It’s not exactly a secret that the NFL is continuing to move away from its running-friendly roots. Through Week 11, this has been the most pass-happy season in league history. Teams are averaging 26.9 rushing attempts per game, which would be the first time NFL teams would fail to hit 27 running plays per game. Receptions per game are also at an all-time high — NFL teams are averaging 21.9 completions per game and 239.6 passing yards per game, which would both be league records. Now 69.9 percent of yards from scrimmage come on passing plays; just wait till Andy Reid and I throw a party when that baby goes over 70 percent!

Fewer run plays would normally suggest that each individual running play would be more effective; as passing plays become more prevalent and teams begin to build their defenses around stopping the pass, rushing should theoretically become more efficient as passing becomes less efficient (even if the two paths never meet). That hasn’t been the case this season, with runners averaging a mere 4.1 yards per carry, the lowest figure since 2007.

The lack of efficiency in 2013 extends to many of 2012’s star backs. It seems like every superstar running back has either experienced a notable decline in efficiency, struggled to stay healthy, or both. I count 24 backs who received 100 or more carries in 2012 and have received that many in 2013. Of those 24, nearly half (11) have lost a half-yard or more off their average yards per carry from last season. Just three — DeMarco Murray, Ryan Mathews, and LeSean McCoy — are averaging a half-yard per carry more than they did a year ago.

What’s ailing those players, though? Is there a consistent problem that stands out? Let’s run through those 11 halfbacks and see what has caused their efficiency to dip in 2013.

Bernard Pierce

2012: 108 carries, 532 yards, 4.9 yards per carry

2013: 103 carries, 279 yards, 2.7 yards per carry

It probably makes sense to discuss Pierce’s backfield mate at the same time …

Ray Rice

2012: 257 carries, 1,143 yards, 4.4 yards per carry

2013: 140 carries, 420 yards, 3.0 yards per carry

Rice’s numbers are buoyed by his 25-carry, 131-yard performance against a second-string Chicago run defense on Sunday, albeit in a game when Pierce could only muster 18 yards on his 10 carries. The comprehensive disappearance of the Baltimore rushing attack this year has been one of the biggest reasons why the defending Super Bowl champions are a mere 4-6 this season. Well, that and Joe Flacco’s deal with the interception devil running out sometime in February.

If it were only Rice struggling, it would be easy to point to the hip injury he suffered early this year as the primary cause of his problems. Rice’s longest carry of the season before Sunday went for 14 yards. He had 11 carries of 15 yards or more a year ago.

A healthy Pierce struggling, though, would seem to indicate that the bigger problem lies with the team. Baltimore has experienced plenty of upheaval since last season, and it has unquestionably affected the running game. The Ravens have swapped out three starting offensive linemen from the unit that created holes for Rice and Pierce during the playoffs; left tackle Bryant McKinnie was dismal before the team traded for Jaguars cornerstone Eugene Monroe, center Matt Birk retired and gave way to 2012 fourth-rounder Gino Gradkowski, and guard Kelechi Osemele went on injured reserve with a back injury, to be replaced by backup A.Q. Shipley. The entire left side of the line has been replaced, and it was reasonable to assume it would take some time for them to jell. Specifically, the Ravens have been weakest running in the interior, which is where they make most of their hay. Sixty three percent of Baltimore’s carries head behind the area between the guards, well above the league average of 54 percent, but Baltimore is averaging the second-fewest Adjusted Line Yards in football on those runs.

The Ravens also traded away the energetic run blocking of Anquan Boldin, but the other guy who has been missing from their attack is a player they actually retained: Vonta Leach. Baltimore’s fullback was released during the offseason before eventually re-signing with the team, but they’ve been hesitant to use him as a regular player. In 2012, Leach wasn’t an every-down player, but he did get on the field for 42 percent of Baltimore’s offensive snaps. This year, Leach has played just 25 percent of the offensive snaps. You might figure that his role is increasing because of Baltimore’s struggles with the run, but he’s actually seeing the field less and less; Leach has played on just 31 of the team’s 226 offensive snaps over the past three weeks, a mere 14 percent. Given how bad the running game was before the Bears game, it seems like it might be time to give Leach the role he previously enjoyed in the offense and see if that pushes things forward.

