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 This means there might be unpublished research out there that continues to support my claim that turf is safe, and the authors just didn’t bother.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: