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28 Jan 2018

What Does 'Injury-Prone' Mean in the NFL?

by Zach Binney

We see the phrase all the time. In the draft. In free agency. In fantasy evaluations. It's always a variation on a theme: "He's a great player, but he's injury-prone." The label is thrown around with ease, but rarely has anyone tried to quantify what it might actually mean in terms of injury risk.

In order to do that, we first need to turn "injury-prone" into something we can approach analytically. When people say "injury-prone," they mean they're concerned that a player's injury history places them at a higher risk for future injury. That's something we can look at -- indeed, previous injury is a well-known risk factor for future injuries that has been heavily studied in sports injury literature.

To see if a prior injury history puts a player at greater future risk, we calculated the one-season risk of missing one or more regular season games due to injury for player-seasons with various two-year injury histories. All injuries occurring in the offseason, preseason, or regular season were counted towards a player's two-year history. We used injuries from only the previous two years to ensure all players had the same amount of time to accumulate an injury history, and because injuries in the distant past may not be as indicative of future risk. We excluded a player's first two NFL seasons because he would not have yet accumulated a two-year NFL injury history. We also excluded player-seasons where the player did not play in the team's Week 1 game but was not on injured reserve for all 16 weeks, as these players may not have been at risk for a full season.

     

This left us with 8,585 player-seasons between 2009 and 2016. Forty percent of these player-seasons included at least one game missed due to injury.

Any Injury History

On average, 40 percent of players who played or would have played in Week 1 missed at least one game due to injury that season. Among those with no reported injuries in the prior two seasons, though, that figure is just 26 percent.

The risk of missing time increases rapidly with a longer injury history. Thirty-five percent, 41 percent, and 46 percent of players with one, two, or three injuries in the prior two years miss time in their upcoming season, respectively. As injury histories get very long (six or more total injuries in the previous two seasons), risks flatten out at around 55 or 60 percent.

Players with the longest injury histories have twice the risk of missing time as those with a completely clean bill over the previous two seasons. However, even a player with five injuries in two seasons has a 50/50 chance of at least suiting up for all 16 games his next season. Does a 50 percent chance of missing time make a player "injury-prone?" They are certainly more injury-prone on average than players with a shorter injury history, but it's important to consider their actual risks. Teams need to understand as precisely as they can the additional risk they might be taking on by drafting or signing a guy with a lengthy injury history rather than just saying "ah, forget him, he's injury-prone."

Figure 1: 1-Season Risk of Missing 1+ Game Due to Injury by Total Number of Injuries in Previous Two Seasons, With 1-SE Error Bars

Specific Injury Histories

The results above naturally lead us to ask whether there are any particular types of injuries that should concern teams more.

     

We looked at each player's history of 17 specific types of injuries over their previous two seasons. A longer history of most types of injuries corresponded to an increased risk of missing time in the upcoming season. What's more, the increases were surprisingly steady: each additional injury of a specific type added to a player's risk.

For example:

  • players with no knee injuries in the prior two years had a 37 percent chance of missing time in their upcoming season;
  • those with one to three injuries had a 45 to 50 percent chance of missing time;
  • and those with a history of four or more had around a 65 percent chance.

Lower extremity (LE) muscle injuries -- calf, quadriceps, hamstring, and groin issues -- followed a similar pattern: risks of 36 percent, 45 to 50 percent, and 60 percent for those with a history of zero, one to three, or four or more of these injuries.

The only two injury histories that weren't associated with future risk were face/eye and upper extremity (UE) bone and joint injuries -- arm, elbow, wrist, hand, thumb, and finger issues. This makes sense as these injuries tend to come from violent contact and are less likely to recur due to some underlying issue with a player.

Figure 2: 1-Season Risk of Missing 1+ Game Due to Injury by Number of Specific Injuries in Previous Two Seasons, With 1-SE Error Bars

Limitations

This analysis of injury histories includes only injuries that made it onto an injury report, which is a threshold with some arbitrariness and variation across teams. It also excludes players' first two NFL seasons, but it may be possible to look at young players' college injury histories to determine how risky they might be.

