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23 Oct 2015

NFL Injuries Part IV: Variation by Position

by Zach Binney

If you haven't read Part I, Part II, and Part III of this series, I highly recommend it. Part I lays out what we're trying to do in this four-part series (describe NFL injuries) and gives some important caveats. Part II investigates how injury risk varies over the course of a season and how injuries have changed in the last 15 years. Part III tackled the question of how injuries change as players age.

Today we're breaking down the top injuries by position. Are any positions more or less likely to get hurt than others? And are shoulders a bigger concern for, say, quarterbacks than for offensive linemen?

Injury Risk and Variation by Position

There seem to be three tiers of positions when it comes to injury risk:

  • First are what I'll call the "mobile" positions. Defensive backs, linebackers, running backs, wide receivers, and tight ends have about a 65 to 70 percent risk of landing on the injury report, a 40 percent chance of missing at least a game, and a 15 to 20 percent chance of missing four or more games in a given season.
  • In the second tier are quarterbacks and linemen on both sides, with about a 60 percent (linemen) or 55 percent chance (quarterbacks) of ending up on the injury report, a 35 percent chance of missing at least a week, and a 15 percent chance of missing four or more games in a given season.
  • All on their own in relative safety are the special-teams specialists. They only have a 25 percent chance of hitting the injury report, a 10 percent chance of missing time, and a 5 percent chance of missing four or more weeks. Maybe with the longer PATs this year we'll get a few more players like Bill Gramatica to even things out. However, we should note that only punters, kickers, long-snappers, and a handful of return specialists are tagged "special teams" in the data, so this doesn't mean that there are actually fewer injuries on special teams plays. But when gunners or blockers are injured, they get coded by their normal position on offense or defense.

So, what are the most damaging injuries for each position on the field? Do some injuries impact certain positions more than others?

As a reminder: the charts below represent regular-season injuries only. The average weeks missed (red bars) are not recovery time estimates for long-term or severe injuries. For example, in our data, an ACL tear in training camp will result in 16 weeks missed, but one in Week 15 will cost the player only 2 weeks. We would need another analysis to assess recovery time, and I'm still working on that. A simple but incomplete solution would just be to calculate the percentage of the season remaining that a player misses -- that would at least put ACL tears at 100 percent. Another more rigorous solution might be a survival analysis, such as Kaplan-Meier curves or Cox models. These are often used to do things like compare median survival times for different cancer drugs, but that could easily be adapted to recovery times for NFL injuries. Anyway, something for the future.

One last quick note: when I say below that an injury seems to be more severe for a given position, that's based strictly on how much time they miss. I did not look at whether injury affected the performance of those who do play varied by position. For example, I can tell you that defensive backs don't tend to miss more time, on average, than other players with a hamstring injury, but I can't tell you if that balky hamstring affects their performance after they do return. It's a familiar refrain by now, but that might be a good topic for a future post.

Also, we have replicated Figure 2 from Part I of the series below if you want to compare any of these charts to the corresponding data for all players.

Quarterbacks:

  • Shoulder injuries, rather than knee injuries, are the biggest bane of quarterbacks, by a large margin. Knee injuries are the biggest problem for every other position and for players overall. Digging a bit deeper into shoulders, "other" shoulder injuries only keep quarterbacks out a couple of weeks on average, but if you hear the word "tear" (e.g. labrum tear, rotator cuff tear), go ahead and weep into your bourbon. Fortunately, as you might expect, comparatively minor shoulder injuries are substantially more common than IR-inducing tears.
  • Elbow problems come in second in overall damage for quarterbacks. This is way higher than these injuries appear for any other position, and they're not even in the top 20 overall.
  • Concussions, when they happen, might be a bit more severe for quarterbacks, who miss 1.6 weeks on average vs. 1.2 weeks for players overall.
  • "Other" shoulder injuries knock quarterbacks out for 2.4 weeks on average, compared to 1.4 weeks for all other players. Shoulder tears are identically problematic at 8.4 weeks for quarterbacks and players overall. If you really ripped up your shoulder, you're out for the rest of the season no matter where you play.

Running Backs and Fullbacks:

  • Broken legs are the No. 6 problem for running backs vs. 14th for players overall.
  • Knee sprains and non-ACL tears, as well as ankle sprains, are also each a few slots higher for running backs than for players overall.
  • "Other" knee injuries are actually about as problematic for running backs (average 1.6 weeks missed) as for players overall (1.7 weeks). Hamstrings, knee tears (non-ACL), knee sprains, ankle sprains, and high ankle sprains follow a similar pattern. Lisfranc injuries, interestingly, appear less severe for running backs than for other positions (7.4 weeks vs. 9.2 weeks for players overall), but this is likely just noise from small sample sizes; we counted only eight Lisfranc injuries for running backs; with some randomly occurring later in the season.

