Understanding Injuries in the NFL Part 1

One of the biggest issues on the minds of Redskins fans right now is the health of the team. For two years in a row, the Redskins’ season has been derailed by a multitude of injuries that seem to be an extreme outlier compared to those experienced by most NFL teams. Some fans are quick to point fingers at the conditioning staff, the field itself, and even supernatural curses in an attempt to make sense of the situation, but I thought it would be worthwhile to look for studies into what correlates with injuries in the NFL as a first step towards understanding our injury situation.

Fortunately, Football Outsiders has an excellent series of articles by Zach Binney doing just that, and I’ll summarize what I think are the most salient points from their articles in this series and provide some of my own commentary. I want to be very clear about what this article is and isn’t though. It is an investigation into what correlates with player injuries in the NFL in general. It is not a look at the Redskins situation in particular in an attempt to ferret out a root-cause explanation as to why we have had so many injuries the last few years (although it does contain a few nuggets specific to the Redskins). I think understanding injuries in general is a good first step in understanding the Redskins injury situation. Also, there’s a TL;DR summary at the bottom for those who don’t want to look at all the details.

Part 1: An Overview of Injuries in the NFL

This article draws from Mr. Binney’s first 4 articles trying to lay the groundwork to characterize injuries in the NFL. I encourage people to read the original articles: part 1, part 2, part 3, and part 4. First, let’s look at how many NFL players lose X weeks to injury in a given season.

The first thing that jumps out of this chart is that a majority of players (61 percent) won’t miss any time in a given year. Another thing that jumps out is that this data is heavily right-skewed (that is, there are way more players who miss little or no time than who miss extended periods).

My take from looking at this chart is that players mostly fall into 3 buckets in a given season: healthy (able to play 16 games, 61% of players in a given year), injured (miss 1-4 games), or seriously injured (season is over). There is a clear curve from 1-4 weeks missed, but the plot is pretty flat after that. I suspect most of the injuries with more than 4 weeks of recovery time are season-ending injuries, and the only variable is which week the player happens to sustain their season-ending injury (and hence, how many remaining weeks he will miss).

Weeks Missed to Specific Injuries

The following two plots show two separate (but related) things:

• Blue - the number of total weeks missed to a specific injury type among all NFL players.
• Red - the average number of weeks missed by a player who sustained a type of injury.

So blue is largely a measure of how frequent an injury is, and red is largely a measure of how severe an injury is. It’s probably best to look at these plots with one color in mind (ie, only look at blue [frequency] or only look at red [severity]).

At the population level (all NFL players), general knee injuries (non-ACLs or other tears) do the most damage, by virtue of their sheer frequency (about 4,500 since the 2000 season). They have cost players almost 7,600 weeks over the last 15 years. Although their severity is about average (1.7 weeks missed per injury), they happen so often their damage builds up quickly. No surprise here.

Some other common culprits round out the top 10 in all-player damage: ACL, hamstring, shoulder (non-tears), ankle (non-breaks or sprains), foot (non-breaks or Lisfranc injuries), groin, and Achilles injuries.

Most of the top 10 cause so much damage due to their frequency, but ACL and Achilles injuries appear high up due to a mix of their frequency and severity (about 10.5 weeks out for an ACL, about 7.0 for an Achilles).

I would only add the following observations:

• If a player is listed with a specific upper-body muscle injury (pectoral, biceps, triceps), they are likely to miss significant time.
• Lower-body injuries are likely to cost less time unless they are a tear or break.

I suspect the above two bullets are a result of reporting bias (ie, players are more likely to play through upper body injuries and not report them unless they are severe).

Injury Risk Over the Course of the Season

Next, we’ll look at how likely injuries are to occur in specific weeks of the season and how that adds up for a team. The following plot shows:

• Blue - the average percent of players on a team who are listed as injured in a given week.
• Orange - the percent chance for a player to gain a new injury in a given week.

The spike at week 1 in orange is simply due to all offseason and preseason injuries being rolled up into week 1 for this plot.

We hypothesized above that injury risk increases over the course of a season. Figure 3, however, shows that the risk of a new injury is roughly flat across all weeks, particularly late in the season. There’s a bit of a spike in Weeks 3 and 4, which makes sense as injuries start to manifest after a couple of tough games. Beyond that there’s only a small amount of variation week-to-week, especially from about Week 9 onwards.

Figure 3 shows us that injury prevalence -- or the percentage of players out with an injury or playing hurt -- increases substantially over the course of the season, which is exactly what we expected. It rises almost 80 percent from 13.1 percent in Week 1 to 23.4 percent in Week 17. It grows quickly from Week 1 to Week 3 as early-season injuries manifest, then slows a bit until about Week 9 before rising more quickly in the second half of the season.

We need to consider the possibility that the flat risk curve late in the season may be due in part to what epidemiologists term survivor bias or the survivor effect. Basically, by late in the season the players prone to any given type of injury will have already gotten hurt. The only players left at risk for a new injury are ironmen with excellent training regimens, good genes, and/or an actual Iron Man suit, and these players are going to have a lower baseline injury risk than those who have already been hurt.

The original article also breaks down injury-likelihood over time by injury type. Rather than re-printing all of those plots, I’ll just include some of the important findings.

Ankle sprains, which I would have expected to fall somewhere in the middle in terms of risk related to time, look to be about flat over the course of the season.

