Turf Type and NFL Injuries: Part I

September 13, 2017, 8:25 pm ET

Photo: USA Today Sports Images

by Zach Binney

As fun as sandlot and back-alley football can be, nothing beats a flawless 160-by-360-foot rectangle of Bermuda or Kentucky bluegrass for a game. Fans want to see a competition between the best players in the game decided by skill, not the whims of the soil, plastic, or rubber under their feet. But NFL stadiums have a surprising array of surfaces on which these players sprint, leap, block, tackle, and cut: two main kinds of grass and eight different artificial turfs have been used between 2007 and 2015.

The impacts of certain kinds of artificial turf on injuries have been investigated before. One study from the NFL's Injury and Safety Panel using the league's own internal injury data from 2000 to 2009 found increased rates of knee and ankle injuries on FieldTurf, the most common artificial turf used in NFL stadiums, versus natural grass. However, studies directly comparing different artificial turfs in the NFL or using a dataset as large and recent as the Football Outsiders injury database are lacking.

Although we have every publicly reported injury from 2000 to 2016, we restricted this study to the four-year stretch from 2012 to 2015. Reported injury rates have changed over time, as have turf types and stadiums, so we picked a narrower period when injury rates were otherwise relatively flat. We did not include 2016 because of the elimination of the Probable designation from injury reports, which prevents comparisons with earlier years.

The injury rates below are given per 1,000 athlete-exposures (AEs). One AE is defined as one player participating in one practice or game. Because we do not have data on injury timing, all injuries occurring during a given week were assumed to occur in-game -- previous studies have shown that more than 80 percent of NFL injuries occur in games. We also do not have data for injuries that occurred in Week 17. AEs are calculated based on the 53-man active roster, with reductions made for players held out of practices or games for ongoing injuries (not including IR, which is separate from the 53-man roster).

A one-unit decrease in injury rate translates to about four fewer injuries in a team-season.

Overall Injury Rates by Turf Type

Figure 1. Injury rates with 95 percent confidence intervals by turf type for all teams (top) and visiting teams (bottom), 2012-2015 regular seasons.

The overall injury rates for grass and artificial turf are 16.9 and 17.2 per 1,000 AEs, respectively -- a modest difference that's attributable to chance. When we stratify by artificial turf type, though, we see a bigger range of differences. Matrix Turf has the best overall injury rate (14.2 per 1,000 AEs), while A-Turf's Titan 50 product brings up the rear at 18.8. Astroplay is also low (15.3) while FieldTurf -- the most common artificial turf in the NFL, used in five stadiums -- was elevated at 18.0. All the other turf types have rates within 0.8 of natural grass surfaces.

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Any conclusions we can draw about Matrix and A-Turf are tempered by the fact that they're only installed at one stadium each -- Dallas and Buffalo, respectively. We may just be seeing the effects of healthy or unhealthy rosters, injury reporting habits, or good and bad training staffs.

One way to control for that is to just look at the visiting teams who play on this turf. Here Matrix still leads the way at 15.6 (injury rates are generally higher for away teams than home teams), but the difference between it and grass (16.7) has been cut in half; maybe Dallas's roster or trainers were having a positive impact. A-Turf looks worse than it did before (21.0 per 1,000 AEs), suggesting Buffalo's training staff also isn't to blame. Momentum Turf, which was installed in Baltimore until last year, also doesn't look good in this analysis with a rate of 19.3, and FieldTurf isn't far behind at 19.2.

Lower Leg Injury Rates by Turf Type

Turf could be expected to have its greatest impact on lower leg injuries, such as knee, ankle, and foot injuries. We zero in on those rates below. To eliminate team effects we just looked at these for visiting teams.

Figure 2. Injury rates with 95 percent confidence intervals by turf type for knee (top), ankle (middle), and foot (bottom) injuries, 2012-2015 regular seasons.

Here we see a bit more variation. For knees, natural grass (2.8 per 1,000 AEs) has the lowest injury rate. Matrix Turf is elevated at 3.2 but includes a lot of uncertainty. Once again Momentum Turf and A-Turf (both 3.8) have the worst injury rates; Field Turf is also substantially above grass (3.6). The difference between Momentum/A-Turf/FieldTurf and grass is about one per 1,000 AEs, which translates to four fewer knee injuries a year for a team strictly working on grass versus these turfs. It's important to take account of the uncertainty in our data, but grass does appear to beat out at least certain kinds of turf for knee injuries.

Ankle injuries are more muddled. Here grass is near the middle with an injury rate of 2.0 per 1,000 AEs. Fieldturf is somewhat elevated (2.4). Momentum is once again at the top with 2.8, but a lot of uncertainty. All other artificial turfs are basically in line with grass.

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Foot injuries follow a similar pattern to ankle injuries, with Momentum again in the worst spot and FieldTurf also elevated. In contrast with its top spot for knee injuries, A-Turf shows a surprisingly low rate of foot injuries.

Conclusions

Unfortunately, our confidence intervals are too wide to draw many firm conclusions, but it may be worth trying out Matrix Turf at a couple more stadiums to see if its low overall injury rate is sustainable in a larger sample. Even when we zero in on lower leg injuries, it doesn't look meaningfully worse than grass.