C.J. Spiller

2012: 207 carries, 1,244 yards, 6.0 yards per carry

2013: 123 carries, 507 yards, 4.1 yards per carry

Spiller’s decline has been of the “little bit of everything” variety. For one, you’d expect Spiller to regress from his gaudy 6.0 YPC because that’s a historic outlier; Spiller was one of only five post-merger players to hit six yards per carry over 200 attempts in a season, which tells you just how rare his 2012 season was. (Another player who executed that feat in 2012 will show up very shortly.) Sheer regression toward the mean would have been enough to bump Spiller down a bit.

Reality hasn’t helped matters, either. The Bills lost their best lineman during the offseason when guard Andy Levitre left for Tennessee in free agency; his replacement, Colin Brown, was so bad that Buffalo straight-up released him a month ago. Journeyman Doug Legursky now occupies Levitre’s abandoned spot at left guard. Spiller has also been slowed by an ankle injury that cost him a game and limited his availability in several others. In all, it has been a disappointing year for one of football’s most exciting players from a year ago.

Willis McGahee

2012: 167 carries, 731 yards, 4.4 yards per carry

2013: 106 carries, 275 yards, 2.6 yards per carry

It turns out there are fewer running lanes with Brandon Weeden at quarterback than there are with Peyton Manning under center. Who knew?

Adrian Peterson

2012: 348 carries, 2,097 yards, 6.0 yards per carry

2013: 194 carries, 851 yards, 4.4 yards per carry

Gauging Peterson’s effectiveness is tough, if only because of the highlight-reel effect. Most people aren’t watching the Vikings play on a weekly basis, so when they see Peterson, it’s on SportsCenter or in a highlight package somewhere. They see his most impressive run of the day, and when Peterson breaks off a big run, it’s downright breathtaking — hard to imagine anybody stopping him. But, too often, even Peterson’s good games end up like his performance against the Lions in Week 1. There, Peterson ran for 78 yards on his first touch of the season. He then carried the ball 17 more times for a total of 15 yards. And if that big run is for 20 yards as opposed to 78 yards, well, the Vikings end up getting three yards per carry from Peterson.

I wrote about Peterson’s likely decline before the season, and he’s hitting a lot of those boxes this year. The big plays are down. The average play is down. He’s beginning to deal with nagging injuries, notably a groin injury that Peterson admitted was slowing him down this past week. (Peterson also had to cope with the tragic death of his child last month.) The quarterback play hasn’t helped, given that the Vikings have played musical chairs under center this year, but their passing attack wasn’t all that great during the second half of the 2012 season, either. That was with Christian Ponder throwing to a group of receivers without Percy Harvin (or 2013 arrivals Greg Jennings and Cordarrelle Patterson), and Peterson still put together the greatest half-season a running back has ever managed. Last year, Peterson ran through nine men in the box and still picked up big plays. This year, nine men in the box have slowed Peterson down. There’s no shame in that. They call them career years for a reason.

Doug Martin

2012: 319 carries, 1,454 yards, 4.6 yards per carry

2013: 127 carries, 456 yards, 3.6 yards per carry

Martin’s incredible rookie season in 2012 was a surprise because of how badly Tampa Bay’s plan had failed. (Little did we know.) The Buccaneers wanted to build around their running game and drafted Martin at the end of the first round to go with two Pro Bowl–caliber guards, Davin Joseph and Carl Nicks. Joseph missed the entire year, and Nicks went down with a toe injury halfway through the campaign. So, in 2013, it seemed reasonable to expect Martin to at least maintain his previous level of performance, given that those guys would be back and healthy.

Oops. Nicks has had to deal with multiple outbreaks of MRSA on his toe, a life-threatening issue that has kept him out of the lineup and limited his effectiveness when he’s been able to play. Joseph, returning from a torn patellar tendon, was downright terrible during the first half of the season; he has only recently begun to show flashes of his former self. And, as you might have heard, the passing game didn’t fare so well under the departed Josh Freeman. Those factors combined to limit the running lanes available to Martin before the second-year back tore the labrum in his left shoulder, ending his season after 127 disappointing carries.

Jamaal Charles

2012: 285 carries, 1,509 yards, 5.3 yards per carry

2013: 186 carries, 803 yards, 4.3 yards per carry

Did you know that Charles has just one 100-yard game this year, and that was a mere 108-yard performance against the Titans in Week 5? In general, 100-yard games are down — there were 117 of them in 2000 and we’re on pace for 85 this year — but it seems impossible that a player as dynamic as Charles could be held in check so frequently, right?

