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A Fantasy Football legend expected to make his return to football this fall. A long-standing icon in Fantasy lineups changing teams for the first time in his career. A beast of a back for years getting another crack at being an every-down guy following a failed Super Bowl run. And a Windy City fixture who might be in his last year as a full-time starter.

And you might not want to draft any of them.

The four guys described above -- Adrian Peterson, Frank Gore, Marshawn Lynch and Matt Forte -- have all been superior Fantasy choices for years. But they all have specific red flags that suggest a breakdown is coming. It's not based on age, though two of them are over 30 while the other two will be 30 by this time next year. Instead, it's based on what we've come to study for a long time: career workload.

The power of TEN

The gauge to determine whether or not a back will slow down has evolved over the years, but we're comfortable with where it's at now. The simple formula: Take a rusher's career reception total (including the playoffs but not the preseason), divide by three and add that number to the player's career carry total (again, including the playoffs). That creates a "Total Evaluation Number" (or TEN) Fantasy owners can glance at to determine if the end is nigh. If that number is at 2,100 or more, emergency sirens should go off in your head.

This isn't complicated -- the more work a back takes on, the more his body wears down. When that work involves catching the ball, the tackles aren't always as punishing, hence why they don't count like a traditional carry. Not to go all doctor on you but it's obvious that as athletes get older, the nicks and bumps they deal with don't heal as fast and impact performance. They might even spur a serious injury, and serious injuries typically slow a back down. It all adds up and eventually a back goes from a big role with big numbers to a decent role with weaker numbers to out of the league. And it happens fast.

But there are a number of running backs who are just as old, if not older, but have a low TEN. Are they safe to draft for Fantasy? Sure, so long as you know WHY they haven't had a lot of work. Darren Sproles has barely registered 1,000 touches over his career but he's been a part-time back since forever. And we've cursed the Panthers for holding back DeAngelo Williams. While their careers have been extended, they also are stuffed into part-time/backup roles. An older back can handle a modest workload of 10 touches per game, but we're not taking players in Fantasy for 10 touches per game. We want more -- the older guys can't hack it.

Let's get to it. Listed below are many of the running backs the majority of Fantasy owners consider "old." Let's see how many of them are truly on the cusp of cracking.

Running back Career carries Career rec. Total Evaluation # Major injury Week 1 age
Reggie Bush 1,312 494 1,477 30
Justin Forsett 630 167 686 29
Matt Forte 1,859 456 2,011 29
Arian Foster 1,496 254 1,581 28
Frank Gore 2,582 360 2,702 32
Shonn Greene 1,092 74 1,117 30
Fred Jackson 1,829 322 1,936 34
Rashad Jennings 554 127 596 30
Chris Johnson 1,908 297 2,007 29
Marshawn Lynch 2,220 249 2,303 29
Adrian Peterson 2,147 212 2,218 30
Darren Sproles 554 462 708 32
DeAngelo Williams 1,455 181 1,515 32

When age lies

There are many backs on the list who are getting close to 30 years old but don't have a ton of wear and tear. Justin Forsett is the poster child -- while he'll be 29 when the season starts, he has 630 carries to Arian Foster's 1,496. Foster is a year younger but several years older when it comes to his body (just ask his soft tissue). You're making a mistake passing on Forsett because he's "old."

You can say the same thing about Rashad Jennings, who hasn't accumulated a ton of work in his career. That doesn't mean there aren't injury concerns for him. Jennings has missed at least one game every year of his career and is a fair-to-middlin' running back in a three-way battle for playing time.

Reggie Bush, Shonn Greene, Fred Jackson, Chris Johnson, Darren Sproles and DeAngelo Williams are all basically role players who carry value as part-time Fantasy players. No one thinks long and hard about spending an early pick on any of these guys.

That leaves four players on the list -- and they're all names people do think long and hard about taking in Fantasy.

Fragile Four or Fantastic Four?

What does this mean for Gore, Lynch, Peterson and Forte? Well, you could call them all worn-out and keep them off your Fantasy team. That's one way to do it. Certainly you'd save yourself from being worried about them holding up through 16 games.

But it's more appropriate to size up each back and develop a realistic expectation. So let's see what we can put together on them, figure out just how risky they are, and what kind of rewards they can deliver.