The same goes for ACL tears. What stands out to me about these, though, is the much higher ratio of Week 1 (i.e., offseason/training camp/preseason) tears to Week 2 tears. For ACLs it’s more than 5:1, while for hamstrings it’s just 2:1. This could be due to a couple of things. First, some ACLs might be holdovers from a previous season (e.g., Todd Gurley and Sen’Derrick Marks). Second, ACLs are season-enders, while other injuries like hamstrings might happen in training camp but disappear by the regular season and not get counted in our data. I might try to look at ACLs in more detail later.

The Redskins have certainly had their share of offseason/preseason ACL tears, but it looks like this is a pattern throughout the league and may be a result of holdover injuries from previous seasons in some cases.

I wanted to look at concussions because of their tremendous health importance and the amount of attention they have received in the media, and this is where things get alarming. Concussion risk clearly goes up over the course of the season, with a pretty clear inflection point around week 9.

Injury Risk by Age

In this section, we’ll look at a player’s injury-risk based on their age. The following plot shows how likely a player is to be injured based on age in several ways:

• Blue - the percent of players of a given age that are listed at least once on the injury report in a season.
• Orange - the percent of players of a given age that miss at least 1 week due to injury.
• Green - the percent of players of a given age that miss at least 4 weeks due to injury.

Note: QBs and special teams players are excluded from these plots due to their tendency to skew old (QBs) or be less prone to injury (ST players).

Your chance of getting listed on an injury report (blue line) rises steadily and substantially from about 56 to 59 percent at the age when most players are drafted (22 or 23 years old) to roughly 70 percent by 30, which is where it stays until around 37, when the vast majority of players have retired.

What might be more relevant, though, is your risk of actually missing any games (orange line) or of missing significant time (at least four weeks, green line). Here we also see increases as players age, though they are smaller than most people would probably guess. For missing any games, we have a pretty steady risk of about 35 to 37 percent through age 29, with a spike to a new normal of about 40 to 42 percent after 30. It’s a clear increase, but be honest: when you’re afraid your team signed an aging veteran who is likely to get hurt, you were thinking the risk increase was more than 5 percent, weren’t you?

There is a clear trend that older players are slightly more likely to get injured, but the difference between older and younger players is much smaller than I would have imagined and is essentially nil. However, maybe the data is being skewed because injury-prone players tend to retire earlier, so the population of older players is more likely to be made up of iron men who don’t get hurt (another form of survivor bias)? The next plot attempts to account for this possibility by separating out groups of players that retire at different ages.

So what does Figure 9 tell us? It tells us that once we eliminate survivor bias we see a much stronger, more consistent, positive effect of age on injury risk. If we focus on the gray line (players who played in the NFL until they were at least 30) we see the risk of missing any time due to injury rising from around 30 percent in your early 20s to 50 or 60 percent by your mid-30s. That’s a much larger rise than the roughly 5 percent increase we observed in Figure 7 over the same time period!

Risk increases grow more pronounced as we require players to have longer careers, which is exactly what we’d expect if survivor bias were at play in Figure 7. If you look at ages 23 to 25, you can see clearly that players who go on to retire later have lower injury risks when they’re younger, suggesting they are a “heartier” group on average. Importantly, this could also partly be due to reverse causation -- if you got lucky with injuries when you were younger, you might go on to have a longer playing career. I’d suspect it’s more that they’re heartier, though.

This certainly makes sense in the context of the Redskins. Two of the healthiest players on the roster are Vernon Davis and Adrian Peterson, veteran iron men who seem to know their bodies well and know how to keep themselves in top shape. However, they are certainly NOT representative of players in general. Vernon Davis is a borderline Hall of Fame player and Adrian Peterson is a 1st ballot Hall of Fame player. Players in general are more prone to injury the older they get, but teams account for this by refusing to sign older players unless they are special players. Hence, the older players still on NFL rosters are more likely to be special players who are no more prone to injury than the young guys.

Injury Risk by Position Group

The following plot breaks down a player’s injury risk by the position he plays.

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... 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.

All in all, this jives with my intuition. “Mobile” positions (LB, DB, WR, RB) are more likely to be involved in big collisions with high momentum, and hence more likely to be injured. That is less true for QB, OL, and DL, and so they are less likely to be injured.

Summary

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

• 61% of players should be expected to start 16 games. If they are injured and forced to miss games, they will either miss 1-4 games or their season is likely done.
• Knee, wrist, and hand injuries are the most common injuries to NFL players.
• If a player is listed with a specific upper-body muscle injury (pectoral, biceps, triceps), they are likely to miss significant time.
• Lower-body injuries are likely to cost less time unless they are a tear or break.
• Players have roughly the same likelihood (between 7-8%) to get injured in any week of the NFL season. They are NOT more likely to get injured as the season wears on, though they are more likely to be playing through injury.
• Players are more likely to get injured as they get older. However, non-special players are also more likely to retire as they get older, so the population of older players still playing is largely comprised of special players who are “iron men” and not much more likely to get injured than the young guys.
• “Mobile” positions (LB, DB, WR, RB) are more likely to be involved in big collisions with high momentum, and hence more likely to be injured. That is less true for QB, OL, and DL, and so they are less likely to be injured.

What do you think? Did I miss any important observations from the data, or do you draw different conclusions from the information presented? Let me know in the comments below.