Understanding Injuries in the NFL Part 2

This is the second installment of a series exploring what actually correlates to injuries in the NFL, based on a series of articles by Zach Binney of Football Outsiders.

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If you haven’t read it already, find part 1 of the series here.

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Although this is a look at NFL injuries in general, it may contain nuggets useful to understanding the Redskins injury situation. This installment has several sections each trying to answer a specific question:

• How much are injuries actually rising in the NFL and how much is due to better reporting?
• Did the CBA have an impact on injury rates by limiting practice time?
• Are players more likely to get injured playing Thursday night?
• Does the field type (and playing at FedEx) play a role in injury rates?

This is a bit of a long article with lots of information, so as always there is a TL;DR summary at the bottom for those who just want the highlights.

Rise in injuries or rise in reporting?

The data in part 1 of the series indicated that there has been a significant increase in reported NFL injuries since 2007. As shown in the original analysis, we can first look at the overall number of distinct injuries that were reported in the NFL in a given year.

Figure 1 shows there is a pretty clear increase in the number of injuries reported over time, confirming what we saw previously. But is this due to a true increase in the number of injuries players are getting, or are injuries just being identified and reported more often? Is a certain kind of injury (for example, concussions or soft-tissue injuries) driving the increase?

To investigate this, we can separate injuries into 3 different types based on severity: injuries that cost a player 1-2 missed games (moderate), 3+ missed games (severe), or no missed games (minor). If the only difference is that injuries are now being identified and reported more often, then we would expect the plots of moderate and severe injuries to be relatively flat (players that missed games were reported in any year), while the plot of minor injuries will show a rise (because the new injuries that are reported were just too minor to report before). In addition, we’ll only look at non-head related injuries, because concussions are a bit of a different animal.

Indeed, the greatest rise is in the plot of minor injuries causing no missed games, while the plots of moderate and severe injuries remain mostly flat, although not entirely flat (there’s a slight increase over time). Mr. Binney quantifies this pattern as follows:

The largest increase by far is for minor injuries, which increased steadily by about 50 percent from 2007 to 2015. Moderate and severe injuries rose steadily from 2007 to 2011 before dropping and then rising again, but the overall difference between 2007 and 2015 is about 10 percent. Taken together, these suggest that much of the observed increase is due to better reporting of minor injuries.

But how much is due to reporting and how much to true changes? To estimate that, we can do some back-of-the-envelope math. First, we’ll assume (generously) that there has been a true increase of 10 percent in the NFL’s underlying injury rate. Overall, there has been about a 34 percent increase in non-head injury rates, so 10/34 = about 30 percent could be due to a true increase and 70 percent due to better reporting.

Head-injuries are a bit different though, as the NFL and medical community at large are learning more every year about concussions, their consequences, and how to better diagnose and treat them. Since we excluded head injuries from the previous analysis, we can look at them exclusively here, using the same three categories of injuries.

The number of head injuries increased regardless of severity. This is consistent with both better identification of head injuries and more careful treatment of those injuries that are identified. We could hypothesize that 1) better identification causes an overall increase in the number of concussions that are reported, and 2) more careful treatment causes increases in the higher severity groups since what would have been a “minor” concussion in earlier years becomes a “moderate” or “severe” concussion in later years.

A majority (at least 70 percent) of the increase in reported NFL injuries appears to be driven by increased identification and reporting of concussions and minor non-head injuries rather than an actual change in NFL injury rates. True injury rates likely increased about 10 percent from 2007 to 2015. That’s not negligible, but it’s much lower than the 35 percent or so that just looking at the overall change might suggest.

Although most of the increase in reported injuries is due to better reporting, there is still a (smaller) increase in the true injury rate in the NFL. So in order to better understand the forces that may be at work there, we’ll investigate some specific factors some people believe are leading to more injuries.

The impact of fewer practices due to the CBA

The Collective Bargaining Agreement (CBA) struck in 2011 by players and owners put limitations on the amount of practice players were forced to attend. Specifically, it cut organized team activities (OTAs) from 14 weeks to 9, eliminated padded two-a-days during training camp, increased off days during training camp, and limited regular-season padded practices from “whatever the coach wants” to 14. These changes were fought for by players because they believed it would actually lead to a decrease in the injury rate, but as we saw in the previous section, that was not the result. Bill Belichick has been quite vocal protesting the practice limitations of the CBA, claiming that it is actually leading to more injuries because players are not conditioned by the time the regular season begins. I’ve heard similar arguments from Chris Cooley over the years, as it is his belief that the extra padded practices helped players to learn better technique (especially while being tackled), and better technique leads to fewer injuries. Since we know part of the reason behind the increase in injuries is due to better reporting, this article looks into the situation first by plotting both the number of injuries and also the total number of games missed due to injury to see if there was a big change after the CBA took effect.