Perhaps most importantly, we only have data on player-seasons where the player was already deemed good enough to either play in Week 1 or stash on injured reserve for a full season by their team. This means we don't have data on players whose injury risks outweighed their skill level, which biases our sample towards higher-skill players with the longest injury histories. For example, despite an extensive history of hamstring issues we have a lot of data on Sean Lee because he's a good linebacker. If he had an identical twin, Jean Lee, with the same level of "injury-proneness" but who peaked as a practice squad player, we might never see his riskiest seasons because he washed out of the league after one hamstring strain.

Another way to think of this is that our analysis bakes in the conventional wisdom of who is and isn't an acceptable injury risk. If all 32 teams passed on a guy because they thought he was too risky, we never get to see his actual outcome. Teams might be right or wrong to filter these guys out; we can't be sure. All our analysis can say is what the risks are for the kinds of players they keep around right now.

Conclusions

It's clear that a lengthy injury history makes it more likely for players to miss time in the future. Such players are indisputably "injury-prone" relative to players with less of an injury history. However, in all but the most extreme cases, there seems to be at least a 50/50 chance the player will be able to suit up for every game.

Much more work remains to be done here -- including figuring out how combinations of different injuries may interact to put certain players at particularly high risk -- but this at least gives us a starting point for when "injury-prone" may be appropriate and what exactly it might mean in terms of the actual risk a team is taking on.

Posted by: Zach Binney on 28 Jan 2018

40 comments, Last at 10 May 2018, 3:14am by Adrian---

Comments

1
by billprudden :: Sun, 01/28/2018 - 9:38pm

Sir - Great effort, and thanks.

2
by nat :: Mon, 01/29/2018 - 10:58am

I'd be interested to see this broken out by position, if sample sizes permit. I would guess there are positions with higher risk of injury, and that this could be a confounding factor in the analysis.

Hypothetically, let's suppose offensive interior line were one of those risky positions. It might be that having a history of injuries puts a player "at risk" of playing an interior line position the next year, which in turn puts him at risk of future injury.

FWIW, I don't dismiss the findings here at all. I'm wondering if this check by position has been done, and whether it makes the "injury-prone" associations a bit smaller than shown here.

3
by Zach Binney :: Mon, 01/29/2018 - 11:20am

There are absolutely differences in injury risk by position (SEE e.g. http://www.footballoutsiders.com/stat-analysis/2015/nfl-injuries-part-iv...). However, it's pretty rare for players to switch out of a position group (with the exception of hybrid DEs/LBs changing from a 3-4 to a 4-3 team or something).

There is some minor variation within position groups (as yet unpublished) mostly due to role (e.g. blocking vs. pass-catching TE, FB vs. HB among RBs), but I would think any confounding would operate in the opposite way from how you've outlined. Unless teams have absolutely no idea what they're doing (OK, possible), you would think they would shift higher-risk players into lower risk roles, such as by only using an older LB on rushing or passing downs. So if anything that should have flattened the associations we found here rather than exacerbated them.

4
by nat :: Mon, 01/29/2018 - 11:51am

I wasn't concerned about teams switching players to higher risk positions.

I was wondering about these relationships:

High risk position (past) => more injuries (past)
High risk position (past) => high risk position (this year) => more injuries (this year)

The same factor - playing a high risk position in the past - increases a player's chances of injury both in the past and in the present. That could be mistaken for a history of injuries being a factor in current season injuries.

In a sense it's true: playing a risky position is bad for your health. But I don't think that's the "injury-prone" concept we're looking for. We want to know if a player is more injury-prone than average for his position group. After all, it's no surprise - and useless to know - that an interior lineman is injured more often than a placekicker.

It's quite possible - even likely - that there is such a thing as being injury-prone. I'd just want to see it among players in the same position group before I trusted the extent of the trend.

Did I miss something here? Had you already accounted for the different injury risks at different positions?

[edit] Nevertheless, you do good work on this topic. Thanks for doing it.

