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.

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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.