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» Audibles at the Line: Week 7

Three shutouts in one day, and the worst defense in the league nearly makes it four. Tennessee survives overtime against Cleveland. Chicago completes four passes and wins. Not much was clear on this very weird day in a very weird season.

22 Sep 2017

Turf Type and NFL Injuries: Part II

by Zach Binney

In the first part of this series, last week, we found that, on average, natural grass has lower injury rates than artificial turf, particularly for knee injuries. However, the turf type itself isn't the whole story: how turf is maintained can play a big part in how it affects injuries. With that in mind we analyzed injury rates by stadium from 2012 to 2015, looking only at injuries to visiting teams to remove any impacts of the home roster and training staff. We also limited the 49ers to Levi's Stadium, excluded Buffalo's two games in Toronto in 2012 and 2013, and ignored the Vikings because of the Metrodome's roof collapse at the end of 2013. The Giants and Jets are combined into the same stadium ("NYC"), and neither one is a visitor when they play there. Results are provided for the Georgia Dome in Atlanta even though the Falcons now play in the Mercedes-Benz Dome since they did not change turf manufacturers and some of the grounds crew likely remains the same. For stadiums that switched turfs during the study period (Cincinnati, Houston, and the NYC stadium), the turf that was there the longest is used below -- in all cases this was three out of four seasons with the indicated turf. Only Houston switched from grass to artificial -- more on that below.

A one-unit decrease in injury rate translates to about four fewer injuries in a 16-game team-season.

Overall Injury Rates

Figure 1. Injury rates with 95 percent confidence intervals by stadium, 2012-2015 regular seasons. The black horizontal line is the overall average.

Looking at Figure 1 quickly can tell us a few things. About half of stadiums are natural grass -- 16 of 31 over this period, to be precise -- and they dominate the lower end of injury rates. The most common artificial turf is Field Turf, which covered five stadiums over this period. The upper end of injury rates is dominated by artificial turf stadiums.

Looking more closely, as expected, visitors to the only field with A-Turf -- New Era Field in Buffalo -- suffer a high overall injury rate. At 23.0, they are effectively tied for the worst overall rate. This would translate to roughly seven or eight more injuries over the course of a full home slate for a team playing in Buffalo instead of a league-average stadium like New Orleans.

Baltimore, the only stadium with Momentum Turf over this period, ranked sixth-worst. As noted above, and perhaps recognizing this, the Ravens switched back to natural grass for 2016.

Indianapolis, New England, and Seattle are three of the four worst stadiums -- and all three of them use FieldTurf. The other two FieldTurf stadiums, Atlanta and Detroit, also had above-average rates.

Rounding out the worst five is Houston. Houston from 2012 to 2014 used natural grass, but they stored it uniquely in 8x8-foot trays that were then wheeled together in the stadium. This left seams that many players complained caused injuries. Indeed, Houston's stadium in 2012 to 2014 had a visitor injury rate that essentially tied them with Carolina for the worst grass rate in that time period. In 2015 Houston got the message and moved over to UBU Sports Turf, though their visitor injury rate remained fifth-worst that year. Arizona, with the fourth-lowest overall injury rate from 2012 to 2015, also stores its grass away from the field but keeps it in a single tray.

San Francisco had the lowest injury rate at 14.7 per 1,000 AEs, though they were measured over only in 2014 and 15 while other teams were measured over the whole period. They had the second-lowest rate when we limited all teams to 2014 and 2015. Levi's Stadium is natural grass.

Washington, which also uses natural grass, is next-lowest at 15.0. Playing a full home slate in Washington instead of New Orleans could be worth about seven or eight fewer injuries.

Rounding out the best five are Tampa Bay and Arizona, which use natural grass, and Dallas, the only stadium to use Matrix Turf. Dallas has the lowest overall injury rate of any artificial turf stadium during this period.

Alternative Explanations

We're looking at these stadium rates through turf-colored glasses, but the rate in any given stadium is a factor of many things. For example, the home team's baseline health and training staff skill could play a role. We have attempted to control for this by only looking at visiting teams. Climate is another possible factor, but it's beyond the scope of this post. A third possible factor is that some teams are more likely than others to injure their opponents. For example, if we had split the NYC stadium between the Giants and Jets, the Giants would have the fifth-lowest overall injury rate, while the Jets would have been in the top half. To investigate this we could control for the injury rate for Team A's opponents when Team A is on the road. However, that is also beyond the scope of this post.

Instead, we decided to repeat our analysis for knee, ankle, and foot injuries -- some of the injuries most likely to be affected by differences in turf quality -- to see if the overall injury patterns remain.

