Does Footwear Influence ACL Injury?

As a return to sport specialist, I often get asked the question, “What cleats should I get?”

Thomson et. al (2019) investigated the differences between six different popular soccer cleats on natural grass throughout the course of one soccer season. These researchers aimed to assess the differences in traction (how well the shoe gripped the ground) between each model of shoe, as well as differences attributed to type of grass and climate variables. What they found was really interesting, and at the risk of over-simplifying a complex topic, I will summarize the findings below.

The article started out by stating that soccer (or “association football”) is the sport with the highest volume of cutting movements – with players completing upwards of 800 cuts per game. We all know that a player’s ability to cut quickly and efficiently largely depends on the grip of their foot or shoe to the ground. What may be less well-known, is that there are two different types of traction that come into play here: translational and rotational traction.

Translational traction is essentially good traction. We want to maximize this variable, as it is the horizontal force required to overcome the resistance between the studs and the ground… Basically, this means that the higher the translation traction, the less straight line or side-to-side motion is allowed between your shoe and the ground. This has been linked to improved acceleration, agility, and overall performance.

Rotational traction, on the other hand, is essentially bad traction. We want to minimize this variable, as it is the rotational force required to release the studs through the playing surface in a rotational manner. Basically, this means that the higher the rotational traction, the more force or load must be generated by our bodies to get our foot to rotate on the ground, something we need to do when cutting or pivoting. The higher the rotational traction, the higher the rotational load through our joints, and the higher the risk of lower extremity injury like ACL tear or high ankle sprain.

All in all: This means that we want to find a combination of high translational and low rotational traction without compromising performance on the field. Since stud/cleat configuration and playing surface (species of natural grass, artificial turf) also effect muscle recruitment patterns and movement strategies, we must think long and hard before choosing which boot to lace up for the big game.

Thomson et al. looked at the six NIKE shoes illustrated below with one artificial grass (AG) model, four firm ground (FG) models, and one soft ground (SG) model. Four of these six models are the most used soccer cleats in the world according to 2018-19 market reports.

After testing these cleats on one Qatar field throughout the course of an entire soccer season, they found that shoe model had no main effect on translational traction. What they did find, however, is that the soft ground (screw-in metal-studded) cleat had the highest rotational traction across all months, while the artificial grass cleat had the lowest rotational traction across all months.

These researchers also found that grass type had a significant effect on rotational traction. Different species of grass have different mechanical properties and traction levels. They, therefore, also have different lower extremity injury risk levels associated with them. If you have ever played a soccer game down south, you know that the grass is much different than that found in New England. This is because grass is obviously effected by temperature, humidity, and soil moisture. Grass growing in Florida, for example, must survive and adapt to much different climate demands than that found in Massachusetts. Much research has shown that heat and drought resistance warm-season grass (i.e. Paspalum or Bermuda) has much higher rotational traction than cold-season (i.e. Rye) grass. Because of this, the rate of ACL injury on warm-season grass is much higher than that of cold-season grass. 

We can’t necessarily change the venue or field we have to play on, and we certainly can’t change the weather. So – when thinking about modifiable factors – all we have immediate control over is shoe type. So which cleat type is the best? 

Unfortunately, there is no “one-size-fits-all” kind of answer. Sure, if you are playing on warm-season grass on a sunny day down in Texas, you probably don’t want to wear soft-grounds. You certainly won’t slip, as your translational and rotational tractions will be off the charts. In the same vein, if you are up in New Hampshire playing soccer on a cold rainy day, you probably don’t want to wear artificial grass cleats. Sure, your rotational traction will be minimal, but your performance will be seriously compromised, as you will be slip-sliding all over the field.

In Table 2 (shown above), the researchers listed each of the six cleats and the amount of rotational traction generated by each cleat. Tiempo SG led the charge with a mean of 52.2 newton-meters of force, followed by Mercurial FG, Magista FG, Tiempo FG, Hypervenom FG, and Tiempo AG. Red values indicate high rotational traction (increased risk of injury), while green values indicate low rotational traction (decreased risk of injury).

That being said, my best advice to you would be to do a little bit of digging. Where are you located? What is the climate? What types of grass are prevalent in your region? If you play on warm-season grass, you may want to stay away from cleat types listed with red values (>46 Nm). Likewise, if you are recovering from ACL injury, you may want initiate your return to sport process in cleats listed with green values (<43 Nm).

While this particular research article certainly has its limitations (as do most), it is a great start to the conversation. It also helps pave the way for future research investigating optimal traction levels for different sports and/or field positions. Furthermore, it begs for the continued examination of different playing surfaces, soil types, and grass species to get a more complete understanding of shoe-surface interaction. While this article only investigated natural grass, please stay tuned for a future post regarding the ongoing grass vs. turf debate.

Thanks for hanging with me, and until next time, be well!

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