6
by Zach Binney :: Mon, 01/29/2018 - 12:50pm

Ah, I misread your comment above. Totally on me. That is an excellent point. I excluded it for the simplicity of the article, but here is Figure 1 stratified by position. Sorry it's very small - might write up a full piece on it later. Anyway, you can still see a strong association between previous injuries and future risk in every position!

Figure 1 by PositionFigure 1 by Position

5
by RickD :: Mon, 01/29/2018 - 12:36pm

The label 'injury-prone' is very likely to be abused by casual fans. If a player has two completely unrelated injuries, that doesn't mean that he's injury-prone.

Consider Rob Gronkowski: he's dealt with all the following types of injuries: broken forearm, blown ACL, and chronic back issues. None of the three has anything to do with the other two.

7
by Guest789 :: Mon, 01/29/2018 - 1:08pm

I mean, back issues can cause problems elsewhere due to compensation and such, so I wouldn't totally discount a connection to the ACL, but generally I'd agree.

8
by Anon Ymous :: Mon, 01/29/2018 - 1:51pm

Given that Gronk's ACL blew because another full speed player put his helmet right on Gronk's knee, I think it's safe to discount the connection in this case. :)

23
by RobotBoy :: Mon, 01/29/2018 - 11:25pm

I've noticed this season that Gronk doesn't run with quite the same sense of abandon as in the past. Used to be he'd run through or over any potential tackler but now, especially when guys are going low on him, he slows, crouches, and sticks out a hand to ward the defender away from his knees.
Brady's numbers drop significantly when Gronk is off the field - he goes from a top 3 QB to maybe 10. Other teams are certainly aware of this.

34
by RBroPF :: Tue, 01/30/2018 - 2:44pm

Certainly Barry Church was aware of it.

9
by Aaron Brooks Go... :: Mon, 01/29/2018 - 2:13pm

Reckless can be part of injury-prone.

Let's pick on someone other than Gronk. Consider Jim McMahon. He missed parts of a ton of seasons with a wide variety of injuries. Hell, the reason he wore the glasses in the first place is because he gouged himself in the eye with a fork when he was 6. Some people just exert a certain injury gravity.

Or Bob Sanders. Sanders missed time due to foot, ankle, knee, and arm injuries. Ultimately, it was repeat knee injuries that got him, but he missed large parts of seasons for three other body parts. Sanders also probably fits under the reckless concept.

11
by Pat :: Mon, 01/29/2018 - 2:57pm

Bob Sanders is an *excellent* example - I don't know why I didn't think of that one. There's an SI article from a few years back about Sanders's playing style, and he even knew that it would probably shorten his career, but it's the only way he knew how to play.

Heck, it's entirely possible that playing style leads to injuries more than anything inherent to the player's body.

12
by nat :: Mon, 01/29/2018 - 3:11pm

Good point.

There are a host of factors that lead to more frequent injuries. Some are what we'd call "injury-prone" and some aren't.

Factors that deserve the injury-prone label:
1) physical fragility
2) personal style of play (recklessness and/or poor technique)

Factors that don't deserve the injury-prone label:
3) position played
4) scheme and role in scheme
5) luck
6) being good enough that opponents target your head or lower legs or otherwise attempt to knock you out of a game or season

It's a matter of opinion how to apply these to Bob Sanders, Jim McMahon, or Gronk.

In my opinion, we can be pretty sure that Gronk plays a risky position (3), with a risky role in the scheme (4), and has been the target of deliberate head shots, leg whips, and direct targeting of his knees beyond what usually happens to an average tight end (6).

His technique is not particularly risky (in my opinion, and once you factor out the nature of his routes through traffic) although he's by no means a cautious player. (2) This is an area in which his play may have matured recently.

With his heavy doses of 3, 4, and 6, it's hard to judge whether either 1 (physical fragility) or 5 (luck) is really a factor. There's too much else going on.

15
by Zach Binney :: Mon, 01/29/2018 - 3:29pm

I love this list. I will say it's important to consider the perspective of whoever is giving the injury-prone label. For example, I would argue (6) is absolutely part of it if you're a team trying to decide whether to sign a guy or not. Am I ever going to NOT sign Gronk because players might go for his knees? Of course not, but it should still factor into my risk equation. (3) and (5) pretty clearly not.

