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.
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.
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.
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.