There is no obvious spike in either the number of injuries or games missed due to injury after the 2011 CBA. If anything, the number of injuries and time missed due to injury decreased in the years immediately following the CBA, before resuming its upward trajectory. But maybe we should do more to separate out different kinds of injuries, since the specific claim by Bill Belichick is that lack of conditioning is the reason players are getting injured. To do this, we’ll separate conditioning injuries from non-conditioning injuries. Conditioning injuries involve soft tissues such as the Achilles tendon, calf, groin, hamstring, biceps, triceps, pectoral, quadriceps, and ACL. Non-conditioning injuries included contact injuries such as fractures, high ankle sprains, and various types of trauma to the face and eye, Lisfranc joint, internal organs, neck, ribs, or toes (essentially, injuries due to trauma). All other injuries (most notably, concussions) are excluded.

There was a slight spike in the number and severity of conditioning injuries in the 2011 season, though it is quickly offset by a reduction in both conditioning and non-conditioning injuries in the following seasons. Rather than being due to restrictions on practice time, this one-year spike might instead be due to the 4.5 month lockout that prevented players from training at team facilities until the CBA was signed. Overall though, it looks like there is no obvious pattern indicating that poor conditioning due to the CBA is the reason behind an increase in injuries. In fact, it looks like the rate of both of these injury types has been relative stable since the CBA. If that is the case, maybe the overall increase in true injuries is largely driven by an increase in concussions (which were excluded from this analysis)?

Do Thursday night games cause more injuries?

Thursday night games have brought a tremendous amount of revenue to the NFL since their implementation, though players have repeatedly gone on record to voice their frustrations at playing Thursday night, claiming the lack of rest makes them more likely to get injured. This article from Football Outsiders investigates the possibility that lack of rest between games leads to more injury. This study separates out leg muscle injuries from other injuries in order to compare the results with a similar study done in soccer that found a 20% increase in injury after a short week in that sport. Leg muscle injuries are defined as any calf, hamstring, groin, quadriceps, or thigh injury. The results are shown below. The blue plot shows all injuries, while the red plot shows leg muscle injuries only. Note, an “AE” described in the plot is an “Athlete Exposure”, defined as players that play snaps in a game (so players on the bench are excluded).

There was no significant increase in the rate of injury for Thursday night football games. If anything, there was a slight decrease in injury rate for these games, although I can’t think of an obvious reason why that might be the case (and it could just be random fluctuation).

Blame it on the field!

One thing Redskins fans will often point to as a possible factor in the number of injuries Redskins players have had the last few years is FedExField itself. Some people claim that the field conditions are so bad that they lead to injury. Before we examine that claim specifically, lets look at the types of fields and how that correlates with injury, originally done here. I won’t describe in detail the properties of these different turf types, but a quick Google search will probably answer any questions you have. Note that FedEx is a natural grass field.

So certain turf types (A-Turf and Field Turf in particular) seem to be more dangerous, but natural grass is one of the more safe fields to play on. But there are a lot more factors at play than just the turf type. Maybe it’s the maintenance of the field that matters? People have often complained about bald dirt patches at FedEx. Or the specific breed of grass? Are we using tangleweed or stranglevine for our grass? I don’t know. The only way to truly account for all factors that could be at play for the field is to look at the the injury rates at the fields of all 32 teams (plus London), originally done here. But to separate out many confounding factors, we will only look at the injury rates of visiting teams, since that will average out factors specific to a team, like the conditioning staff of the home team. Let’s see what that tells us.

That’s right folks, FedExField is one of the safest fields in the NFL! BOOM!!! Queue the fireworks! It’s not even close, this is not just a freak effect due to few data points. Maybe those bald dirt patches are protecting people, I don’t know, but visiting players are actually much less likely to get injured here than at almost any other stadium. It’s only the home team that seems to get injured. I realize this will be tough for some fans to swallow, so let’s just be 100% sure. Since the field is only expected to contribute to knee, ankle, and foot injuries, let’s look at those specifically.

Nope, FedExField is still one of the safest in the NFL, no matter how you look at it. Don’t let appearances or your hatred of the stadium experience fool you folks, our players are not getting hurt because of FedEx. Also, note that grass is the safest field type in general, with artificial turf dominating the highest injury rates.

Summary

The TL;DR of this information to me is the following:

• Injury rates in the NFL have risen quite a bit since 2007, but most of that increase is due to better diagnosis and reporting of injuries.
• A majority (at least 70 percent) of the increase in reported NFL injuries appears to be driven by increased identification and reporting of concussions and minor non-head injuries rather than an actual change in NFL injury rates.
• True injury rates likely increased about 10 percent from 2007 to 2015. That’s not negligible, but it’s much lower than the 35 percent or so that just looking at the overall change might suggest.
• There is no significant increase in injuries associated with the practice time restrictions of the new CBA.
• Players are no more likely to get injured in Thursday night games than in other games. In fact, they may be slightly less likely to get injured on Thursday night, though it’s not a big effect.
• Natural grass fields are generally far safer than artificial turf fields.
• FedExField is one of the safest fields in the NFL!

I hope you found some of this interesting. Let me know in the comments if you interpret any of this information differently, or what you found interesting in particular (if anything). The third (and final) installment in this series will look at player-specific factors that influence injury likelihood, namely: are smaller players more likely to get injured and are certain players injury-prone?