The Eagles made wave after wave in free agency, and while the off-season, and with it the implementation of Chip Kelly’s vision for the team, is far from over, there is one common thread that all the major moves the Eagles have made have: an injury history, in particular a soft tissue injury. Nowhere is this more prominent than in the backfield, where DeMarco Murray, Ryan Mathews and Sam Bradford have combined for four full seasons in their careers. Additionally, Byron Maxwell, Kiko Alonso and Walter Thurmond all missed time in 2014, the latter two significantly. In a contact sport, this common denominator is concerning. Setting aside Bradford, who may not even be on the Eagles by the draft, the most concerning health issues are at running back. DeMarco Murray has been injured in three of his four seasons, Ryan Mathews in four of his five.

Multiple studies have been done on running back workloads, they aren’t encouraging. Football Outsiders has the most notable study, "the Curse of 370". It claims:

A running back with 370 or more carries during the regular season will usually suffer either a major injury or a loss of effectiveness the following year, unless he is named Eric Dickerson.

This study is flawed, 370 carries is a quintessential example of ‘arbitrary end points’, it does not account for receptions--which they realized after the fact, it does not account for the additional wear and tear of playoff games--which they also realized after the fact, and hand waving away Eric Dickerson is just plain bad statistical methodology. Additionally, some of the declines that happened to high workload RBs the following year had nothing to do with their workload. Terrell Davis tore his ACL, but did it trying to make a tackle on an interception return. Corey Dillon's production dropped off after his 370 season, but at that point he had the 25th most carries in NFL history, his tank was running dry anyway. John Riggins was 34 and never stayed healthy. Much of the study’s conclusions are intuitive: we already know running backs have a short shelf life, and if a player is carrying the ball a historical amount, they are having exceptional health and most likely good production that season. A regression to the mean of missing time should be expected. But how much of a regression should we really expect?

A more thorough study has been done by Advanced Football Analytics, rebuking some of Football Outsider’s findings, better answered that question:

In the 25 RB seasons consisting of 370 or more carries between the years of 1980 and 2005, several of the RBs suffered injuries the following year. Only 14 of the 25 returned to start 14 or more games the following season. In their high carry year (which I'll call "year Y") the RBs averaged 15.8 game appearances, and 15.8 games started. But in the following year ("year Y+1"), they averaged only 13.0 appearances and 12.2 starts. That must be significant, right? The question is, significant compared to what? What if that's the normal expected injury rate for all starting RBs? If you think about it, to reach 370+ carries, a RB must be healthy all season. Even without any overuse effect, we would naturally expect to see an increase in injury rates in the following year.

The study would go on to compare RBs with 370+ carries vs a similar sample size, RBs with 344-369 carries:

The differences are neither statistically significant nor practically significant. In other words, even if the sample sizes were enlarged and the differences became significant, the difference in games started between the two groups of RBs is only 0.4 starts and 1.0 appearances. RBs with 370 or more carries do not suffer any significant increase in injuries in the following year when compared to other starting RBs.

That’s obviously encouraging. We shouldn’t expect DeMarco Murray (and Ryan Mathews, but he is not coming off a high workload) to be any more or less injury prone than he was prior to the 2014 season. But therein lies the problem: prior to the 2014 season, Murray couldn’t stay healthy. In 2011, he fractured and sprained his ankle, ending his season with three games to play. In 2012, he sprained his foot, sidelining him for six games. In 2013, he sprained his MCL, missing two games. We can find some encouragement though through sports science. From Football Outsiders' Adjusted Games Lost:

Philadelphia, noted for Chip Kelly's foray into sports science last year, had the best AGL in 2013 and ranked fifth this season. They are only the third team since 2002 to lead the league in AGL and finish in the top five the following season. Maybe this team is onto something with preventing soft tissue injuries.

Murray’s injuries were soft tissue ones. And Ryan Mathews, who has missed time with two broken bones (collar, hand) and a concussion, but also soft tissue injuries of a calf, ankle and MCL sprain, Sam Bradford (two ACLs), Walter Thurmond (pectoral), Byron Maxwell (calf), and Kiko Alonso (ACL). Again, all these additions (and some hold overs) have had problems with soft tissue injuries. It's only been two seasons, but if Kelly’s methods can prevent many soft tissue injuries, then there’s reason to believe that these injury plagued players can stay relatively healthy.

So perhaps DeMarco Murray can stay on the field in 2015. Heavy workload running backs seemingly encounter another fall off besides health. Their production drops. From Football Outsiders’ revised 370:

All players with 390 or more carries, no matter how these carries were split between the regular season and the postseason, averaged a 33 percent drop in total yards, and an 11 percent drop in yards per carry.

And now we're back to discouraging. This ESPN piece has a good illustration of that:

But there's a problem with these studies: they only look forward, not backwards. We should expect that a player's historical carry season be one with a good yards per carry average. Coaches generally aren't going to feed the ball that much to a player who is struggling, though seasons like Ricky Williams falling forward 392 times in 2003 exist. Should that 0.4 yards per carry decline be expected regardless of workload? Somewhat.

Prior Season 370+ Season 4.3 ypc 4.5 ypc

In 16 of the 26 "370" seasons (two of the 28 seasons in ESPN's chart were not applicable because they were the RB's rookie year, George Rogers and Eric Dickerson), the running backs had a higher yards per carry in the 370 season they they did in the previous season. So we should expect a high workload running back's yards per carry to regress even without considering wear and tear.

But then there's DeMarco Murray. His high workload 2014 season had a very good 4.7 yards per carry, but his 14 game 2013 season had an even better one, 5.2. The fabled Cowboys offensive line might have helped keep him healthy, but they didn't make him more effective, his career yards per carry entering the season was 4.9. There have been cases of players having a higher yards per carry in the season following a 370 year: Eric Dickerson in 1984 and 1986, Emmitt Smith 1993, LaDainian Tomlinson 2003 and Michael Turner 2009 to name a few. There are always exceptions to the rule.

So should we expect DeMarco Murray to stay healthy? Probably not. For all the Eagles apparent advancements in preventing soft tissue injuries, only two seasons by one team is not enough to conclusively say that the Eagles have a decided advantage. They very well may, and there is reason to believe he can. But we shouldn't raise our expectations on it. Should we expect Murray's production to fall of a cliff? No. He won't lead the league in rushing again, because even if healthy he won't carry the ball as much. But when Murray does get the ball, he should continue to be effective.