A-Turf, Momentum Turf, and FieldTurf raise eyebrows, but these differences could be due to chance or improper maintenance rather than issues with the product itself. Still, Baltimore appears to have its own concerns about Momentum Turf, as they switched back to natural grass in 2016. They then rocketed from 30th to 11th in adjusted games lost (AGL) before suffering a rash of injuries during the early part of training camp, though many of those did not occur on the stadium turf. It will be interesting to see how the Ravens fare over the rest of the season.

In the second part of this article next week, we will focus on the turfs of individual stadiums to get a better idea of how these averages translate to actual NFL games.

Zach Binney is a freelance injury analyst and a Ph.D. student in epidemiology focusing on predictive modeling. He consults for an NFL team and loves Minor League Baseball. He lives in Atlanta.

Comments

18 comments, Last at 18 Sep 2017, 5:40pm

#1 by TomC // Sep 13, 2017 - 8:55pm

Omigod, error bars!

In general the level of statistical analysis here (including the subjective stuff like "this thing might be significant but this other thing is clearly in the noise") is quite high. Nice job.

#2 by Will Allen // Sep 13, 2017 - 9:26pm

Good effort in working with inadequate volume of data.

#3 by davew // Sep 14, 2017 - 2:30am

dave walter -At some point work in the turf's ability to increase performance: speed, cut ability, i.e. the trade-offs.

#4 by Megamanic // Sep 14, 2017 - 2:36am

But that affects both teams equally and if your team is playing 8+ games a year on a surface that significantly increases their chance of a season ending injury it's not a balanced trade IMO.

#5 by Aaron Brooks G… // Sep 14, 2017 - 8:15am

Any chance you could throw game temperature in as a variable?

Expecting a material to respond the same in Dallas (indoor) versus Buffalo (godforsaken moonscape) is silly.

#6 by Zach Binney // Sep 14, 2017 - 8:46am

That's a great point. Climate is certainly an alternative explanation for some of the data seen here. Basically, that'd require some more sophisticated modeling and I just wanted to start with a simple descriptive look. I'll be the first to admit there's much more work to be done before establishing a causal link between any of these kinds of turf and injuries. However...

I'll tease the second part of this series by saying that stadiums with disparate climates but the same turf have shown similar injury rates. That should be coming out soon-ish.

#15 by LionInAZ // Sep 15, 2017 - 10:09pm

You neglected to mention Chicago's goat track. Maybe precipitation ought to be included also.

#7 by roguerouge // Sep 14, 2017 - 9:33am

Hm. If the injury data at the college level is sufficient, that would be a bigger sample. Are the injury reports reliable and available?

#8 by Zach Binney // Sep 14, 2017 - 10:12am

The NCAA does have an Injury Surveillance Program which produces quite a bit of research. I'm not aware of a study on this particular question yet, but I'd love to see it done. Getting in touch with them is on my to-do list.

EDIT1: There are actually 3 studies on specific injury types from the NCAA ISP: this one (https://www.ncbi.nlm.nih.gov/pubmed/22491794) found higher rates of ACL tears, this one (https://www.ncbi.nlm.nih.gov/pubmed/23339895) higher rates of high ankle sprains, and this one higher rates of turf toe (https://www.ncbi.nlm.nih.gov/pubmed/24334272) on artificial "fill" turfs, of which Field Turf is a common type. So, there's your larger samples.

EDIT2: There's also this (https://www.ncbi.nlm.nih.gov/pubmed/20075177) study from 2010 that looked at 24 Division I football programs from the 2006-07 seasons. It did not use the NCAA ISP. It found LOWER overall injury rates for Field Turf vs. natural grass. However, as far as I can tell (the article is opaque and the Appendix seems to be missing) there were no such differences specifically for knee, ankle, or foot injuries. Also, the study was funded by FieldTurf. This doesn't invalidate the study, of course, but you always want to consider the funding source and motivations of any researcher (myself included - and I have no conflicts of interest to report).

#9 by Zach Binney // Sep 14, 2017 - 10:17am

Also note the study from the NFL cited in the article that found higher rates of knee and ankle sprains on Field Turf. That's not really a new finding, but it's good to have replications from independent sources (and more recent data).

There's a long, long literature of higher lower limb injury rates of various kinds on artificial turf vs. natural grass. What's been missing so far in both college and the NFL, to my knowledge, is a comparison across different types of artificial turfs. I'm hoping this post spurs some more interest in those questions.

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#10 by Kevin from Philly // Sep 14, 2017 - 11:45am

Throw out results from Gillette Stadium, as injury reports skew low for "unknown" reasons.

#11 by Zach Binney // Sep 14, 2017 - 11:49am

Oh man, you are gonna LOVE next week's stadium-specific results...

#12 by Independent George // Sep 14, 2017 - 11:55am

Any theories on causation? How are Matrix, Field Turf, and A-Turf constructed differently?

#13 by Zach Binney // Sep 14, 2017 - 1:04pm

Outstanding question, and to be quite frank: no, at least when it comes to separating artificial turfs. I don't have that level of expertise. I'm hoping others will take this data and chime in/work on this question with or without me.

For artificial turf vs. natural grass, the main hypothesized mechanisms are turf stiffness (more stiffness on turf --> less give --> greater impact force per stride) and greater friction between shoe and turf than shoe and grass --> increased forces for stopping/starting/cutting.