Turf and Injuries: The Data Hurts
/By DMP
TL;DR: Turf doesn’t cause injuries
One of the most peculiar matches of the 2018 regular season occurred on August 18th. The LA Galaxy were already stretched thin from injuries to both dos Santos-es and Romain Alessandrini (their three DPs) and defender Michael Ciani for Sigi Schmid’s return to Seattle. But when they showed up in town, there was a huge name - perhaps the biggest name in MLS - missing from the lineup. That name was Zlatan, and by all indication his absence was voluntary.
By the end of the afternoon, the Galaxy really could have used one of the greatest players ever to kick a soccer ball. They ended up suffering their worst loss of the season and Seattle notched their best (5-0). Oh yeah, and the Galaxy missed the playoffs by less than three points.
We all know why he missed that game. It’s because the Sounders play on FieldTurf. There’s a perception out there that playing on artificial grass increases the risk of injury, and Zlatan had hurt his knee not long before (not on turf).
The superstar is not alone in his perception. I remember being disappointed not to see Thierry Henry play at CenturyLink Field in 2013. In fact, a group of Canadian researchers surveyed 99 MLS players back in 2011 and found that the vast majority (93%) said they believe third-generation artificial turf (FieldTurf) increases the risk of injury.
But here’s the rub (heh): it doesn’t.
Over the next who-knows-how-many-words-this-will-end-up-being, I’ll try to convince you that there’s no difference in injury risk playing on FieldTurf vs natural grass. I’m going to do it the only way I know how: with science.
Part 1: The Mechanics
Admittedly, this is the part I understand the least. The basic premise here, and what I suppose players are worrying about, is that artificial turf is more grippy on your foot, and that leads you to exert more torque on your knees and ankles. That torque doesn’t necessarily injure you, but it may increase your risk.
There’s no denying the science that if an athlete’s foot sticks to the ground better, the risk of rotational injury is higher. Footwear makers know this and have been changing their practices for decades since Temple University professors published their landmark research study on cleats.
But does your foot stick to turf differently than grass? This is where the sports engineers out there have a heyday. I’ll review the biggest and best studies on the subject and see what they say.
First up, a 2006 research study published in the American Journal of Sports Medicine. I won’t do it full justice, but basically these scientists from departments with fancy names such as Applied Biology and Biomedical Engineering built a gizmo that pretends to be a twisting foot, then tested it on a bunch of different surfaces. They found that “peak torque” (measured in newton metres) was actually higher on FieldTurf than grass, regardless of whether their robot was wearing turf shoes or cleats. This is the kind of thing that I assume scares the players.
But then, this lone wolf badass Paul Fleming from Loughborough University published this ridiculous 64-page magnum opus on the mechanics of shoes and turf. This guy went deep. I mean so deep that on page 61 he devolves into a discussion about climate change deep (not kidding). I didn’t understand half of the 12 figures, but he explored the properties of the little rubber pellets themselves and learned that when grass isn’t maintained well it begins to show more “dilatant” behavior, but turf doesn’t, and that’s good for turf.
Fleming found the same thing as Japanese researchers Fujikake, Yamamoto and Takemura in 2007: that the condition of the surface matters a lot, and that all told, rotational resistance is actually higher on grass than turf until you get to peak rotation, then it’s about the same.
That first point is worth repeating. The condition of the surface matters a lot. We can test torque with robots all we want, but as the stuff begins to fall apart it degrades differently: grass gets more clumpy and FieldTurf get more compact. These guys (and basic reason) are pointing out that simulations of torque on robots are a simplification of the experience of players, and that torque alone isn’t the only factor at play here. Injuries are a convergence of many risk factors and bad luck.
Mechanics alone have yet to prove beyond doubt that turf is or isn’t one of those risk factors.
To conclude this segment, I’m not a biomechanical engineer. Here’s a scorecard though:
| Turf bad | Turf not bad | |
|---|---|---|
| X | ||
| X | ||
| X |
Part 2: The Epidemiology
Phew. Made it through the mechanics. Now on to something I understand, and something that’s frankly more direct to the point.
If playing on turf is one of the risk factors that increases your risk of injury, then you should see more injuries on turf in the long run, right? That’s not to say that any single game should have more injuries, but if you kept careful data for a long time, the driving force that is turf would tip the scales towards more injuries, and when we tallied it all up we’d see a difference that isn’t likely chance alone. A wonderfully testable hypothesis.