The truth is that Charles’s workload has expanded to the point that he’s the only featured offensive weapon in the lineup. The Chiefs have little faith in fumble-prone backup Knile Davis, and given that they’ve been ahead for most of the 2013 season, they’ve been happy to hand the ball to Charles, avoid turnovers, and play field position with their dominant defense. And, truthfully, Charles hasn’t exhibited the same big-play ability he has in the past. Charles’s 35-yard run against the Broncos on Sunday night was his longest carry of the year and just his second attempt all season to exceed 20 yards. That from the same guy who had three rushing touchdowns of 80 yards or more last season alone! Charles is still a valuable, talented player, but the increase in his workload has been met with a drop-off in efficiency.

Chris Johnson

2012: 276 carries, 1,243 rushing yards, 4.5 yards per carry

2013: 167 carries, 632 rushing yards, 3.8 yards per carry

This was supposed to be Johnson’s return to the top of the rushing charts; the Titans had spent big money on the aforementioned Levitre and used a first-round pick on Alabama mauler Chance Warmack, and the improvement in their passing attack was supposed to get guys out of the box and out of Johnson’s way. That just hasn’t happened. Jake Locker has been injured, Warmack has been ineffective, and Levitre is still developing some rhythm with the guys around him.

Again, though, it seems like the big-play acceleration that was once Johnson’s calling card is gone. There have been five games this year in which Johnson has failed to produce more than 10 yards on even one carry. Like many others on this list, he had his biggest run of the season last week, and even that was merely for 30 yards. Johnson has always been a boom-and-bust back, but this year, the boom has fast-forwarded to later.

Pierre Thomas

2012: 105 carries, 473 yards, 4.5 yards per carry

2013: 107 carries, 413 yards, 3.9 yards per carry

This is another case of efficiency being traded for usage. Thomas actually has more carries in 10 games this year than he had during his entire 2012 campaign, when he suited up for 15 contests. That comes thanks to the alternately injured and ineffective Mark Ingram (again, last week aside) and the departure of Chris Ivory to the Jets. Thomas is averaging 10.7 carries per game, the second-highest average of his career. The last time he averaged this many carries per game was in 2010, when he averaged 13.8 carries per game and could only muster 3.2 yards per rush before an ankle injury ended his season. With Ingram back, Thomas’s workload should decline, which will help boost him back over four yards per carry.

Marshawn Lynch

2012: 315 carries, 1,590 yards, 5.0 yards per carry

2013: 208 carries, 925 yards, 4.4 yards per carry

It seems fair to blame injuries along the offensive line for Lynch’s troubles in 2013. Seattle’s expected line from training camp has been ripped to shreds during the season. Star left tackle Russell Okung has played just three games because of a foot injury. His counterpart, right tackle Breno Giacomini, has lined up for four. And center Max Unger, the team’s best interior lineman, has played in eight games. When your offensive line is in shambles and running a bunch of backups out there in place of Pro Bowl–caliber linemen on a weekly basis, it’s going to drop your yards per carry. The good news is that Okung, Unger, and Giacomini are all back in the lineup, so the Seahawks — and Lynch — should be in much better shape going forward.

BenJarvus Green-Ellis

2012: 278 carries, 1,094 yards, 3.9 yards per carry

2013: 156 carries, 522 yards, 3.3 yards per carry

The Law Firm has never exactly been a hyperefficient back — his career rushing average was just over the 4.0 yards per carry mark heading into the season — but the arrival of Gio Bernard has seemingly turned Green-Ellis into an old fogey of a running back. Green-Ellis seems to hit the hole these days and hope that he’ll get lost in the shuffle so the Bengals keep paying him. He hasn’t been a great player in 2013, and while he does offer a nice change of pace for Bernard, BJGE has been an issue.

Of course, there are other backs with disappointing seasons on their shoulders. Trent Richardson has done Trent Richardson things for the Colts this year. Arian Foster has actually averaged 4.5 yards per carry this season as opposed to the 4.1 figure he hit a year ago, but injuries have sapped his availability and led to Foster’s appearance on injured reserve after back surgery. Frank Gore, who narrowly misses out on this list at a drop of 0.44 yards per carry, hasn’t been as consistent as he was at making people miss a year ago. In all, many of the league’s halfbacks are exhibiting signs of decay and inefficiency relative to where they were a year ago, even as the leaguewide rushing workload declines.