On the topic of scheme - I have some really interesting stuff I should publish soon on size and position interactions that get at that, but I'd like to work in some more actual usage data first.

18
by nat :: Mon, 01/29/2018 - 5:51pm

Getting targeted for being a great player doesn't make you personally injury-prone in the usual sense. But it does put an upper limit on your value, I agree. You can hide a valuable QB from dirty players to some extent. A tight end is available to cheap shot artists, and there isn't much he can do to avoid them without losing a lot of value.

I'm looking forward to the scheme topic. Sounds hard, but interesting.

16
by Eddo :: Mon, 01/29/2018 - 3:32pm

Why would not consider factors #3 and #4? Isn't the point of figuring out which players are "injury prone" to understand how well you need to put some depth around them? If nose tackle is a particularly injury-prone position (factor #3), then you need to make sure you have depth there. If you plan on using your top WR as a lead blocker, and that leads to more injuries (factor #4), then you need to have more depth there.

17
by Anon Ymous :: Mon, 01/29/2018 - 3:40pm

Nat isn't saying those factors shouldn't be considered, he's talking about whether an individual player deserves to be considered "injury prone".

19
by Eddo :: Mon, 01/29/2018 - 6:16pm

But why else would you be tagging a player as "injury prone" if not to determine how much you need to worry about depth?

21
by nat :: Mon, 01/29/2018 - 7:20pm

Draft position. Trades. Cutting him or not. How much to offer him...

Only a complete jerk would offer a player less because he was willing to run routes into traffic and blocked aggressively, even at the risk of more frequent injury.

27
by Eddo :: Tue, 01/30/2018 - 11:30am

All those things are about player acquisition (which, to be fair, I did not explicitly say).

But it wouldn't be a jerk, it would be a smart GM.

Hypothetically, if the position of nose tackle was more likely to suffer injuries than the position of defensive end, and a team had players they value equally when healthy at each position, they should absolutely be willing to pay the DE more, because it's more likely that player stays healthy.

31
by Anon Ymous :: Tue, 01/30/2018 - 12:52pm

You're still missing the point. Of course any GM is going to have contingency plans for positions that entail elevated injury risk, but that isn't what being "injury prone" is about. The "IP" label is for players that have elevated injury risk above and beyond the norm for their position.

To put it another way, Player X is not injury prone for suffering 4 major injuries if that is the median for his position. Player Y is more likely to be injury prone for suffering 3 major injuries if his position has a median of 1... even though his total is less than Player X's.

32
by Eddo :: Tue, 01/30/2018 - 1:14pm

When comparing players at the same position, yes, you're absolutely right.

But, using your Player X and Player Y as examples, if I think they provide the same amount of value as each other when healthy, I'm still paying player Y more, as he is less likely to miss time with injury.

37
by Anon Ymous :: Tue, 01/30/2018 - 3:17pm

Sure, but recall that this traces back to your first comment and this line:

Why would not consider factors #3 and #4?

That said, I think we've finally found common ground. :)

28
by Pat :: Tue, 01/30/2018 - 12:14pm

"Only a complete jerk would offer a player less because he was willing to run routes into traffic and blocked aggressively, even at the risk of more frequent injury."

Um, no - I'm pretty sure that's just math and business. It's supply and demand. Again, Bob Sanders is a great example here. Not as many teams went after him compared to someone else who was as on-the-field skilled, simply because he just wasn't going to be on the field as much. I mean, this is just math: if you've got a player who's likely to be injured, you need to spend more on his backup. So the net cost of that player is more than just their own salary.

It's literally not much different than why you don't pay a lot for a running back. They're not going to be around as much, so dedicating resources to them isn't a great choice.

29
by nat :: Tue, 01/30/2018 - 12:45pm

You're missing the point. Bob Sanders was injury-prone compared to average for his position, and because of his own style of play, not the scheme - or let's suppose so for this discussion. Having him on your roster meant you'd be playing backup players more than other teams. That limited his value. Injuries came with the package.

