Much electronic (and acutal) ink has been spilled over the years on the topic of running back workloads. This year's Pro Football Prospectus has an article about whether receptions and playoff workloads are worth adding to the list of considerations, concluding no and yes respectively.
The question is vaguely reminiscent of the debates over pitcher workload that rage in the baseball stat geek community. At least they used to rage back when I was keeping up; I assume they still do. In some sense, the situations are similar. In both cases, you're dealing with positions where serious --- often career-ending --- injuries are more likely than for others in the same sport. Also in both cases, it appears that some of the injuries or drops in effectiveness might plausibly have been caused by workload while others were most likely just dumb luck. On the other hand, the situations are different in some ways. Most notably, pitching injuries are about repetitive stress while running back injuries are about violent contact.
Or are they? It is certainly intuitive that a high workload for a running back would lead to a greater probability of injury or loss of effectiveness, but it's not clear to me what the exact link is. What exactly happens on those 350 carries that might cause problems the next year? Is it the hits? The cutting? Is it muscles? Joints?
Question 1 - forgetting the empirical evidence for a minute, what is the theoretical basis of these ideas? Does a high workload in Year N mean an increased likelihood of a torn ACL in Year N+1? Receivers do a lot of cutting. Linemen do a lot of rolling around in piles. I could be wrong, but I don't think players at those positions tear ligaments at the rate running backs do. But running backs do a lot of both (cutting and rolling in piles). Does that combination cause ligaments get "tired" in a way that five or six months of complete rest doesn't fix?
Also, should the injury question be separated from the loss-of-effectiveness question? If the medical argument for high workloads contributing to injury is that ligaments get slowly weakened by a high-workload season and don't have time to heal, then does that same medical argument explain loss of effectiveness as well?
Question 2 - Could we learn something by looking at not only raw numbers of carries but also the distribution of those carries?
Question 2A - if it is true that the offseason isn't a long enough time to heal whatever damage is done during the year, then should we weight late-in-the-season carries heavier than early-in-the-season carries? If so, this might help explain Jamal Lewis' torn ACL in training camp of 2001. In his rookie year (including playoffs), he had 286 carries from week 10 on. In particular, maybe postseason carries should not only be counted, they should be counted extra.
Question 2B - is a consistent N carries per game less damaging than the same number of carries accumulated in a less consistent way? That is, is a 30-carry game twice as damaging as a 15-carry game? Or moreso? Last year, Clinton Portis had 385 rushes and Larry Johnson had 336. But Johnson had five games of more than 30 carries and Portis had none. If you assume the first 20 rushes of each game are "free" (such assumptions are often made in baseball --- the damage starts to pile up only after 70 or 80 or 100 pitches --- I have no idea if those assumptions are justified), then you might argue that Johnson was the most overworked back in the NFL last year. Here are the leaders in "Rushes over 20"
Larry Johnson 2005 336 84
Edgerrin James 2005 373 74
Shaun Alexander 2005 430 71
Tiki Barber 2005 370 60
Willis McGahee 2005 325 53
Cadillac Williams 2005 308 52
Clinton Portis 2005 385 52
Rudi Johnson 2005 350 46
Thomas Jones 2005 334 43
LaDainian Tomlinson 2005 339 37
Julius Jones 2005 257 34
Domanick Davis 2005 230 32
Lamont Jordan 2005 272 29
Samkon Gado 2005 143 26
Fred Taylor 2005 202 25
Reuben Droughns 2005 309 23
If you look at Rushes Over 15, the leaders are Alexander, James, Johnson, and Portis. If you look at Rushes Over 25, it's Johnson, Alexander, James, and Cadillac Williams. Considering that Johnson also had most of his carries late in the year (though not as late as Shaun Alexander), might he possibly be the back most at risk of serious injury or decline this year?
Questions to get the discussion going: Who is more at risk of serious injury this year, Larry Johnson or Clinton Portis? Which of them is more at risk of severe decline? Also, whatever you estimate the probability of injury or decline for each to be, how much of that is attributable to their workloads from last season? In other words, say the following factors contribute to the chance of Portis getting hurt this year: his own unique physical characteristics (genetics, etc), his workload from last year, and dumb luck. What are the relative weights on those three (or however many) factors?
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