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Running back deterioration: age or mileage?

Posted by Doug on June 15, 2006

Question: do NFL running backs wear down because of age, or do they wear down because of mileage? Should a 27-year-old running back with 1700 previous career rushes, for instance, be considered "older" than a 28-year-old running back with only 1000?

This seems like a simple question, but I have had a tough time designing a study that sheds any light on it. Today, tomorrow, and Monday I'll describe a couple of studies that I think have a bit of promise. I'm not completely happy with either of them, though, and suggestions are welcome.

The first idea is simple. I find all pairs of running back seasons where the two backs had very similar production at the same age, but where the previous mileage of the two backs was significantly different. Then I see whether the low mileage back had the longer career after that season. Here is an example:


Name YR age Prev RSH YD After
===================================================
Thurman Thomas 1993 27 1376 | 355 1315 | 1146
James Wilder 1985 27 758 | 365 1300 | 463

I am trying to keep things very simple here by considering only rushes and yards. Once I have an experimental design I'm happy with, I can think about adding other things like receptions, possibly era adjustments, height, weight, and so forth. The RSH and YD columns demonstrate that Thomas' 1993 and Wilder's 1985 were in fact quite similar. There are, of course, some differences not captured in the rushing totals, but I'm not too concerned about that. I'm just trying to identify pairs of seasons that were pretty close in terms of rushing workload and quality. The Prev column shows how many career rushes each of them had before that season. So you can see that Thomas had substantially more mileage on him at that point. The After column shows that Thomas had 1146 rushes after his age 27 season while Wilder had only 463. If the low mileage backs had significantly longer careers, that would be evidence that wear and tear do play a role in the decline of running backs.

In principle, I like this study quite a bit. In practice, there are some complications.

First of all, "similar" is not a binary concept. How similar do two seasons need to be for me to include them as a pair in this study? And how to measure similarity? The usual tradeoff presents itself: you can require that the seasons be extremely similar and get a small sample size, or you can include pairs of seasons that aren't quite as similar and expand the sample. There is no correct answer. If this were baseball, with 100 years of history to fall back on, you can get a big enough sample without letting in any pairs of seasons that don't feel similar enough. With NFL data, I'm reluctant to include anything before 1978. And since we can only look at backs whose careers are over (or very close to it), we're left with a pretty small window.

Second of all, I said I wanted to match pairs of seasons that were similar, but where the previous workloads of the two backs were significantly different. What does "significantly different" mean? 500 previous career rushes? 300? 800?

Here is another complication I wasn't expecting. Emmitt Smith, for example, has lots of seasons that are similar to lots of other guys.


Name YR age Prev RSH YD After
===================================================
Charlie Garner 2000 28 736 | 258 1142 | 543
Emmitt Smith 1997 28 2334 | 261 1074 | 1814

Emmitt Smith 1997 28 2334 | 261 1074 | 1814
Harvey Williams 1995 28 499 | 255 1114 | 267

Terry Allen 1995 27 641 | 338 1309 | 1173
Emmitt Smith 1996 27 2007 | 327 1204 | 2075

Dorsey Levens 1997 27 162 | 329 1435 | 752
Emmitt Smith 1996 27 2007 | 327 1204 | 2075

And so on. If I included all these, the pairs would not be independent. Would that necessarily bias the results one way or the other? I'm not sure, but I think it might. Guys that had long careers (like Emmitt) might tend to be over-represented because their long careers mean more years to potentially match up with other backs.

So I decided to only include each back once, with his best match. So only the Emmitt-Garner match would be included. This leads to some arbitrariness. Suppose Back X's best match is to Back Y. Then we'd include the X-Y comparison in the study. Now along comes Back Z, whose best match is also to Back Y, but that match isn't as close as the X-Y match. So Back Z doesn't get included in the study or he gets paired with someone who is not quite as comparable as Back Y.

In general, there are a lot of parameters to tweak. And tweaking them just a little changes who gets included and who gets paired with whom. That's unfortunate.

All that said, though, I have yet to find a collection of settings that shows any evidence for the low mileage backs having longer careers. Here is the collection of pairs that I'm currently happiest with, listed in order of the strength of the match.