That's not true of a player who is used (by the coaches) in ways that increase injury risk compared to average for his position. If, for example, your scheme puts your slot receivers at extraordinary risk of injury, then it's not the slot receivers who are forcing you to devote more roster spots to receivers. It's your scheme.

Cutting (or low-balling, with possibly the same effect) your supposedly "injury-prone" slot receiver just replaces him with a free agent or someone similar. If that replacement runs the same high-risk scheme, he probably gets injured at a similar frequency.

And the next year, you cut that guy for being "injury-prone" too? Yes, that would be jerkish. Also stupid.

24
by ssereb :: Tue, 01/30/2018 - 1:45am

I think two factors you missed (though I'm not asserting them in the case of any particular athlete) are conditioning and quality of medical care. If a player neglects care of his body in such a way as to make himself more vulnerable to injury, that probably belongs among the factors a player is responsible for, whereas a medical staff that misdiagnoses or mistreats injuries belongs with the latter group of factors outside a player's control.

13
by jtr :: Mon, 01/29/2018 - 3:18pm

Just generally playing a violent style of football like Gronkowski does is likely to increase a player's injury risk. And I suspect there's a correlation with size--ligaments and tendons don't get linearly bigger and stronger as the humans who depend on them get bigger and stronger.

14
by Anon Ymous :: Mon, 01/29/2018 - 3:29pm

Perhaps, but this doesn't align with the injuries sustained. Gronk broke his arm blocking on an extra point and had his knee blown out while making a catch. The back is tougher to say, because some of his surgeries have been maintenance, but the lost season last year was just a brutal tackle. Had he been injured while trying to carry three tacklers into an end zone that would be more in line with that description.

10
by Pat :: Mon, 01/29/2018 - 2:49pm

Eh. I mean, it's super-hard to say that without being Gronkowski's doctor. I'm sure it's possible for those three to be medically related, and I'm sure it's possible for those three to be totally unrelated.

Even *related* injuries don't necessarily make a player injury-prone: it's always possible for the first injury to be improperly healed, of course, or for doctors to take multiple attempts to figure out how to fix something permanently. Correll Buckhalter was a running back for the Eagles and missed 3 full seasons in 4 years due to a torn left ACL, torn right patellar, and another torn right patellar. And then he never missed more than 2 games/season for the final 5 years of his career.

But even if they're not medically related, that still doesn't mean they're not causally related. I think, if anything, "injury prone" is probably more appropriate when you have a *player* who doesn't necessarily protect themselves in how they're playing, like RG3. So it's hard to say that Gronkowski doesn't have some of that.

20
by rpwong :: Mon, 01/29/2018 - 6:25pm

Players definitely need to be coached to protect themselves more, rather than making high-risk moves to gain a few extra inches at the end of a play. I know that it's "in the moment", but as a coach I'd want the player's instinct to be, "get as much as I can and go to the ground safely."

I'm specifically looking at the players who think, "I'm going to hurdle over top of this guy and keep going". It looks great on film, but if the defender plants his feet and straightens up, you could easily come down on your face and break your neck.

22
by RickD :: Mon, 01/29/2018 - 9:26pm

Well we know exactly how these injuries happened. His ACL was blown out when TJ Ward targeted his knee at full speed. It's a bit much to put that one on Gronk for being "reckless" and certainly not as part of some structural weakness to his skeletal/muscular frame.

Certainly the prolonged issues with the arm were related to each other. And his back issues have been around since he was an undergrad.

25
by Aaron Brooks Go... :: Tue, 01/30/2018 - 9:43am

When I was talking about Gronkowski as reckless, it's the forearm issues I was thinking of.

The injury in November 2012 was just one of those things.
The reinjury in January was likely partially attributable to coming back too soon.
The infection surgery in February 2013 is probably at least partially on Gronk, for failure to properly maintain his arm and for getting caught on film wrestling guys in a bar in his cast. That's just reckless and an own-goal.