Frank Gore, Colts
Risk factor: High Just look how much work he's had! No one currently in the NFL has as many career touches as Gore, and he's had at least 250 carries in each of his last four seasons. It is completely fair to wonder how much longer he will last (though we've wondered that for years). A pair of tough matchups to start the season at the Bills and vs. the Jets won't help his case.
Reward factor: Medium For all he's done, Gore has finished anywhere from 11th to 17th among running backs each of his last four seasons. You could argue he'll have a shot to end up in the Top 12 in the Colts' offense, particularly if he winds up being used as Ahmad Bradshaw was last year. As for his health, he hasn't missed a game since 2010. Plus his rushing average actually went up last season and his receiving skills are better than what he's been allowed to do the past four years.
Worth taking: Round 3 If you're going to take a player with high risk, do it with a pick that won't burn you as badly if things go south. By Round 3 you'll have two other playmakers and can still add a number of rushers after taking Gore to cover yourself.

Marshawn Lynch, Seahawks
Risk factor: Medium Between picking up heavy amounts of work for the run-focused Seahawks and deep playoff runs, Lynch has amassed 1,345 carries and 134 receptions over his last four seasons with one game missed. That's equally remarkable and scary. Lynch's TEN went from 1,946 to over 2,303. Whereas last August we knew he was overused but still had gas in the tank, now we know he's surpassed the red-flag number and still has those over-use concerns.
Reward factor: High Lynch doesn't play around -- he's finished as a top-five running back each of the last four seasons. There isn't another running back who has done that. And you sort of have to give Lynch the benefit of the doubt since he's never actually suffered a major injury (as far as we know). He's just one of those freaks of nature. The workload will eventually catch up to him but the Seahawks didn't give him $12 million guaranteed for 2015 just to be nice.
Worth taking: Round 1 Maybe your knees will knock a little bit trusting Lynch as a first-round pick, but the only strike against him is our research -- and even we are overlooking it to put this rare talent on our Fantasy rosters. Just draft Robert Turbin in Round 11 to protect the investment.

Adrian Peterson, Vikings
Risk factor: High On the surface, no sane human would take a running back with a TEN of 2,218 coming off a year where he was forced to sit out 15 games. But Peterson's a rare talent reportedly in excellent shape. It's particularly rare for a player to be forced out of action for a year or longer and quickly return to form. Peterson already has a TEN of over 2,200, which puts him right in the cross-hairs of a breakdown, but there's no denying his skills.
Reward factor: High Peterson's track record might be enough proof for you to chase him on Draft Day. He rushed for nearly 2,100 yards after tearing his ACL and happens to have at least 10 touchdowns in every season he's played in, including the ones cut short by injury. He's earned plenty of credit for being a Fantasy legend.
Worth taking: Round 1 The bottom line is that Peterson is in position to be one of the few 20-touch backs with goal-line work. And he'll do it in an offense that is better than the one he played with last. That is the No. 1 reason why he's still valuable in Fantasy. Even if he averages a modest (for him) 4.4 yards per carry and 5.7 yards per catch, he'll still post a lot of yards and double digits in touchdowns. And while he's amassed a lot of work over his career, the year off should do wonders for his durability. I'd look for him with a Top 5 pick.

Matt Forte, Bears
Risk factor: Medium Forte has had a lot of work the last three seasons -- 803 carries and 220 catches -- but is still shy of the 2,100 red-flag TEN warning. That will change after this season, which should be his eighth straight with a sizable workload. Pair that with a declining rushing average (3.9 yards per carry in 2014 thanks to a mostly weak final five games) and there has to be some concern that the wheels will fall off of Forte sooner than later.
Reward factor: High The workload he has taken on is real, but it's also benefitted him since he's used the volume of touches to produce excellent stats. Helping Forte is a coaching staff that seems willing to run the ball, particularly if Jay Cutler plays erratically as everyone expects. And a floor has been established for his numbers -- he has had at least 1,400 total yards every year of his career with at least 10 total touchdowns in each of his last two seasons.
Worth taking: Round 1 He's consistent, he's durable and he's not quite ready to be a candidate for disaster. All indications are that we will be able to squeeze out at least one more year of numbers out of him before Father Time settles in. That early-season schedule (Green Bay, Arizona, at Seattle) is nasty, though.

Warning track

It'll take some time for the next wave of running backs to worry you about their workload catching up with them. LeSean McCoy is within striking distance of a TEN of 2,100 but would need to get seriously overworked in 2015 to get there. Figure he's two years away along with Jamaal Charles. And while DeMarco Murray is at least three years away from the danger zone, there's major concern about him holding up in 2015 after taking on 436 carries and 61 catches over 18 games last year.

Running back Career carries Career rec. Total Evaluation # Major injury Week 1 age
Joique Bell 483 143 531 29
Jamaal Charles 1,349 263 1,437 28
Ryan Mathews 923 146 972 28
LeSean McCoy 1,608 309 1,711 27
Darren McFadden 1,038 211 1,108 28
DeMarco Murray 978 174 1,036 27
Jonathan Stewart 1,081 135 1,126 28