Name YR age Prev RSH YD After
===================================================
Jerome Bettis 1999 27 1807 | 299 1091 | 1373
Adrian Murrell 1997 27 560 | 300 1086 | 515

Eddie George 2002 29 2078 | 343 1165 | 444
James Stewart 2000 29 765 | 339 1184 | 374

Terry Allen 1995 27 641 | 338 1309 | 1173
Walter Payton 1981 27 1865 | 339 1222 | 1634

Earl Campbell 1981 26 1043 | 361 1376 | 783
Herschel Walker 1988 26 360 | 361 1514 | 1233

Rodney Hampton 1995 26 1241 | 306 1182 | 277
Antowain Smith 1998 26 194 | 300 1124 | 1290

Charlie Garner 2000 28 736 | 258 1142 | 543
Emmitt Smith 1997 28 2334 | 261 1074 | 1814

Ottis Anderson 1983 26 1105 | 296 1270 | 1161
Greg Bell 1988 26 597 | 288 1212 | 319

Priest Holmes 2001 28 459 | 327 1555 | 948
Curtis Martin 2001 28 2010 | 333 1513 | 1175

Gary Anderson 1988 27 323 | 225 1119 | 321
George Rogers 1985 27 995 | 231 1093 | 466

Raymont Harris 1997 27 317 | 275 1033 | 92
Curt Warner 1988 27 1189 | 266 1025 | 243

Roger Craig 1989 29 1274 | 271 1054 | 446
Dorsey Levens 1999 29 606 | 279 1034 | 358

Tony Dorsett 1984 30 1834 | 302 1189 | 800
Lamar Smith 2000 30 480 | 309 1139 | 534

Thurman Thomas 1993 27 1376 | 355 1315 | 1146
James Wilder 1985 27 758 | 365 1300 | 463

Mike Anderson 2000 27 0 | 297 1487 | 568
Wilbert Montgomery 1981 27 835 | 286 1402 | 419

Anthony Johnson 1996 29 330 | 300 1120 | 186
Mike Pruitt 1983 29 1137 | 293 1184 | 414

Jamal Anderson 2000 28 992 | 282 1024 | 55
Lewis Tillman 1994 28 355 | 275 899 | 29

Pete Johnson 1981 27 762 | 274 1077 | 453
Harvey Williams 1994 27 217 | 282 983 | 522

Marshall Faulk 1999 26 1389 | 253 1381 | 1194
Robert Smith 1998 26 646 | 249 1187 | 516

Craig Heyward 1995 29 683 | 236 1083 | 112
Freeman McNeil 1988 29 1306 | 219 944 | 273

Barry Sanders 1994 26 1432 | 331 1883 | 1299
Chris Warren 1994 26 513 | 333 1545 | 945

Eric Dickerson 1988 28 1748 | 388 1659 | 860
Christian Okoye 1989 28 262 | 370 1480 | 614

Chuck Muncie 1981 28 923 | 251 1144 | 387
Bernie Parmalee 1995 28 226 | 236 878 | 105

Edgar Bennett 1995 26 398 | 316 1067 | 401
Gerald Riggs 1986 26 928 | 343 1327 | 718

The high mileage backs averaged 775 carries during the rest of their careers. The low mileage backs averaged 529. This certainly fails to provide evidence that low mileage is a good thing. But it doesn't prove that high mileage is a good thing either, for reasons I'll explain shortly.

First, I need to mention some fine print.


  1. As you can see, the matches are pretty weak near the bottom of the list. That's why I sorted them that way, so you can draw the line wherever you like.
  2. I included players who were born in 1973 or earlier, so a few active guys are on the list. Antowain Smith, Priest Holmes, and Mike Anderson appear as low mileage guys. Curtis Martin and Marshall Faulk appear on the high mileage side.

As you browse through the list, you'll notice that the pairs of comparable players often don't feel all that comparable. I mean, Tony Dorsett comparable to Lamar Smith? C'mon. Walter Payton and Terry Allen? Emmitt Smith and Charlie Garner? It just doesn't seem right to call some of those matches matches.

It could be hindsight playing tricks on us. Now that we know how Walter Payton's career turned out, it's easy to say he wasn't comparable to Terry Allen. But he had more than 1800 carries prior to his age 27 season. That's a bigger workload than LaDainian Tomlinson, who some people are worried about. Would it really have been that surprising if Payton had started to decline around that point?

Even accounting for the hindsight effect, though, I think the problem is real. Even though the guys had similar seasons in that particular year, that doesn't mean they were similar. Lamar Smith was a journeyman who put together a good season at age 30. Tony Dorsett was Tony Dorsett. Despite the fact that their numbers were similar at age 30, Dorsett was just plain better. And it's no coincidence that the better player had the bigger previous workload.

In short, it's very difficult to find truly comparable pairs where the previous workloads were significantly different. That's why this study isn't enough for me to rule out that workload plays a role in aging running backs.

More tomorrow.

This entry was posted on Thursday, June 15th, 2006 at 4:21 am and is filed under Fantasy, General. You can follow any responses to this entry through the RSS 2.0 feed. Both comments and pings are currently closed.