# Piecing Together the Injury Puzzle

By Max Mulitz

There’s been some really great work from a variety of sources on injury rates.  I’m gonna try to synthesize the current research on injury rate and hopefully further the discussion by addressing a couple areas that have so far gone unnoticed.

Football Outsiders and Pro Football Logic both peg the percentage chance of a player missing at least one regular season game with an injury in a given season at ~38%, meaning 62% of players go uninjured. However, when you limit your sample to players who were on a team for the whole 16 game season, only 54% of players are able to avoid injury the entire season. Football Outsiders also notes that rate of players missing time with injuries is generally increasing over time, though Pro Football Logic’s data is from 2015. In total, a base rate of about a 58% chance of not missing time for the average player in a given season is probably about right. Another way to address this problem would be to look at the per game injury rates and then take the probability of not getting injured in a given game and extrapolate it to a full season (15 games, since injuries suffered during week 17 can’t cause the player to miss any regular season games.) Using Pro Football Logic’s injury rate per game of 4.1%, we can extrapolate an expected 47% chance of a player missing at least one game with an injury.

One question might be why the implied chance of a healthy season is a little lower than we’d expect, 53% instead of 58%. One contributing might be injury recurrence. If players who miss games at some point in the season with an injury are more likely to get re-injured later in the season than the average player, than the total injury rate will cause us to expect a greater number of players to sustain at least one injury than is prudent. A player with a 58% chance of staying healthy to dress for 16 games has a 3.6% chance of being injured in a given game (assuming all games carry an equal risk of injury).

## Injury Proneness

Injury Predictor is an NFL injury analysis prediction website that has done research demonstrating that “injury proneness” seems to be predictable and documenting the increased risk of injury recurrence over a season.

It strikes me as inherently obvious that some players are injury prone. To give an extreme example, it became clear that Arian Foster was more likely than the average player to get injured in his final few seasons, which led to his in-season retirement. While I don’t agree with all of Injury Predictors’ methodology (predicting Drew Brees only has a 1% chance of missing a game with an injury this year is silly) they do claim to have demonstrated the ability to predict which players are at the highest risk of injury (66-75% chance of missing at least one game) and which players are at the lowers risk (~30% chance of missing time).

It may seem strange given a base rate of about 40% injuries to see injury predictability of injury prone players 20-30% above the baseline but for lower risk players to only be injured 10% less than average, but actually I think it makes intuitive sense. If you imagine me, or any person of average athleticism, playing in an NFL game where I either receive 20 carries or play until I am too injured to continue, the chance I would actually be healthy enough to touch the ball 20 times are vanishingly small. This should make it clear that injury likelihood can scale up to essential 100% depending on situation. On the other hand, even the healthiest player is one awkward helmet to the side of the knee away from being out for the rest of the season, football is a brutal game on the human body, and there simply aren’t people who are impervious to it’s risks.

Anyway, my sense right now is that it is possible to identify high injury likelihood, but players with an extremely low rate of injury are probably mostly lucky. Also, because injury likelihood increases with age, by the time you identify a player as being particularly unlikely to get injured, his increased age has probably offset the benefit. Actually that’s exactly what we see in the Football Outsiders piece, where the injury rate is flat across all ages, but when you look at players with long careers their probability of getting injured increases each year of their career. Research suggests the negative physical effects of aging begin at around age 23, so each year of a players career is going to bring accumulated damage and increased risk.

## Closing Thoughts

In an upcoming post, we’ll look at average injury length across position and then combine injury rate with injury lengths to look at total games lost due to injury. Understanding how many injuries a team can expect to have is integral to team building and achieving balance between acquiring top-end starters and maintaining quality depth through the lineup so the team can continue to function if injuries do occur.