Coming out with a chronic back issue sets you up for an injury-prone label, though. That's not the sort of thing that usually heals.

26
by Kopalec :: Tue, 01/30/2018 - 11:01am

Interesting topic, I wonder though...

Take someone like Marshawn Lynch, likely playing the most "injury prone" position in the NFL. He also played with what many would deem reckless abandon, making an effort for every inch of grass. Yet, from my admittedly arbitrary point of view I wouldn't consider him injury prone. He suffered 15 injuries throughout his career judging by a quick search, but many didn't cause him to miss games, mainly practice time or a reduced workload. His initial retirement after 2015 was due to a series of LE injuries adding up to reduced production. Even his reoccurring issues with his back were something he treated and played through. What would your thoughts on him be? Do I have rose colored glasses, or is he an outlier?

30
by Aaron Brooks Go... :: Tue, 01/30/2018 - 12:45pm

Lynch hasn't had an especially high career workload. His career touches are comparable to McCoy or Forte, and less than Peterson.

He has about 1000 fewer career touches than a Gore or a Bettis, and about 2300 fewer than Smith.

I would argue most of those guys played comparable styles. Most backs don't last this long (either because of quality or durability), but he's not especially an outlier.

33
by jtr :: Tue, 01/30/2018 - 1:39pm

There is also a general "toughness" factor that probably keeps some players on the field over others. Although that doesn't necessarily help the team; for instance, I can think of a few games where Kurt Warner played through something just to hurt his team.

Played through a broken hand in Philly in 2002 and ended up with 3 fumbles, 2 INT's, 8 sacks, and only 218 passing yards on 42 attempts:
https://www.pro-football-reference.com/boxscores/200212010phi.htm

Played through a concussion in 2003 at NYG and ended up fumbling 6 times, with a pick and 6 sacks to boot:
https://www.pro-football-reference.com/boxscores/200309070nyg.htm

35
by RBroPF :: Tue, 01/30/2018 - 3:06pm

I'm always interested in this topic and really appreciate the work you've done here.

I always look at this in terms of projecting player value. If you're going to continue developing this research, it seems like it'd be pretty straightforward to get a lot more information about the probable reduction in value due to injury history. If you calculated the mean number of games missed due to injury, rather than just whether or not they played all 16 games, that would be really illuminating.

e.g. While a guy who misses one game is 94% as valuable as a guy who didn't miss any games, this analysis considers one to be healthy and the other injured. On the other hand, guy who misses 13 games is only one-fifth as valuable as a guy who misses one game, and this analysis looks at both as being equally injured.

36
by Zach Binney :: Tue, 01/30/2018 - 3:14pm

Couldn't agree more. This is just one of many dimensions you could use to look at the issue. It's always tough to decide what to leave on the cutting room floor. Much more will be available in the academic paper version of this article, I promise.

38
by ChrisS :: Wed, 01/31/2018 - 12:35pm

How hard is it to separate the effect of coming back "too early" from an injury from the effect of being more injury prone?. If I get an injury I am not going to resume my normal athletic activities until I am at 100%. An NFL athlete is likely to come back well before that because at 75% healed he is likely better than his replacement but at some cost of higher probability of re-injury.

39
by medelste :: Thu, 02/01/2018 - 2:20pm

Great analysis, Zach! handclapemoji handclapemoji handclapemoji

Let's also save a handclap for Conventional Wisdom! So often you're wrong, CW, but there may be something to this "injury prone" label you've been using forever. Then again, a broken clock is right twice a day...

40
by Adrian--- :: Thu, 05/10/2018 - 3:14am

Really interesting article, thanks. I'd be interested in some analysis of how many injuries are suffered by players according to size (preferably by position). I often read draft profiles with comments like "slender frame may make him vulnerable to injury" but I'd love to see the injury rates for 180-189lbs WRs, 190-199lbs WRs, and 200lbs+ WRs (and every other position).

I think the injury risk to smaller players is generally overstated because often smaller players are more flexible, and many of the worst injuries are affected by the amount of weight placed on a leg.