I did two things to get at the research question from this angle. First, I spent hours combing the internets for peer-reviewed, scientific studies that compare injury rates on turf vs grass. Second, I went out and got my own data about injuries in MLS and checked for myself.
2.1: Existing Research
First of all, I’m not %100 sure I found every study ever made on this topic. I did however find one group based out of the Sports Performance Research Institute of New Zealand (yes, their acronym is SPRINZ), who did exactly this systematic review in 2011, and I found more studies than they did, so there.
In the end, I limited my search to peer-reviewed scientific journals and studies that compared turf vs grass for soccer specifically. There are lots of other studies about American football, but the scientific community agrees that soccer injuries and football injuries aren’t very comparable because of all the collision.
So long story short, I found 10 research studies (see table below) besides that SPRINZ study. They all analyzed different datasets about the rates of soccer injuries on turf and grass. Eight were cohort studies, one was an analysis before and after the introduction of turf at a single field, and one was a case-control study. These studies included youth athletes and top-flight professionals, men and women, and included Americans (none in MLS) and various European countries. They all were intelligent enough to look at the rate of injuries, not the number, and their methods generally passed my sniff test (I am an epidemiologist).
By and large, they found no difference between injury rates on turf and grass. For due diligence though, I should mention that three of the ten studies found a detectable pattern of potentially more ankle injuries on turf, but none of the recent studies have seen it. The SPRINZ study concluded the same thing as I did when looking at mostly the same publications.
Even though I said I wouldn’t go there, I did happen to take notes on five American football studies. These are more varied. Two found that there are significantly more knee and ankle injuries on turf (one was from 1992 though, when artificial turf was still in its “second generation” before today’s FieldTurf), two found significantly fewer, and one found no difference.
I think this conclusion is a little bit clearer than the previous section: most people who have looked at it find no difference in injury rates on turf vs grass.
Here’s the scorecard:
| Turf bad | Turf not bad | Explanation | |
|---|---|---|---|
| X | X | More ankle sprains on turf, all other types same on turf vs grass | |
| X | Women in the study actually had fewer injuries on turf, but not significant | ||
| X | X | More severe injuries and ankle sprains on turf, all other types same on turf vs grass | |
| X | No differences between turf and grass | ||
| X | No differences between turf and grass | ||
| X | X | Fewer quad strains for men in the study on turf, more ankle sprains on turf, all other types same on turf vs grass | |
| X | Fewer ankle injuries on turf, more back/shoulder injuries on turf, all other types same on turf vs grass | ||
| X | Significantly fewer injuries and fewer “substantial” injuries on turf, all other types same on turf vs grass | ||
| X | No differences between turf and grass | ||
| X | Injuries went up before they added rubber chips, then down after they did | ||
| X | No differences between turf and grass except maybe ankle sprains |
I wouldn’t consider this science if I didn’t at least ponder the limitations of the evidence. One huge limitation of any systematic review is a thing called publication bias: less-interesting studies are less likely to get published. There are probably several ways that publication bias might influence the conclusions I draw here, but I actually think it skews the literature towards claiming there is a risk, not the other way around. A bigger headline would be “we found a significant effect” than “we found no difference”. This means there might be unpublished research out there that continues to support my claim that turf is safe, and the authors just didn’t bother. In a bias-free world there might be 12 studies that say turf is safe, not ten.
2.2: One New Analysis to Add to the Mix
Finally, I get to do what I really wanted and analyze my own data.
MLS makes a great subject for the turf-and-injuries debate because only six teams play on turf out of 23. That means that 17 teams play the majority of their games on grass, and six teams play about half their games on turf. I would expect that, if turf causes injuries, those six teams would tend to have more injuries than everyone else if we looked at enough data. I also can’t imagine that there’s something systematically different about all 17 grass teams that would make them collectively less prone to injuries besides their playing surface either, so don’t talk to me about confounding.
But where to get the data? I’ve spent a long time now in dark alleys and sleazy bars trying to get my hands on some good, reliable data about injuries in MLS. And judging by the fact that none of the above research studies included any teams in MLS, I’m guessing there’s no systematic internal database of injuries for me to beg for. It’s been an elusive dataset to say the least because the official MLS page gets updated haphazardly, and there’s no good way to look at it back in time. So I decided to put my webscraping mettle to the test and pull out all the information about injuries and absences from the Match Preview pages on mlssoccer.com. Then, it was just a simple excruciating matter of wrangling all that text into analyzable data.