Offensive Line:

  • Everybody in the pile! "Other" ankle injuries and ankle sprains are a bigger problem for the Big Uglies, taking second and ninth place in overall damage vs. fifth and 16th for all players, respectively. Also, broken legs take eighth place for offensive linemen vs. 14th for all players. Maybe these injuries are correlated more with ending up at the bottom of the pile at the line and having guys fall or roll on top of you?
  • Triceps injuries probably show the most drastic difference: they're sixth in overall damage for offensive linemen, but they don't even crack the top 20 for players overall. It makes sense that pushing hard on very heavy objects with your arms dozens of times a game is correlated with triceps injuries.
  • Back injuries are also more problematic, taking fourth place in overall damage for offensive linemen vs. eighth place overall. This could be pile-related, too, but the difference is less dramatic here than it is for other injuries.
  • Huff-puff, big man! Hamstrings are a smaller problem, ranking 11th for offensive linemen but third for all players in overall damage. I'd have to guess that's due to the much lower frequency (but much greater awesomeness) of a guard rumbling 30 yards or more downfield than a running back or wide receiver doing for the same.
  • Back injuries, in addition to being more frequent for offensive linemen, appear to be slightly more severe as well, costing them 1.6 weeks on average, compared to 1.2 for all players. Achilles injuries seem to be less severe for offensive linemen, knocking them out for 4.6 weeks on average compared to 6.5 weeks for all players. This might just be small sample size again (we only counted 16 Achilles injuries for offensive linemen), or it might be due to the lower running demands of the position.

Wide Receivers and Tight Ends:

  • Receivers (including both wide receivers and tights ends) have a pretty similar distribution of overall damage from injuries to those of all players. Knee sprains (eighth for receivers vs. 12th for all players) and ankle sprains (tenth vs. 16th) are somewhat bigger problems for receivers than for other players, probably driven by running demands.
  • Notably, ACL tears take over the top spot in overall damage from "other" knee injuries, their severity just making up for their lower frequency in this group. ACL tears are more frequent relative to "other" knee injuries among receivers: ACL tears make up 14.3 percent of the total of "other" knee injuries among receivers, vs. 10.1 percent for all players.
  • Thumb injuries are substantially worse for receivers than they are for the average player, sidelining them for 3.0 weeks on average, more than double the 1.4 weeks for players overall. Knees are a mixed bag, with sprains (2.1 weeks missed for receivers vs. 1.7 weeks for all players) seeming more severe but non-ACL tears (5.6 vs. 6.5 weeks) and "other" knee injuries (1.3 vs. 1.7 weeks) somewhat less severe for receivers versus all players. Hamstring, groin, Achilles, ankle, and foot injuries have about the same severity for receivers as for all players, despite the greater demands running might put on these body parts.

Defensive Linemen:

  • Defensive linemen have roughly the same most common injuries as all players combined, with a couple small differences: calf injuries just crack the top 10 in overall damage for defensive linemen, but are 18th for players overall, while Achilles injuries are fifth for defensive linemen vs. ninth for all players. These might be pile-related, too, but the differences in rankings aren't quite as big as they are for offensive linemen.

  • Similar to offensive linemen, though, hamstrings are a much smaller problem for defensive linemen than for other players, ranking eighth in overall damage for the defensive trenchmen vs. third for players overall. Again, this is likely due to the lower running demands on the defensive line.
  • "Other" shoulder injuries knock defensive linemen out for 1.1 weeks on average, slightly less than the 1.4-week average of other players. Hamstrings are also slightly less of a problem (1.3 vs. 1.6 weeks), while calves are slightly more of a problem (1.4 vs. 1.1 weeks). Interesting but completely pointless is the fact that eye injuries are tripled in severity for defensive linemen (average 3.7 weeks missed) versus all players (1.3 weeks). I'm all but certain this is random error from small sample sizes -- we only found nine injuries among defensive linemen -- but it's fun to think this is because they have to constantly watch the running back or quarterback or something.

