and kept track of the boots they wore to play American football.
The group of athletes who wore the boots that were most firm against the ground suffered almost five times as many leg injuries as the group whose boots were least firm.
We also know this from a large Norwegian handball study, which examined the slip resistance of the floor in different handball halls.
The result was clear: in the halls with the particularly slip-resistant floor, more than twice as many cruciate ligament injuries occurred than in the halls with the normal wooden floor.
Fortunately, this led to a change, and today the floor may no longer be as 'sticky' as it was then.
In general, more joint and ligament injuries (including cruciate ligament injuries) are also seen when the courts are dry than when they are wet, in countries where there are large differences in weather.
In tennis, you also see many more injuries on hardcourt compared to clay courts.
Know the different football boots
- Football boots for artificial turf (AG) are characterised by having many short and round studs. The studs are also both shorter and more flexible, allowing them to flex against the ground. These football boots are always referred to as 'AG' for artificial grass, while this designation is usually also marked in writing on the sole.
- Football boots for general grass (FG) are characterised by having knobs moulded in hard plastic, which are typically either round, triangular or elongated (so-called 'blades'). These boots are designed for dry grass pitches.
- Football boots for wet pitches (SG) are characterised by having the lowest number of studs, which are both the longest and often also have a number of metal studs.
- The two artificial turf pitches in the Superliga are both '3rd generation pitches', consisting of long synthetic fibres ('blades of grass') on a surface held down by sand (bottom) and a layer of elastic rubber granules (top).
Could Kevin Mensah have avoided his injury?
Although we know that Mensah was playing in the wrong football boots the day he accidentally tore his anterior cruciate ligament against Silkeborg, it is far from certain that the right artificial turf boots would have saved him.
In other words, we do not know whether there is a direct causal link between the choice of football boots and whether you sustain an ACL injury.
Unfortunately, we will never know, as we obviously cannot repeat the situation with Kevin Mensah, where he is wearing the right type of football boot.
In addition, it would also be ethically indefensible, and gambling with the welfare of sportsmen and women, if we were to conduct a clinical lottery trial repeating the situation of last Sunday.
So we can't ask half of them to play in normal grass football boots when we already have the prior knowledge that these boots increase the load on the foot, ankles and knees - and thus probably increase the risk of an ACL injury occurring.
But we know enough to make a responsible choice
Nor do we know from clinical lottery studies that winter tyres and seat belts save lives and limb.
No, our knowledge and perception of winter tyres and seat belts is based on manufacturers' 'know-how', data from insurance companies and mechanical laboratory tests - i.e. how fast can the car brake, how fast can it turn without skidding and basic crash tests.
It's the same level of knowledge we have about using correct football boots on artificial turf.
This information is far from new.
FIFA and the English FA have long strongly recommended that 'normal' football boots should not be worn on artificial turf - precisely to avoid unnecessary sports injuries.
This is particularly important for footballers who, like Mensah, have a previous cruciate ligament injury.
Ultimately, it's about risk mitigation.
Just as we are likely to wear a seatbelt or change to winter tyres even if we don't expect an accident, we have a responsibility to ensure that our children, young people and professional footballers alike wear the right boots when playing football on artificial turf.
In this video you can see the situation where Mensah got injured and Filip Gertz Lysdal talks about prevention and rotation on the surface in different boots.
*The article was originally published on Videnskab.dk's Forskerzonen, where the researchers themselves communicate. Click here to see the article.
