DISTILLING DECADES OF NON-CONTACT ACL INJURY RESEARCH

Researchers established the injury-causing link between athletic cleats and the playing surface decades ago. While that connection is unquestioned, how to alleviate the problem has remained unsolved. Dozens and dozens of studies, funded by renowned health institutions, professional and amateur leagues, and private industry have sought to determine the cause of non-contact ACL and other lower extremity injuries. 

One of the reasons we were compelled to pursue our quest to develop a solution was the overwhelming evidence that athletic cleats, by and large, were the culprit. While some parties have pinned the blame solely on artificial turf, the truth is that non-contact injuries are only 20% more likely to happen on artificial playing surfaces than on natural grass. To us, the finger instead points squarely at athletic cleats, and at the shoe companies perpetuating the problem by blindly increasing traction. 

 

Let’s walk through the science.

Natural grass is often used as a benchmark in these studies, since grass has a breaking point, literally, that artificial turf really does not. Grass roots and rhizomes break (called shear failure) during peak loads. So, comparing how cleats on artificial turf behave with how they behave on natural turf is often used, as in this 2015 study published in Sports Biomechanics1 that states that:

“A significant (p <0.05) relationship was found between the peak force and torque on natural grass. Almost all of the cleats caused shear failure of the natural surface, which generated a divot following a test. This is a force-limiting cleat release mode.”

 

KEY TAKEAWAY:

Natural Grass is the ‘weakest link,’ and it breaks (before the shoe, or a body part does).

 

When they tested those same cleats on artificial turf, they found that:

“Only one cleat pattern, consisting of small deformable nubs, released on the artificial surface and generated force (3.9 kN) comparable to the range observed with natural grass. These findings (1) should inform the design of cleats intended for use on natural and artificial surfaces and (2) suggest a mechanical explanation for a higher lower-limb injury rate in elite athletes playing on artificial surfaces.2” 

And yet, cleat manufacturers keep increasing traction, most recently with cleats placed closer to the soles’ edges, and/or with blades replacing more forgiving conically shaped cleats. Again, from that same study:

“… with the artificial playing surface all but one of the cleat patterns resulted in a degree of engagement of the surface that caused forces and torques reaching the limit of the test device (approximately 4.8 kN [1079 lbs] force and 200Nm [147.5 ft-lbs] torque).3” 

A 2009 article in The American Journal of Sports Medicine4 includes,

“The edge-cleat design produced significantly higher rotational traction and was associated with an ACL injury rate 3.4 times higher than that of all other designs combined. (emphasis added)” 

 

KEY TAKEAWAY:

When the cleat and playing surface don’t give: the athlete’s ACL or ankle does. 

 

The Next Step: Testing 

We engaged biomechanical engineering experts who have developed laboratory tests using a machine purpose-built for testing cleat designs as well as different types of turf (artificial and natural). 

The machine connects a vertical pneumatic rod to the forefoot part of a given cleat, and then ‘pre-loads’ the cleat with various amounts of weight. It then tests different shoe-surface interactions. One is a translational test, where the pre-loaded cleat is dragged along the playing surface. The second is rotational, where the cleat is twisted as in a change of direction or cut. 

In the same report from above5, when discussing tests performed on this machine, the authors state:

“In both tests, the loads measured at the foot-form (cleat forefoot) can be substantially below these values if some load-limiting mechanical interaction occurs at the cleat, surface, or cleat-surface interface (e.g. the surface fails in shear at a lower load or the cleat releases and moves relative to the surface).” (Emphasis added) 

 

KEY TAKEAWAY:

A cleat that moves relative to the surface can “substantially” reduce the load on the foot, ankle, and knee. 

 

A 2017 NIH-funded study6 echoed this conclusion, saying:

“... in an ideal world where a certain level of force is known to cause injury, a cleat-turf interface may be designed to “release” at such a point that the foot never experiences the injury load level.” 

 

We Put Our Tech to The Test.

We engaged these biomechanical experts to put one of our prototypes, incorporating our SmartStudsTM technology, on their machine. We used one of the worst-performing existing cleats, with longer-than-usual studs, an aggressive stance, and less-than-flexible uppers. The results were plotted against a wide range of currently available cleats. 

This graphic shows the results of our putting Caddix SmartStudsTM tech on one of the lowest-rated, most aggressive cleats on the market. 

 

KEY TAKEAWAY:

We improved an underperforming cleat’s release score by 17% … a significant improvement. 

 

The Caddix cleats now in production incorporate optimal cleat shape, length, and configuration on par with some of the best-performing cleats on the market … WITH the added (and unique) advantage of our SmartStudsTM being able to flex to disperse some of the injurious torque other shoes just aren’t built to do. The shift Caddix SmartStudsTM make under stress is monumental in reducing torque, but subtle enough to be imperceptible to the athlete counting on them for the traction they need. 

 

KEY TAKEAWAY:

Change is here. 

 

1, 2, 3, 5: Richard Kent, Jason L. Forman, David Lessley & Jeff Crandall (2015): The mechanics of American football cleats on natural grass and infill-type artificial playing surfaces with loads relevant to elite athletes, Sports Biomechanics, DOI: 10.1080/14763141.2015.1052749.  Epub 2015 Jun 26. PMID: 26114885.

4. Villwock MR, Meyer EG, Powell JW, Fouty AJ, Haut RC. Football Playing Surface and Shoe Design Affect Rotational Traction. The American Journal of Sports Medicine. 2009;37(3):518-525. doi:10.1177/0363546508328108

6. Jastifer J, Kent R, Crandall J, Sherwood C, Lessley D, McCullough KA, Coughlin MJ, Anderson RB. The Athletic Shoe in Football. Sports Health. 2017 Mar/Apr;9(2):126-131. doi: 10.1177/1941738117690717. Epub 2017 Feb 2. PMID: 28151702; PMCID: PMC5349396

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