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Running back injury rates by week

Posted by Jason Lisk on June 5, 2009

The NFL is considering expanding the regular season to eighteen games, and one of the big points of discussion is what impact extending the regular season will have on injury risk to players. Brian Burke of Advanced NFL Stats and Bill Barnwell of Football Outsiders have already weighed in with some interesting thoughts on the topic, and the NFL’s study regarding injury rates.

Since it now seems somewhat relevant, I thought it would be as good of a time as any to dust off some data I had lying around from last off-season that never got published, and add in the new individual games info we have going back further than 1995 (as well as add in last season’s numbers).

I know the discussion on injury effects will center on more than the running back position, but it is this position, where the players take as many hits as any other and have shorter careers, that we probably all think of when we consider who may be most impacted by additional games.

I pulled all running backs who (1) had 200 or more rushes in a season from 1988-2007, and (2) played in the first game of the following season. This hopefully excluded all retirees and guys who came into the year injured. (On second thought, I'm extremely positive it did exclude all retirees). I then recorded which game, if any, each of the qualifying backs first missed a game that next season. So when you are looking at the chart below, the "Game 2" line tells you that 365 backs met the criteria by playing in game 1 the season after they had at least 200 rushes, 9 of those backs did not play in game 2, with 1 of them missing all remaining games starting with game 2. As you can see, the "Total" number is decreasing as the weeks progress because I am subtracting out the players who missed games. This is trying to look at the rate at which players who had not missed any games to that point then miss the next game. I should point out that not every game missed is due to injury, but I'm comfortable enough that the vast majority are missed games caused by injury.

GAME		Total	Missed	Missed All	Missed/Total
2		365	9	1		0.025
3		356	21	3		0.059
4		335	19	2		0.057
5		316	14	3		0.044
6		302	23	3		0.076
7		279	10	3		0.036
8		269	11	2		0.041
9		258	14	3		0.054
10		244	5	1		0.020
11		239	10	3		0.042
12		229	13	5		0.057
13		216	11	3		0.051
14		205	8	4		0.039
15		197	7	5		0.036
16		190	14	14		0.074
play all	  176				

Judging by these numbers, there’s about a 4.7% chance that your established healthy running back who has not missed any games so far will then miss the next game, and a little less than 50% chance that a veteran back will play a full 16 games. I don’t see any particular trend that suggests that the injury rate is increasing in the final weeks of the season. You may be inclined to think it is, based on the percentage of guys who missed game 16. However, the group of backs who played 15 straight games then missed the final regular season game is over-populated with star backs on really good teams who had already clinched playoff spots. I don’t know individually how many would have still missed the final game if there was any incentive to play, but I feel pretty comfortable saying that the true numbers are a lot closer to the week 14 and 15 rates.

Will there be more injuries with two extra games? Sure there will, even if the rates don't increase. The powers that be on both sides of the negotiating table will have to decide what the appropriate level of risk is. Judging on these numbers, I don’t see that the rate of injury should be expected to increase dramatically in games 17 and 18. I will note, though, that this is looking only at the injury rate for guys who had not missed any games previously that same season. We could potentially have a situation where the running back injury rates are a little higher overall, even if the injury rates for previously healthy backs and injury rates for previously injured backs are steady, because there are more “previously injured” backs in game 16 than in game 4. (This assumes that players who have already missed games with injury and return within the same season are at an elevated risk of additional injury).

A bigger concern for me is not whether the additional games create more injuries in just those two weeks, but whether they increase the risk of serious injury going forward in the future. I’ve done research on injury rates in the past, including looking at end of season workloads and playoff games played.

That data was from 1995-2006 (I probably need to update that, but don’t have time right now). It looked at all players who averaged at least 15.0 carries a game over the final six regular season weeks and played in all games over that stretch. Pooling that data, the guys that didn’t play on playoff teams (n=104) suffered a season ending injury within the first 6 games of the following season 7.7% of the time. The playoff performers (n=86) had a season ending injury 7.0% of the time—roughly equal. The non-playoff guys missed at least one game within the first six weeks 19.2% of the time, versus 18.6% for the playoff guys. Average games played the following year was 13.5 for the non-playoff guys versus 13.3 for the playoff guys.

Now, if you look at the guys who played 3+ playoff games on top of the regular season, you might be concerned about increasing the game totals. I'll just note that of the 18 guys that averaged 15.0 or more carries and played in 3 or more post season games, the only 3 to suffer season ending injuries in the first month (and thus lower the games played dramatically for the group) were also the only 3 to average 25.0 or more carries a game over the final six weeks of the regular season. For backs with a more moderate starting running back workrate who were extended deep in the playoffs, the average games played the next year was 14.1.