Knee, Ankle, and Foot Injury Rates

We refer to these injuries below as "lower leg" injuries. Overall we see similar patterns with grass stadiums dominating the lower end and artificial turf dominating the upper end of rates.

Figure 2. Knee/Ankle/Foot injury rates with 95 percent confidence intervals by stadium, 2012 to 2015 regular seasons. The black horizontal line is the overall average.

Baltimore -- the only stadium with Momentum Turf over this period -- jumps from sixth-worst to worst, with a lower leg injury rate of 9.2 per 1,000 AEs versus an overall average of 6.8. This translates to around four or five more of these injuries over the course of a full home slate. As a reminder, Baltimore transitioned back to natural grass in 2016 and immediately improved from 30th to 11th in adjusted games lost (AGL).

Dallas -- the only stadium with Matrix Turf -- looks downright average for lower leg injuries. They held the fifth-lowest spot overall. As in our previous post, Matrix doesn't look as good for lower leg injuries as it does for others. It's possible the low overall rate could be driven by other factors like the pleasant indoor climate or a lower propensity of Dallas to injure its opponents.

Meanwhile, Buffalo and its A-Turf look better. They have a perfectly average rate of lower leg injuries despite having the second-worst overall rate. Perhaps A-Turf isn't as bad as it first appeared, and the elevated overall rate is due to other factors like climate or the propensity of Buffalo to injure its opponents.

FieldTurf still looks bad, though -- those stadiums hold the second-, third-, and fourth-highest rates of lower leg injuries. The other two FieldTurf stadiums also rank in the top half.

The stadiums with the six lowest lower leg injury rates are all natural grass. Houston remains essentially tied with Tennessee for the worst grass ranking.

Conclusions

All else being equal, natural grass fields seem the safer choice, especially for lower leg injuries. The issue, of course, is that they may not be suitable for all climates and stadium types. There's also a lot of stadium-to-stadium variation -- if you choose grass but use Houston's tray system, that's not helpful. Getting the right turf can pay huge dividends, as the difference between average and bad turf can be around half a dozen injuries per season. Any extra upfront costs of making the right choice pale in comparison to the cost in wins and game checks of getting it wrong.

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.

Posted by: Zach Binney on 22 Sep 2017

5 comments, Last at 24 Sep 2017, 12:31pm by bonesjackson

Comments

1
by Richie :: Fri, 09/22/2017 - 6:39pm

Green Bay doesn't use natural grass?

This is not natural grass?

https://i.ytimg.com/vi/uJXXEosOou0/maxresdefault.jpg

2
by dbostedo :: Fri, 09/22/2017 - 6:48pm

From the Packer's website :

In 2007, the Packers installed an entirely new playing surface, including a completely new drainage and heating system, bringing the latest technology in field management to the famous stadium. Chief to the system is DD GrassMaster, a natural-grass surface reinforced with man-made fibers. All existing levels of the field were removed and the new system began with a clay sub-grade level, compacted and graded (with a .6 percent slope), including drain tile, irrigation pipe and thermostat wiring for the heating system. The second level consists of 5½ inches of pea gravel. On top of the gravel layer is 43 miles of ¾-inch tubing for the heating system, which can maintain a root-zone temperature of 55-plus degrees to keep the ground from freezing during the season’s latter months. Level three consists of 12 inches of root-zone sand and Kentucky bluegrass turf. DD GrassMaster’s synthetic fibers are stitched into the surface, providing strength and stability to the field. Fibers extend approximately seven inches below the surface, are exposed approximately one inch above the surface, and are spaced every three-quarters of an inch. Approximately 20 million individual stitches make up the process. The slope equates to a crown of about 5½ inches on the new surface. To further enhance the surface, a system of grow lights is used in the fall to extend the growing season.

3
by Richie :: Fri, 09/22/2017 - 8:47pm

I don't understand why the grass looked dead in that 2013(2014) playoff game. If there were synthetic fibers in there, wouldn't we see them? Instead of just seeing yellow?

(I realize you are just the messenger.)

4
by bonesjackson :: Sun, 09/24/2017 - 12:28pm

The synthetic fibers only stick out 2 cm from the surface. Their primary use is as root support system for the natural grass as it is pushed 20 cm deep. The roots of the natural grass connect to the grassmaster fibers..

Keep in mind the natural grass should never been cut to 2 cm for a football game. You wouldn't see it unless you are on the field trying to see it.

5
by bonesjackson :: Sun, 09/24/2017 - 12:31pm

Also keep in mind that they quit giving the grass light in early December since it wouldn't help as the temperatures are too cold to allow the grass to grow really no matter what they tried to do to keep it warm and growing