Patience is an often-underrated virtue in today’s day and age.

It’s been four months since running back Arian Foster -- who suffered a groin tear and Achilles’ tendon rupture last season -- first worked out for his new team, the Miami Dolphins. He signed with them last week, almost exactly a year after the two injuries ended his season. The chance to sign with another NFL team took plenty of steps and even more waiting for the former Houston Texans star.

Yet, a nine-month recovery is definitely on the short end of the spectrum for a serious injury. Modern medicine has done some amazing things for athletes.

Even 20 years ago, it would have been a miracle for an NFL athlete to return from major injuries like these and almost unheard-of to consider that this player might become dominant once again. Foster could be yet another impressive comeback story if 2016 breaks his way, but we have to be patient; there is a lot of risk that comes with this.

What effects do major leg injuries have on running backs' careers?

The Pace of Nature

Part of the reason why I mention patience right away in this article is that it was a major component of the research behind this piece. I wanted to be sure to take the time necessary to do the research properly in handling this sensitive topic. There is a lot of noise to parse through in order to get to the signal, but hopefully this can provide an analytical perspective on how we can approach severe running back leg injuries as fans and fantasy players.

With that in mind, I want to assess major leg injuries among running backs over the last 15 years and determine whether or not there are career-damaging injuries for the running back position.

We will do this by charting the course of running backs’ careers from the last 15 years before and after injury via numberFire’s signature metric, Net Expected Points (NEP). NEP helps us take the numbers we get from the box score and shows how that player did versus expectation. For more info on NEP, check out our Craig Zumsteg of Footballguys.com -- who specializes in injury data research -- I pulled the records of players who suffered their injury in 2001 or later and then mapped out their annual rushing attempt totals, Rushing NEP per attempt rates, and Rushing Success Rates over the course of their careers.

Is there anything we can learn from this?

Don’t Cross the Bridge Until You Come to It

I grouped the average annual rushing attempts, Rushing NEP per attempt, and Rushing Success Rate of these 49 running backs by their years before and after the initial severe injury.

The table below shows these averages for all running backs in this study, grouped into two time periods: before injury and after injury.

Are there any patterns we can find?

Unfortunately, this isn’t as conclusive across the board as it could be. There is a clear diminishment in per-play rushing value; Rushing NEP per attempt plunges from above-average to well below-average (-0.03 is the mean NFL Rushing NEP per attempt since 2000).

However, there is no discernible difference in average carries per season or in Rushing Success Rate.

If we just look at the 20 running backs who had an average of 150 carries or more in seasons prior to suffering one of these injuries, the data shows only a slight change -- but an important one.

The per-play rushing value change is backed up by looking at these former high-usage runners only, but here we also see a significant decline in average carries per season, in the order of a 25 percent loss of work in the years after injury.

It is worth noting, too, that 11 of the 49 running backs since 2000 who suffered these injuries had more than one such severe injury -- 22.45 percent of the sample. While this is obviously not an overwhelmingly large percentage, it is significant, and studies have shown that injuries to soft tissues increase the likelihood of both repeat injuries or compensation injuries.

While these results clearly don’t show a night-and-day change from before a running back’s injury to after, there are hints here that seem to suggest there is a value decline and workload in the years following a major injury.

Woosah

Minnesota Vikings running back Adrian Peterson sustained a simultaneous ACL and MCL tear at the end of the 2011 season and promptly came close to breaking the single-season rushing record right away in 2012. Foster is returning to action nine months removed from both a groin tear and Achilles’ rupture.

We football spectators almost expect this accelerated timetable these days, though it’s fairly clear that backs such as Peterson and Foster are the exceptions and not the rules. The average time for a return after an ACL or MCL tear (knee ligaments) is about seven to nine months, and a full return to pre-injury form -- if ever possible -- can take almost two years. For an Achilles’ rupture, the average return to NFL action is 11 months.

As we’ve seen knee injuries become ever more prevalent in the NFL, surgeries have gotten better and better to help repair them and rehabilitate players quickly. Los Angeles Rams running back Todd Gurley tore his ACL before he played a down of NFL football, and he’s still a star.

However, studies have shown that no running back -- Mikel Leshoure and Kendall Hunter both come to mind -- has ever fully recovered from an Achilles’ tear.

Does the data bear this out? The table below shows the before-and-after changes in value for each injury type, on average, as well as the average years played after suffering such an injury.

When we break the injuries apart by subtype, knee injuries (ACL, MCL, PCL, etc.) are actually the least severe of all injuries these days, in almost every way. There are so many instances of successful knee ligament surgeries today that it’s not unlikely a running back returns to form easily after this kind of injury.

The worst kind of injury for a running back, in terms of average value diminishment, is the Achilles’ rupture. These injuries suffer the largest drops in Rushing NEP per attempt and Rushing Success Rate, as well as the lowest average years playing after suffering the injury.

It’s true, there is a only small sample size of Achilles’ ruptures in the last 15 years, but the results have been devastatingly bad for those players. Vick Ballard hasn’t registered a carry since rupturing his Achilles’ tendon in 2014, and while Ryan Williams registered 58 carries the season after tearing his, he sustained an impossibly bad -0.33 Rushing NEP per attempt over those carries.

Players’ bodies are different, and their mindsets during recovery are different, so there’s always a chance players like these rebound.

According to history, though, it’s a slim margin. Be patient with them -- you might strain something if you're not careful.