UO researchers study how cleats could contribute to disproportionate number of ACL tears in female athletes

A team of researchers at the University of Oregon is currently studying the relationships between soccer cleat composition and risk of injury in female versus male athletes.

Female soccer players are three times more likely to suffer from major knee injuries than male soccer players, and a recent English study found that ACL injuries are 2-6 times more likely in females than males. This group of students and faculty at UO have been running tests for the past five years testing cleat stud length, traction and stability. They have created a model for a new type of cleat designed specifically for the female foot anatomy, which is in the beginning stages of development.

We’ll hear more from Emily Karolidis, a PhD candidate and lead researcher in this study, and Mike Hahn, professor of human physiology at UO.

The following transcript was created by a computer and edited by a volunteer:

Dave Miller: From the Gert Boyle Studio at OPB, this is Think Out Loud. I’m Dave Miller. Female soccer players are about three times more likely than their male counterparts to suffer from major knee injuries. This has been known for a while now and a variety of physiological differences have been put forward as reasons. But over the years, less scientific and apparel industry attention has been paid to the fact that soccer cleats were designed for male athletes. A team of researchers at the University of Oregon has spent the last five years studying the relationship between soccer cleat composition and the risk of torn ACLs. Now they’ve created a model for a new type of cleat designed specifically for the bodies and the physics of female athletes.

Emily Karolidis is a PhD candidate at U of O and the lead researcher of this study. Mike Hahn is a professor of human physiology at the school. They both join me now. It’s great to have both of you on Think Out Loud.

Emily Karolidis: Thank you so much for having us.

Miller: Emily, just how common are ACL tears among girls and women who play soccer?

Karolidis: Females, we’re finding, are at about a two to three times greater risk of ACL tears than their male counterparts. It’s historically been a problem and we’re continually seeing that, even up until last year at the women’s World Cup – about 22 of the best female soccer players in the world were taken out by ACL injuries alone.

Miller: Mike, what are the reasons that have been put forward for this discrepancy in ACL tears, by gender, in the past?

Mike Hahn: That’s a good question. In the past it has been chalked up to rather simple explanations such as differences in anatomy, differences in neuromuscular control of the body, and then also hormone cycles. None of these things have been found to be strong predictive factors. But we know that it is a multifactorial problem. So these are all contributors, but we feel like the traction dynamics with soccer cleats on the ground have been long overlooked.

Miller: And that’s what you focus on. We’re going to turn to that in just a second.

Emily, I’ve also read that one potential piece of this puzzle could be that, for girls and women who play soccer, there has been less of an emphasis by their coaches or trainers on certain kinds of strength or agility training. Has that had an effect?

Karolidis: That’s a great question. We can look at the research and if we look at female athletes alone, we find that they’re quadricep dominant. So where we have male athletes that have this equal ratio of quadricep to hamstring strength, females are already at a disadvantage that their hamstrings are just not quite as strong. And so if they’re quad dominant, that puts a lot more force on their knee. And if they’re not adequately training their hamstrings, then this increases their ACL injury risk factors even more. So if we’re not having adequate training and we’re not specifically targeting these muscles, we can see that there’s just this downstream effect.

Miller: I guess the short version of that is that it’s not like you’re saying that a shoe alone, a better designed shoe, is a totally complete solution. But your argument, based on some of the work you’ve done, is that it should be at least part of a solution. How much do we know now about the connection between current cleat options and ACL injuries?

Karolidis: To go back to your first point, it’s absolutely multifactorial. So we don’t necessarily think that we’re going to eliminate all ACL injuries here. But if we look at the interface of the surface and the athlete, which is ultimately the cleat, we believe that it is overlooked. What was the second part of your question?

Miller: What do we know about the connection between the current cleat options for athletes – women and men – and ACL injuries?

Karolidis: The market available cleats right now span from turf cleats, which are supposed to be worn on turf – they have a bunch of different studs that are shorter in length – all the way up to artificial grass, artificial ground, firm ground. These cleats are longer in length, sometimes they’re different in material composites. So these cleats are associated with greater traction essentially. By introducing these cleats on the wrong surface, ultimately, we could end up increasing these multiplanar joint injuries. Unfortunately, right now, a lot of female athletes are playing in their firm ground cleats on turf surfaces. And so this mismatch between turf and these very, very aggressive stud patterns could potentially be exacerbating injury as well. Matching surface to the cleats is an important factor.

