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?

In this article I'll describe a couple of studies that might give us some clues, but I want to stress that they are not definitive.

In fact, I'll go ahead and state right now that this article is not an attempt to quantify the concept of mileage or workload for running backs. Mileage consists of NFL rushing attempts, plus high school and college workload, preseason and postseason workload, rushes, receptions, blocking assignments, practice time, and so on. Trying to measure all that accurately is practically an impossible task.

Impossible tasks are something I've never been very good at, so I am going to settle for something less ambitious. In particular, I will seek to answer the question:

If we already know his age, does a running back's number of career-to-date NFL rushes tell us anything about his future career length?

Method 1: matched pairs

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. 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? Yes it would. Guys that had long careers (like Emmitt) would 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.

Method 2: Grouping

The idea here is to find all backs who met a certain performance benchmark during their career. For example, we might look for all runners who finished in the Top 12 (by fantasy points) at least four times. Now that's a pretty exclusive group. Lamar Smith isn't in it, for example.

After throwing out the still-active players, we are left with 28 runners. Now we'll count up each of their career-to-date rushes through (and including) their age 27 season. We'll order them from least to most and then divide them into three groups: low mileage, medium mileage, and high mileage. Here they are:

Player          Rsh Thru Age 27
-------------------------------
James Brooks            426
Wendell Tyler           583
Terry Allen             641
Earnest Byner           672
Herschel Walker         721    LOW MILEAGE
Chuck Muncie            748
Roger Craig             749
William Andrews         793
Lydell Mitchell         801

Wilbert Montgomery      835
John Riggins            928
Chuck Foreman           939
Neal Anderson           947
Lawrence McCutcheon     950
Ricky Watters           990    MEDIUM MILEAGE
Tony Dorsett           1026
Franco Harris          1135
Curt Warner            1189
Marcus Allen           1289

Terrell Davis          1343
Eddie George           1360
Thurman Thomas         1376
Ottis Anderson         1401
Earl Campbell          1404    HIGH MILEAGE
Eric Dickerson         1465
Barry Sanders          1763
Walter Payton          1865
Emmitt Smith           2007

Mileage   N  RshAfter27
=======================
Low       9    1070.9
Medium   10    1204.7
High      9    1385.2

Obviously, the benchmark had two arbitrary paramters in it: top 12, and four times. If we change it to something else, we'll get similar results. Here are a couple of examples:

Benchmark: top 20 at least five times

Mileage   N  RshAfter27
=======================
Low       9     1070.1
Medium   11     1192.2
High      9     1381.3

Benchmark: top 5 at least once

Mileage   N  RshAfter27
=======================
Low      19      544.4
Medium   20      708.5
High     19     1169.7

I have tried several sets of cutoffs but have not found one where the high mileage group does not come out on top. But I'm still left with the same feeling I had in the first study. Namely, the premise of this study is that we are identifying groups of comparably skilled players. But if you look at those lists you still don't get that feel. Wendell Tyler wasn't a one-year wonder, but he wasn't Eric Dickerson either. Despite meeting the same benchmark, the groups are not truly comparable.

So let's try to stack the deck in favor of the low-mileage group. Let's look at all running backs who finished in the top 24 at least once, and then classify them as either low-, medium-, or high-mileage as above. But then let's throw out any low- or medium-mileage back who was in the top 24 only once. So we're comparing low-mileage backs who finished in the top 24 at least twice to high-mileage backs who finished in the top 24 at least once.

Mileage   N  RshAfter
=====================
Low      29    497.9
Medium   40    470.5
High     70    656.3

Still the high-pre-age-28-mileage backs had the longest post-age-28 careers.

This design helps to minimize some of the concerns with the earlier studies, but it has problems of its own. In particular, the group of backs who made the top 24 at least once is pretty large and contains a lot of fringe types. This lets in a lot of guys with low mileage, which drives the "high-mileage" cutoff down to a point where it's not really high-mileage in an absolute sense. In other words, the categories might more properly be called super low, low, and other instead of low, medium, and high.

Conclusions

1. Trying to separate the effects of chronological age from the effects of workload-related age is very, very difficult.
2. The reason it's so difficult is because workload is such a good marker of quality. If a running back has a very high workload, that probably means he's a very good running back.

So back to the question that started the article: Should a 27-year-old running back with 1700 previous career rushes be considered "older" than a 28-year-old running back with only 1000?

After all this, my answer is: I don't know and don't really care which one is "older," but I'm confident that the 27-year-old with the higher workload will have a longer career from here on out. The workload difference tells me that that the 27-year-old is probably better. Not definitely --- there are always exceptions --- but probably.

Or, if you phrased it another way, the question might be: given two backs of equal quality and age, but significantly different past workloads, do you expect them to have different career lengths from here forward? The past data seems to be telling us that there are very, very few examples of backs of truly equal quality having signficantly different workloads at the same age. Any back who is good enough to compile a scary high workload at a young age is probably just flat out better than the low-mileage alternative you're considering. Sure, he might blow out a knee like Terrell Davis did. Or he might hit a wall like Eddie George did. But I see no evidence that he's a higher risk to do so than his low-mileage counterpart.