Linebackers:

  • Bro, do you even bench press? Pectoral injuries rank eighth in overall damage for linebackers but 21st for all players. They are usually more severe injuries for linebackers too, knocking them out for 8.4 weeks on average, compared to 6.0 for other players. Frankly I'm not sure what to make of it since a majority of pectoral injuries are IR-inducing tears. Do they just tend to occur earlier in linebackers? There were 34 pectoral injuries among linebackers so I'm less keen to chalk this up to random error.
  • Achilles injuries are sixth in overall damage for linebackers and ninth for all players. This makes sense given the demands that linebackers place on that part of the body.
  • On the other side, "other" foot injuries are a substantially smaller problem for linebackers, ranking 11th in overall damage at that position vs. sixth for all players. That wasn't what I expected to see for a run-heavy position, and it is driven by slightly lower severity: the average absence for linebackers who suffer this injury is 1.2 weeks, vs. 1.5 overall. Back injuries are also less damaging, ranking 12th for linebackers but eighth overall.
  • Lots of collisions and rough tackles going on here, right? Turns out concussions for linebackers are right in line with the rest of the players: 10th in overall damage for each. Curiously, severity is a bit higher for linebackers, who average 1.6 weeks out with a concussion vs. 1.2 overall.
  • Linebacker injury severity doesn't vary much from all-player averages for most injuries. As noted above, "other" foot injuries are a bit less severe for linebackers for some reason, while "other" knee injuries are a smidge worse (average 1.9 weeks missed vs. 1.7 overall). Hamstring, "other" ankle, Achilles, and groin injuries are about the same for linebackers as they are for all players overall. Turf toe injuries exhibit the biggest difference (3.6 weeks for linebackers vs. 1.9 weeks overall), but that's probably due to the small sample size of nine turf toe cases among linebackers.

Defensive Backs:

  • Hamstrings snag the No. 2 spot in overall damage for defensive backs, beating out ACLs. This isn't surprising for a position that runs so much. What is surprising is that hamstrings keep defensive backs off the field only about as long as the average player. I would have expected defensive backs to be hampered for longer. Of course, their performance could still be degraded more severely than other positions who suffere that injury -- we didn't look at that.
  • "Other" foot injuries knock defensive backs out for 2.0 weeks on average vs. 1.5 for all players. Non-ACL knee tears (7.5 vs. 6.5), high ankle sprains (4.1 vs. 3.4), and turf toe injuries (2.6 vs. 1.9) all also seem to hit defensive backs a bit harder than players at other positions. Hamstrings, "other" knee, "other" ankle, groin, and Achilles injury rates are about the same for defensive backs as they are for other players.

Next Steps and Comments

This concludes our four-part series on describing injuries in the NFL. That was a lot of information. My hope was for this to serve as a useful reference post and not just a one-time read. We have taken a look at injuries overall and tried to describe how they have varied over the last 15 years and by position, age, and week of the season.

We welcome any comments or suggestions for future directions below. Gut reactions or deep cogitations upon whatever caught your eye are great. Injuries are a major area of concern in the NFL right now, and we want to make sure we're staying on the forefront.

Zach is a freelance injury analyst and a PhD 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 23 Oct 2015

5 comments, Last at 30 Oct 2015, 9:51am by brian30tw

Comments

1
by dbostedo :: Fri, 10/23/2015 - 9:08pm

Awesome as usual... just wanted to say thanks.

2
by Digit :: Fri, 10/23/2015 - 11:18pm

I'm thinking maybe separating statistics from pre-2006 (or whenever the Brady / Carson Palmer rule kicked in) against post-2006 might be an interesting comparision.

Would that sort of separation tell us anything about the frequency of shoulder/knee injuries for QBs?

My thought was to see if knee injuries for QBs fell off drastically post-rule, and if shoulder/elbow injuries went up because QBs were getting hit higher and landing poorly that way.

Or would the results be skewed by selection bias/small sample size coming in play?

3
by Tomlin_Is_Infallible :: Sat, 10/24/2015 - 11:24am

Total Weeks missed & Average Weeks allows one to compute # of injuries, but with the scale differences of the axes it is not readily apparent. Plot of Injury incidence (frequency) and average duration would allow one to determine total weeks missed, but tell more readily on the plot meaningful information.

It's also not that interesting to see data with positions grouped into bulk.
LB concussion risk , for example, should be significantly lower for a 4-3 Tampa2 cover ILB vs a tradition 3-4 thumper ILB.

Also, the stats would be more interesting if scaled somehow to a normalized snap count.

/ps
EXCEL plots still make baby jesus cry.

--------------------------------------
The standard is the standard!

4
by pm :: Sun, 10/25/2015 - 2:11pm

I would like to see a fantasy component integrated into this. How does a player who is listed as probable/questionable/doubtful do statistically in comparison to when he is "healthy". How about players who missed time with injury, what are their stats in the first game back and the rest of season.

5
by brian30tw :: Fri, 10/30/2015 - 9:51am

Just because Rodgers and Romo recently suffered broken collar bones, I'm curious if this injury would fall into one of the two "shoulder" categories, or if it's in "other."