What, AP worry? Workload concerns for 2015's top fantasy RBs

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In a season like this, with the top tier of running backs so closely bunched, every seemingly insignificant edge helps.

Here's an oddity, looking at this year's top 20 at the position in terms of ADP (average draft position): They're a somewhat older bunch than usual, with two 28-year-olds (Jamaal Charles and Jonathan Stewart), four 29-year-olds (Marshawn Lynch, Matt Forte, Justin Forsett and Joique Bell), a 30-year-old (Adrian Peterson) and -- gasp -- a 32-year-old (Frank Gore).

An older crop of running backs -- relatively speaking, that is -- rekindles the career-workload debate, as history serves an excellent reference point for age/workload-related risk factors. Perhaps, in fact, it can serve as our proverbial "tiebreaker" for some of these oh-so-tight rankings battles.

Let's be very clear about this up front: What follows measures historical workloads for reference points, rather than serving a "bust" decree of the player. History is also littered with examples of players who bucked past trends, beginning with the very first name on the list.

Still, as you consider any of these aging or heavily used running backs, don't tread lightly into the decision without knowing the facts.

Adrian Peterson

Workload worry: He's 30 years old, and he's coming off a 2014 season during which he appeared in only one game -- that in Week 1 -- before landing on the commissioner's exempt list.

The closest comparisons we have -- those defined by career workload, age at the time of return and starting role -- are John Riggins, Garrison Hearst and Stephen Davis, and even then, each fails to represent a perfect parallel. Riggins was two years older and had 388 fewer career carries than Peterson entering 1981, Hearst was the same age, but with 888 fewer carries entering 2001 (which followed two years absent) and Davis one year older with 329 fewer carries entering 2005. Still, the group represents a small indicator as far as worry: They averaged 210 fantasy points in the season before their extended absences, and 147 fantasy points during their comeback years. Even if we adjusted their numbers projecting full, 16-game campaigns, they'd have suffered a 59-fantasy-point drop from their pre- to post-absence years.

Expanding the pool to any player with at least 1,500 career carries at the time of a "comeback campaign" -- that defined as a season following one of 50 or fewer carries -- we find 14 historical comparisons. Here is their average comeback season:

Caveat one: The average age during which this group missed a year (the sub-50-carry year) was 31, with diminishing roles at advancing ages influencing the numbers. Still, the three youngest players for whom this happened, Ottis Anderson, Davis and Ricky Williams, suffered an almost identical decline in fantasy production during the comeback year; they dropped by an average of 53.2 total fantasy points and 3.2 fantasy points per game. Age, apparently, did not weigh in the player's comeback attempt.

Caveat two: A 14-player sample is precariously small and diminishes the strength of the data. It does, however, illustrate the rarity of the Peterson workload example as he enters his 2015 campaign.

Caveat three, and perhaps the most important: Peterson is a once-in-a-generation talent who has already bucked historical trends by nearly setting a single-season rushing record while scoring a career-best 297 fantasy points in 2012, his first since returning from a major knee reconstruction. If anyone can rebound at age 30 following such a lengthy absence, it's Peterson, who no doubt has as high a statistical ceiling as anyone in football.

But bear in mind his draft price. As a certain first-rounder and top-five overall pick, Peterson will need to defy the odds for a second time in four seasons, putting forth another historic campaign, to be a success story. That's not to say he can't; it's a reminder that his basement expectation is arguably the lowest of anyone currently going in the first round. It is the primary reason that I cannot support him as my No. 1 overall pick for 2015 or, frankly, No. 2-4.

DeMarco Murray

Workload worry: He carried the football 392 times and touched it 449 times last season, those the eighth- and sixth-largest single-season totals in NFL history. Perhaps you've heard the phrase before, "The Curse of 370"?

This is the one with a sizable enough sample to be of concern, as there's compelling evidence that heavily-used players one year are at increased risk of regression the next. For this study, let's expand the pool even deeper: We'll use 360 carries rather than 370, as that calculates to 22.5 per game over a full, 16-game season, which gives us 43 examples before Murray's. Again, these are average seasons among the 43 players included, and the player's pre-360 campaign is included as an additional barometer:

Pick your comparison point: Either players in this group suffer declines of roughly three games, 90 fantasy points and three fantasy points per game (if compared to the 360-plus year), or 1.5 games, 32.5 fantasy points and eight-tenths of a fantasy point per game (if compared to pre-workhorse levels). Either way, there's a downturn in production and it's not necessarily the result of late-career aging curves, as the average age of the 43 players was 25.

Among this group, the three closest comparisons -- again defined by career workload, age in the given season and starting role -- represent a mixed bag as far as Murray's future prospects: James Wilder, Jamal Anderson and Larry Johnson, all of whom are better parallels than in the previous Peterson examples.

