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Don't worry, this will all make sense by the end. I think.

Don't worry, this picture's presence will make sense by the end. I think.

Two years ago, I wrote this post on running back aging curves. One conclusion from my research was that age 26 was the peak age for running backs, which was immediately followed by a steady decline phase until retirement. In that study, I only wanted to look at very good-to-excellent running backs in the modern era; as a result, I was forced to limit myself to just 36 players. I’ve been meaning to update that post, but wasn’t quite sure what methodology to use.

Last year, Neil wrote a very interesting post on quarterback aging curves. In it, Neil computed the year-to-year differences in Relative ANY/A at every age. While reviewing that post, a lightbulb went off. We can greatly increase the sample size if we only look at running backs from year-to-year, and not just the best running backs on the career level.

There are 723 running backs since 1970 who had at least 150 carries in consecutive seasons and who were between 21 and 32 in the first of those two seasons. For each running back pair of seasons, I calculated how many rushing yards the player gained in Year N and many yards he gained in Year N+1. Take a look:

Age#Rsh Yds Yr NRsh Yds N+1Diff
2116989107889
22431057986-70
2392993106774
2411510461017-29
251241043106926
2610711211070-51
278311051007-98
28591067994-74
29371081982-100
302611181008-110
31131104906-198
3281006769-237

Just so we’re clear on what that table says, let me walk through an example. There were 43 running backs who were 22-years-old and had at least 150 carries, and also had at least 150 carries at age 23. [1]Careful readers will quickly recognize that this opens us up survivorship bias issues. Running backs who decline greatly from Year N to Year N+1 won’t have their failures recognized in this … Continue reading Those running backs averaged 1,057 yards at age 22 and then 986 yards at age 23. While these numbers may be useful for reference purposes, if you’re like me, a table like this isn’t particularly intuitive; a graph would be much better at explaining what the data tell us.

If we assume that a running back will rush for 1,000 yards at age 21, and then gain or lose the amount of yards showed in the table above each season (i.e., will rush for 89 more yards at age 22, then 70 fewer yards at age 23, then 74 more yards at age 24, and so on), he would produce the following career curve:

rb aging

The dotted blue line tracks our fictional player who rushed for 1,000 yards at age 21. But the smoothed black line is probably the more useful one, which is presumably a better representation of the effects of age on a running back’s production. What’s interesting to me is that despite using several different variables and measuresh, this study comes to a pretty similar conclusion to the last one: running backs peak at age 26, and then begin a steady decline. It also suggests that for older running backs, some dropoff should always be expected from year-to-year, even if they have done very well in the prior season.

Do you know which running back’s career most closely resembles our set of averages? The answer: Buffalo Bills Hall of Famer Thurman Thomas. The graph below shows Thomas’s yearly rushing averages from ages 22 (when he entered the league) through 33: as you can see, the curve is shifted upwards because he was better than the average back, but the shape of the curves are pretty similar. If nothing else, may you remember that Thurman Thomas aged like your typical running back.

thurman

Thomas had a great five-year peak from ages 23-to-27 on which his Hall of Fame career was built. That was followed by a respectable but inferior three-year period; after that, his production fell off a cliff sharply, beginning after his age 30 season.

What can we take from today’s post? The survivorship issue [2]You guys always read the footnotes, right? has not been resolved, although I don’t think it’s a huge issue here. If anything, my guess would be that it understates the degree of magnitude by which older running backs decline. Another conclusion is that running backs don’t take very long to progress in their careers: a running back in the draft is not far behind, if at all, from a running back who is 25 or 26. [3]Of course, we’re focused just on production in the running game here, not things like pass protection. This is your 1,283rd reminder that giving big money contracts to older running backs is one of the riskiest moves an organization can make. Three years ago, a 26-year-old Maurice Jones-Drew led the league in rushing. Over a week into free agency, the 29-year-old running back is still looking for a job. Last season, a 26-year-old Knowshon Moreno gained 1,586 yards from scrimmage and scored 13 touchdowns. He’s still looking for a job, too.

Update: For those curious about the choppiness at the beginning of the curve, the table below shows the results for the 22-year-old running backs:

Running backYearTmageYr N GYr N RshYr N RshYdYr N+1 GN+1 RshN+1 RshYdDiff
LeSean McCoy2010phi22152071080152731309229
Jonathan Stewart2009car2216221113314178770-363
Knowshon Moreno2009den221624794713182779-168
Ray Rice2009rav22162541339163071220-119
Rashard Mendenhall2009pit22162421108163241273165
Kevin Smith2008det221623897613217747-229
Maurice Jones-Drew2007jax22151677681619782456
Adrian Peterson2007min22142381341163631760419
Steven Jackson2005ram22152541046163461528482
Kevin Jones2004det2215241113313186664-469
Clinton Portis2003den22132901591153431315-276
LaDainian Tomlinson2001sdg22163391236163721683447
Edgerrin James2000clt221638717096151662-1047
Ron Dayne2000nyg221622877016180690-80
Ricky Williams1999nor2212253884102481000116
Fred Taylor1998jax2215264122310159732-491
Fred Lane1997car221318280914205717-92
Warrick Dunn1997tam221622497816245102648
Karim Abdul-Jabbar1996mia2216307111616283892-224
Mario Bates1995nor221624495114164584-367
Marshall Faulk1995clt2216289107813198587-491
Curtis Martin1995nwe22163681487163161152-335
Jerome Bettis1994ram2216319102515183637-388
Natrone Means1994sdg2216343135010186730-620
Rodney Hampton1991nyg2214256105916257114182
Emmitt Smith1991dal22163651563163731713150
Johnny Johnson1990crd221423492615196666-260
Reggie Cobb1990tam221615148016196752272
Barry Sanders1990det22162551304153421548244
Sammie Smith1989mia221320065916226831172
John Stephens1988nwe2216297116814244833-335
Thurman Thomas1988buf2215207881162981244363
Greg Bell1984buf2216262110016223883-217
Marcus Allen1982rai229160697162661014317
Tony Collins1981nwe22162048739164632-241
Andra Franklin1981mia22162017119177701-10
Joe Cribbs1980buf22163061185152571097-88
Ottis Anderson1979crd22163311605163011352-253
Clark Gaines1976nyj221415772414158595-129
Mike Thomas1975was2214235919132541101182
Walter Payton1975chi2213196679143111390711
Franco Harris1972pit2214188105512188698-357
John Riggins1971nyj221418076912207944175
Average15250105714.1243986-70

References

References
1 Careful readers will quickly recognize that this opens us up survivorship bias issues. Running backs who decline greatly from Year N to Year N+1 won’t have their failures recognized in this study if they fail to hit the 150-carry threshold. Unfortunately, I can’t quite think of a good solution right now, but perhaps one will come to me (via the comments?) by the time Part III is produced.
2 You guys always read the footnotes, right?
3 Of course, we’re focused just on production in the running game here, not things like pass protection.
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