(Photo by Toni L. Sandys/The Washington Post)

Running backs are not as valuable as they used to be. Teams are spending less on free agents at the position and more and more teams are moving to a running back by committee.

Some of it has to do with the short life span of an NFL running back. Looking at the 170 RBs, aged 21 to 32, since 1970 with at least 150 carries in consecutive seasons, we can see there is a sharp decline after they turn 27 years of age. The example below uses the average loss of yards per year for a hypothetical 21-year-old running back with 1,000 yards rushing in his rookie year.

With some of the league’s best running backs staring at the wrong side of the 27-year-old mark, you have to wonder how much they have left in the tank. And that includes Minnesota Vikings tailback Adrian Peterson.

Purple Jesus had the fifth-most rushing yards (1,266) in the NFL last season but also missed two games and yielded far below his 2,097-yard campaign from 2012. He is currently 27th on the all time rushing yards list and is being penciled in for a 1,400 yard, 11 touchdown season in 2014. Couple that with the anterior cruciate ligament injury he suffered in 2011 and you start thinking about how many more carries he has in him.

Knowing a running back’s previous workload and production — defined here as the Value Based Drafting metric — can help us predict his future career length using the formula: Future rushes =~ 3203 – 104*age + 2.3*VBDLastYr + .813*PreviousVBD – .13*PreviousRsh

For the purposes of this discussion, the key number is the -.13. It says: all else equal, every rushing attempt you had before last year will cost you .13 predicted future rushes. So if two backs are completely equal in every way, but one of them had an extra 500 rushes when he was young, you would expect the player with the higher workload to have 500*.13 = 65 fewer rushes during the rest of his career. The 104 next to “age” indicates that, all else equal, a player who is one year older will expect to have 104 fewer carries left in the tank. Combining these two numbers, we could infer that it would take about 800 previous rushes to age a back as much as one chronological year does.

Plugging Peterson’s numbers into the formula projects he has 786 future rushes in his career, or a little less than three seasons at last year’s pace. If he was to suffer an injury similar to the one in 2011, that could decrease even further.

Two running backs have already defied the odds: Willis McGahee and Fred Jackson. Based on their workload and production to date, you would have thought they would be out of the league by now. And while history suggests they should be, they show there are exceptions to the rule. History suggests we are also seeing the end of the line for DeAngelo Williams, Pierre Thomas, Steven Jackson, BenJarvus Green-Ellis and Frank Gore.

Gore and the 49ers have already started preparing for this day. San Francisco traded up in the second round of the 2014 NFL draft to select Ohio State’s Carlos Hyde,  the second running back taken, and Marcus Lattimore, a 2013 fourth-round pick last year, is expected to return from the knee injury he suffered in October 2012.  Kendall Hunter and LaMichael James are also part of the crowded backfield.

“I’ve been out there competing ever since I left high school,” Gore said. “I’ve been with top guys who have been in the league. It’s all to get each other better, and I’m up for it. One day, they’re going to have to get this role. But while I’m here, I’m going to look at it as a challenge.”