Miller: Oh, so that’s actually different from the work you were doing to try to come up with cleats that are more matched to a particular body. You’re saying that even some improvement could be found right now if athletes would just wear the right shoes for the right surfaces?

Karolidis: It’s kind of a combination of both. If we look historically at male athletes, they’re playing in firm ground cleats and they’re playing on the correct surface. Female athletes are playing in firm ground cleats that are not designed for female athletes and they’re playing on the wrong surface. So it’s even worse.

Miller: When you’re saying cleats that are designed more for male bodies, what happens if a woman wears them? What is the physics problem there?

Karolidis: Yeah, we kind of look at it in a bunch of different lenses. Females have a much smaller mass. So if you design cleats to withstand a greater mass, you’ve obviously got inherent problems there. We can also look at the musculoskeletal susceptibilities of the female athlete. So females tend to have greater knee valgus, which means that their knees are kind of pointed inward, which causes a lot of rotational problems.

So if you look at these musculoskeletal capabilities and then you place an athlete in shoes that have very high rotational traction, the combination of these two factors can be very problematic. So it’s almost like there’s no safeguard for the future athlete. The cleats are validated and tested on male athletes. But then you put a female body that’s more susceptible to ACL injuries to begin with, and then they get these ACL tears. We think it’s exacerbating the risk of injury.

Miller: Let me make sure that I understand what’s happening because if I’m a little bit confused, I think listeners might be as well. Is the problem, in terms of mass for example – on average, male athletes might be heavier than female athletes – that these shoes are so good at sticking where someone has stepped, that they’re too good at that? The shoe stays and the lower foot stays and the upper body moves, and the knee suffers. Have I simplified it too much?

Karolidis: No, no, that’s exactly what we are thinking. So oftentimes, cleats are designed for maximal performance because that’s ultimately what sells the shoe. You want to have these cleats that give you the most amount of traction so you feel like you can plan and twist and go as fast as possible. But we also need to consider the injury risk that’s associated with high traction. And high traction designs are inherently associated with multiplanar joint injuries: joints of the ankle, joints of the knee. Ultimately, we think that there needs to be this balance of performance/injury risk.

Miller: So just to take it to a kind of absurd physics experiment extreme – on the one end, on the far end, you’d have a frictionless ice skate, and on the other, you’d have a boot that you just bolt to the ground in the middle of running. Obviously, you can’t have either one of those. But where you draw the line is a tricky thing.

Karolidis: Super tricky and that’s a great example. We do not want a shoe that has zero traction because the athlete is gonna skid. And we don’t want a shoe that’s gonna have too much traction that causes this foot fixation and then, all of a sudden, you’re having all these injuries. So it’s a complex problem, but it’s a fun one.

Miller: Well, how did you actually set out to figure out what kinds of cleats would be best for female soccer players?

Karolidis: Good question. I work out of the Bowerman Sports Science Center at the University of Oregon. We, as a lab, don’t have the capacity to necessarily manufacture cleats, and come up with all these brilliant ideas, and make them, and put athletes in them. So what we needed to do is control for individual variables within the shoe, to be able to identify, for example, the optimal stud length. So we would dremel down a couple of different pairs of Nike material cleats and identify which cleat length is the best for the female athlete, specifically, her new mechanics.

We also ran a separate study that was looking at stud shape. We compared elliptical studs, which have less rotational resistance, to a stud that was bladed in shape, which has greater rotational resistance. And we could compare the two and also look at what the knee mechanics of the female athlete would look like.

And then lastly, we could also look at stud positioning. So we have these fancy planar pressure insoles that kind of map out where pressure is being distributed across the forefoot and heel as an athlete is wearing them. And so we can optimize the positioning of the studs according to this data as well. So independently looking at each of these different controlled factors, you can then kind of turn it into this conglomerate of what would the ideal cleat look like. And now we’re fortunate enough to have an industry partner that’s manufacturing those for us.

Miller: How different are the shoes you ended up with from the ones that are currently on the market?