It’s still a work in progress, but I now have about 99% of the MLS injuries from 2017 and 2018 in a neat, tidy dataset. I even have the official description of the injury, so I can figure out which ones are leg injuries, which of those are sprains, and which ones are severe enough to list a person as “out” (rather than “questionable”).
In all, I can now tally up 4,984 gameday absences due to injury. 2,268 of these injury-related absences occurred in the 2017 season, and 2,716 occurred in 2018. These include 4,013 injuries that mentioned legs, knees, ankles, feet and the many other terms associated with lower extremity injuries. Among them are 2,998 injuries that mentioned strains, ruptures, tears, tendons etc. in combination with a lower-extremity description. The injuries tend to be more common in the middle of the season than the start and end, with 788 occurring in July, but only 416 occurring in March and 454 in October. The most injured team was RSL, who suffered 400 gameday absences due to injury in the last two seasons, while New England reported the fewest (108 injury absences).
What I don’t know is exactly where/when these MLSers got hurt, just what type of injury they have, what team they play for, and when they were absent. It would certainly be better to analyze rates of injuries that occur on turf fields vs grass fields, but alas, I make do with what I have.
There are other limitations to the new data I’m presenting here too though. A more important limitation than not knowing the circumstances of the injury is reporting bias. I mentioned that New England only reported 108 absences in the last two seasons, whereas RSL reported 400. As far as I can tell, the teams are mandated to release a matchday report to the league prior to every game, but there’s no way to know how forthcoming they are when they release it. It could be that New England is just really conservative in who they list as injured, maybe because they’re just more cagey about showing their cards. This is a concern that probably weakens the validity of my data. On the other hand, though, I have no reason to suspect that the grass teams are systematically less likely to report their injuries than the turf teams; it probably has more to do with the personality of their coach than the surface they play on. Perhaps there’s a coincidence that less-forthcoming coaches happen to play on grass, but I don’t know how to measure it. I’m sure if we thought about it more, we could come up with a litany of other things to worry about. But I’ll go ahead and guess that few of them have anything to do with playing surfaces. All that said, I’m still confident that we would see a difference between the turf teams and grass teams overall if there was an effect.
And guess what? We don’t. Any way I slice it, the teams that play on turf have almost the exact same rate of injury as the teams that play on grass. Sometimes actually fewer. For example, the turf teams in MLS between 2017 and 2018 averaged 3.51 players per game listed as injured. The grass teams? A bit higher at 3.58. This is true for injuries that list a player as “out” (2.42 players per game vs 2.58) and leg sprains/strains etc. (1.97 vs 2.2). Leg injuries in general (not just sprains) were the only way I could find marginally-more injuries among turf teams (2.90 vs 2.85). It’s not just a sample size thing, if that’s what you’re wondering; none of these averages are statistically different from each other except leg sprains, which is actually significant in favor of the turf teams. Here’s what that all looks like in a graph:
In the above, you can see just how close the turf teams and grass teams are in terms of injury rate, and also that the turf teams tend to have slightly fewer injuries, depending on how you look at it. It’s noteworthy that there almost appears to be a protective effect of turf for leg sprains (the last two bars, for which turf injuries are lower by the widest margin), but I wouldn’t make much of it considering those confidence intervals.
To repeat myself, MLS teams that play on turf average three and a half injury absences per game, and so do MLS teams that play on grass. There is no difference in the rate of injuries between turf and grass teams. To bring it back to the original point, no difference in rates implies that there is no difference in underlying risk, i.e. no excess harm attributable to turf. They don’t prove it, but these data do add the mounting evidence against the hypothesis.
Ok, that’s it. I think I’ve belabored this point long enough. I’m not going to say that I’ve absolutely proved it, just that the bulk of scientific literature says there’s no relationship between turf and injuries in soccer. Even the sports engineers sort of agree. What I will say is that this is the first time anyone I know has checked MLS injury data for risks associated with turf. Presumably the teams are doing this themselves and just not saying. But when I did, I found no evidence that turf is more dangerous than grass either.
You might get a turf burn, but your ACL is equally safe. See you in Seattle, Zlatan.