This year's free agent running back market has the potential to be salivatory for fans of the ground game. With guys like DeMarco Murray and Adrian Peterson possibly up for grabs, there's going to be some serious moola flying.

Teams are preparing to put forth a decent chunk of cash to bring these guys into their respective franchises, as they should. They're gross athletes who, when they've been on the field, have been among the best in the sport. But does that mean it is smart to bring them in via free agency?

I wanted to take a look back through the last couple of years to see how teams have fared when they have spent money on a running back in free agency. I took each of the free agent running backs that had earned at least $1.5 million per year in their new deals and recorded their performance the year before they hit the open market and their first year with the new team.

The statistic I used to evaluate how the backs performed before and after their contracts was numberFire's Net Expected Points (NEP). On each play, there is an expected number of points a team will score on that drive. If a running back gets stuffed for a loss, that number decreases. If he busts off a 35-yard run, then it increases. Those changes in the expected points on a drive are totaled to find a player's Net Expected Points.

I should note that NEP is different at running back than other positions. A third-down or pass-catching running back can have a higher NEP total than a guy who carries the ball on first and second down because he has better opportunities to add a good chunk of expected points while not carrying a ton of risk. With these guys, their roles will change from team to team, but not drastically. They are the same running back, after all, so we'll roll with it and see what we get out.

The following applies to Tuesday's LeSean McCoy trade. Philly owed him buku bucks that they could not allocate elsewhere. It's opportunity cost, which is the same thing we are dealing with in free agency. So keep that in mind as we go through the following as it does relate to Shady and the Bills.

A Decline in Efficiency

Overall, from what I could find, there were 34 running backs from 2009 to 2014 who accepted contracts worth at least $1.5 million dollars per year. I looked at their NEP numbers the year before and the year after the deal, the number of touches they recorded in both seasons, and how the team, as a whole, fared with those players. The most definitive takeaway from all of this was that the players signed in free agency became less efficient.

The year before hitting free agency, the average running back of these 34 recorded a 9.32 Total NEP, which takes into account both their points added on rushes and receptions. For reference, Murray was at 23.54 last year and Peterson was at 16.02 in 2013. So while they were higher than the average free agent, it's not like these guys are just swimming in efficiency.

In their first season with their new respective teams, the backs saw their efficiency drop to just a 0.64 Total NEP. This means they were only 0.64 points above expectation on the year and 8.68 points worse than last year.

The interesting thing about this is that it's not a result of just a few players experiencing significant drop-offs. Of the 34 players in the grouping, 26 of them saw their Total NEP decrease after signing a free agent contract. And one of those guys who improved, Joseph Addai, only increased because he was cut before the season after signing with the Patriots, upping him from a -5.31 rating the previous year to zero.

This is a trend that is getting even more exaggerated. Last year, there were eight running backs who signed a free agent contract worth at least $1.5 million per year. Every single one was worse in 2014 than 2013, as they dropped by an average of 20.94 Total NEP per player. Grossness.

In fact, over the past three offseasons, the only player that fit the qualifications that saw his NEP increase (outside of Addai) was Reggie Bush after he signed with the Lions. The other 20 all went down. That's not exactly going to make teams investing in Murray or Peterson burst with giddiness.

But efficiency isn't everything. If a player can jack up his volume with his new team, then maybe he can still be worth the contract. Unfortunately, even volume takes a hit after players sign via free agency.

Vanishing Volume

I didn't want to focus too much on volume because, when guys are signed as free agents, they aren't expected to maintain the same number of carries. But it's still worth noting, in my mind, as the drop-off was so grotesque.

The 34 players on the list averaged 184 touches in their final year with their old team. After signing their new contracts, they averaged only 123. That's a decrease of just less than one-third at 33.15 percent.

For perspective, let's just pretend that Murray were to experience a similar decrease in his touches. He would go from touching the ball 449 times in 2014 to just 300 times in 2015. While his situation is different from that of others as he is expected to maintain his role as a feature back, it would be foolish to think he can duplicate his 2014 volume and efficiency on a new team.

Overall, only 7 of the 34 running backs saw their touches increase with their new team. Of those seven, Bush, Darren Sproles, and LaDainian Tomlinson also managed to increase their Total NEP.

While this may not relate to Murray, it can serve as a cautionary tale for guys like Toby Gerhart. Gerhart actually had decent efficiency when he was with Minnesota. That efficiency did not translate over to Jacksonville, and that resulted in decreased volume relative to expectations.

Although it's obvious that the players themselves experience a down-turn after free agency, that's not entirely what matters here. If the production the new player provides is better than the alternative, then it's possible the signing would be a positive thing. So let's take a look at how the teams as a whole fared when they brought in a new face to the backfield.

Opportunity Cost

What I did to evaluate the teams was to look at their Adjusted NEP per play (which is adjusted for strength of schedule) and Adjusted Rushing NEP per play both the year before and the year after the signing. This can't tell us whether the new player was the reason for any deviation, but it can show whether or not there is a drastic increase when a new back is brought in.

We'll start with the offense's overall efficiency. In the year before bringing in a new running back, the average offense was exactly that -- average. They had an average Adjusted NEP per play of 0.00, which is exactly at expectation.

In the year after signing their big-buck back, the teams averaged an Adjusted NEP per play of 0.01. So, they improved slightly here overall. That might be something to keep in mind.

If we focus exclusively on the ground game, we see more of the same. The teams went from an average Adjusted Rushing NEP per play of -0.02 the year before signing the free agent running back to -0.01 the year after. Again, a slight improvement, but nothing earth-shattering.

The problem here is that we have seen previously that investment in other positions can result in larger increases than the ones here. Back in January, I looked at the effects drafting an offensive lineman in the first round has on a team. From 2001 to 2014, the 73 teams that took an offensive lineman in the first round of the draft saw an average increase in their Adjusted Rushing NEP per play of 0.014. That's more than the teams saw from bringing in a free agent running back.

What this means is that, by this analysis at least, a rookie offensive lineman provides a larger short-term boost to an offense than a veteran running back. One of them is going to provide you significantly more long-term value than the other. And this is only in the running game as offensive linemen also increased the Adjusted Passing NEP per play 0.027 in their rookie season.

While signing a running back does not exclude a team from drafting an offensive lineman, there is still opportunity cost. That's money that the team could have spent in their defensive backfield, at wide receiver or even on either line. Considering the lack of a benefit to the team after they sign the running back, it's not a stretch to say that would be money better spent elsewhere.

If the player fills a very specific niche and comes at a low price, they can certainly validate their contract. But if you're signing a guy like Murray to be your main horse for a decent chunk of change, the odds that he lives up to the commitment are low. History has shown that he will likely produce less and record a lower volume of touches.

All of this is not to say that Murray and other free agent backs are not valuable pieces. It's more just showing that signing free agent running backs to big-money deals is a huge risk. They often drop in productivity as they are damaged goods and their usage declines with that productivity.

It's entirely possible that there is a team that only has a need at running back and money to spend. Then they should go for it. But the odds of that happening -- when they could not have spent the money on another position of higher importance -- are about as small as the production that these aged free agent backs provide to their new teams.