Hahn: I’ll jump in on this one to give Emily a little bit of a breather. The difference will be something that, if you were to have the shoe sitting on the table looking at it, you wouldn’t be able to tell the difference. But if you flip it over and look at the base of the shoe, you’ll see a variety of stud types, as Emily was talking about, with the elliptical versus the bladed conformation. The positioning will look a little bit different too. Instead of just a classic kind of perimeter of the boots, you’ll have some different positioning of studs, inside of that space as well.

And then some of the height differences that Emily talked about might be detectable to some people. Maybe not. That’s one thing we’re hoping to actually test, [whether] someone can perceive the difference. So if an athlete is doing these soccer based maneuvers and says, “Ah, this feels different” or “No, this doesn’t.” If we see the mechanics are different, but the person doesn’t feel the difference, that’ll be quite telling.

Miller: That was my next question. I’m just wondering, if you give these to elite athletes who obviously want a number of different things … which might be butting up against each other. They want to have the best performance they can on the field. They want to be able to run, and twist, and turn, and keep going, and win, and make money. They also don’t want to have career-ending injuries or career-delaying injuries. Is it possible that some top-level athletes are going to want to stick with the larger cleats they have for the feeling or reality of control it gives them, the grip and performance, even if it means a higher risk of injury?

Hahn: That’s a really good question. It’s something we wrestle with a lot. We often, in my lab, start to talk about how you keep things in balance and keep things on a spectrum. So if you have high performance on one end, but it comes at the cost of high risk of injury, clearly the balance is off. But if you have something that’s so great at keeping you safe that you can’t use it to actually play a sport, clearly, that’s not what we want either. So there will be some compromise between the design characteristics and the performance metrics. We would predict that without any question.

Some of the debate we’re having, especially with our industry partners, is [whether] it is all or nothing, or is it a slider that we can move left and right on the spectrum – the 15-year-old developing athlete who’s quite competitive, what do they need versus the premier level athlete that’s playing for the women’s national team? Do they need a different cleat for the task?

At the moment, it’s based on the price tag. So if someone wants to have a more, well designed cleat, they’re going to pay a lot more money. But if the well-off 15-year-old can get ahold of that same cleat, they may be playing in something that’s designed for the premier athlete and is not well suited to the 15-year-old.

Miller: Huge apparel makers only really change the way that they make products if consumers demand it and if they feel like they can make money. That’s the job. That’s what shareholders demand of them. Do you think these multinational companies will see the business case for this change?

Hahn: We naively hope so. We’re university people. Sometimes we don’t understand business, logistics and why things happen the way they do. We hope though, in this case, that once a new product comes to market and once a new design element comes into visible space, then if athletes start using it, if we can show the evidence to say that the knee mechanics are improved, the injury risk is reduced, and the performance is not compromised, then it starts to become a competitive advantage.

We know from past innovative designs in footwear and other sportswear products that once that advantage is demonstrated, all the other companies get on board very quickly. Within six months to a year, maybe a little longer, everyone is designing into the same space. And we see that on the Olympic stage, this month, where a lot of footwear products are a result of that competitive pursuit over the last 10 years. So we’re hopeful that in the female soccer athlete space, this might be the first icebreaking opportunity to get the industry to change a little bit.

Miller: And it’s actually not hard to imagine the ad campaigns that these huge companies could run – maybe made in Portland, the ads themselves – if they have this technology.

Emily, just briefly, we’ve been talking about soccer but it’s not the only sport that uses cleats. Could what you’re doing be used for rugby or lacrosse or field hockey?

Karolidis: Yeah, absolutely. Anytime you’re looking at field sports, you’re wearing high traction footwear. So historically, a lot of athletes that are playing rugby or lacrosse, as you mentioned, are wearing these same soccer cleats. So there’s that same, almost disconnect between the female athlete research and the footwear that’s being worn, not only in soccer, but also in these other field sports.

Miller: Well, thank you both very much for joining us. Mike Hahn and Emily Karolidis, thanks very much.

Karolidis: Thank you.

Hahn: Thanks for having us.

Miller: Emily Karolidis is a PhD candidate and lead researcher on this study at the University of Oregon, where Mike Hahn is a professor of human physiology.

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