Wilder had 191 fewer career carries than Murray entering 1984, played at Murray's same age of 26, during which Wilder amassed another 407 rushing attempts. Wilder would rush 365 more times in 1985 and score a healthy 212 fantasy points, only 79 off his 1984 total, but he had only 463 total carries from 1986 on, steeply declining thereafter.

Anderson had 21 more carries and entered 1998 only a few months younger than Murray, and rushed 410 times plus another 70 during the playoffs: This is important as, counting the postseason, Anderson's 480 carries (second-most all-time) were only 44 more than Murray's 436 (seventh-most). Anderson tore his ACL in only the second game of his follow-up 1999, and returned to appear in only 19 total games of 337 carries from 2000-01.

Johnson had 66 fewer carries than Murray entering 2006, also played at the same age, then set an NFL single-season record with 416. Johnson suffered a steep downturn in production in 2007 and was limited to eight games due to a foot injury; he played in only 37 games and had 535 carries from 2007 on.

Beyond the historical concern, Murray has two other factors working against him: First, his injury history. Since becoming a starter midway through 2011, he has missed 11 of 58 team games due to ankle, foot and knee injuries.

Second, Murray is only the second player with at least 360 rushing attempts in a season who then changed teams before the following year; the other was Edgerrin James, who suffered a 96-fantasy point decline as a 28-year-old in his first season for the Arizona Cardinals in 2006. And by doing so, Murray left the Dallas Cowboys and their vaunted offensive line for the Philadelphia Eagles, a team that has a considerably greater competitor for carries in the backfield in Ryan Mathews (not to mention veteran Darren Sproles). The Eagles are probably mindful of Murray's workload -- and his aforementioned injury history -- and will spell him more often than the Cowboys did.

It's a primary reason I've ranked Murray firmly in my second round, behind the top four wide receivers.

Marshawn Lynch

Workload worry: He's now 29 years old and has 2,033 career carries. Only 12 players in history had more entering their age-29 seasons.

Two of those 12 entered 2014 the same age as Lynch: Maurice Jones-Drew and Peterson. Extracting them from the study -- they haven't yet provided us their aging evidence -- here are how the other 10 fared from ages 28 forward. The "#" column indicates how many of the 10 remained in the league at that age:

Granted, there's a noticeable decline in production from this group of 29-year-olds, but at the same time, Lynch's three closest comparisons, Thurman Thomas, Eddie George and Jamal Lewis, combined to lose only 4.2 percent of their fantasy production from their age-28 to age-29 seasons. George, in fact, was the most productive 29-year-old in the study, with 211 fantasy points in 2002.

Perhaps this makes Lynch a riskier early first-round pick than our rankings indicate, but at the same time, he has also been one of the game's most productive goal-line backs, with NFL bests in both scores (17) and opportunities (37) within the past three seasons, and competitive numbers in both in 2014 alone. (An aside: Super Bowl "recency bias" shouldn't convince you otherwise.)

I was awfully divided between my No. 4, 5 and 6 ranked players -- Lynch, Peterson and Antonio Brown -- for this reason, but those touchdowns are difficult to ignore.

Peterson, Frank Gore and Rashad Jennings, among others

Workload worry: All enter the 2015 season aged 30 or older, a threshold historically known for steep declines in running back production.

History shows a distinct aging curve, with a noticeable decline for players typically at the age of 28, then another, more devastating one, at age 31. This deviates from the lessons of the past; previously we had thought of the age-30 season as the swift, steep decline year.

Not so. The following chart collects all 53 players who have managed at least 1,500 career carries, having played the entirety of their careers during the 16-game era (1979 forward), and averages their production by age. The "#" column indicates how many of the 53 were in the league at that age:

The following players will play the majority of 2015 at the age of 30: Peterson, Jennings, Reggie Bush, Pierre Thomas, Danny Woodhead. Thankfully for this quintet, the age-30 data provides a hint of hope. I'm no longer quite so fearful of 30-year-olds simply because of that number three-oh; there needs to be some other sort of mitigating factor.

The following players will play the majority of 2015 aged 31 or older: Gore (32), DeAngelo Williams (32), Fred Jackson (34), Sproles (32), Steven Jackson (32).

Gore is the most notable name of concern in this group, with 2,442 career carries on his legs, 17th-most in history before his 32nd birthday. Among the top 20 in career carries by that age, the eight who got there during the 21st century averaged 98.3 fantasy points as 32-year-olds, with Jerome Bettis (172, 2004), Corey Dillon (159, 2006), Emmitt Smith (120, 2001) and Curtis Martin (102, 2005) the only ones to crack the century threshold.

The move to the Indianapolis Colts could thrust Gore into a much more fantasy-friendly situation, but at the same time that's a pass-heavy offense and he has suffered a steep decline in receiving production as he has aged, his receptions dropping from 46 in 2010 (age 27), to 28 in 2012 (29), to 11 in 2014 (31). If you're wondering why I've got Gore ranked lower than most, there's your answer.