PHILADELPHIA — Adrian Peterson struggled with a new system, so he got what he wanted. DeMarco Murray had trouble in a different offense, so he was benched.

More NFL teams are running the ball out of shotgun and pistol formations instead of an I-formation or a split-back set. Power football has become more old-fashioned than old school.

It’s been a tough trend for some of the league’s elite running backs.

After missing nearly the entire 2014 season, Peterson returned to the Minnesota Vikings and found the team operating much of its offense with quarterback Teddy Bridgewater in the shotgun. Peterson wasn’t comfortable getting carries without a full head of steam. He said he was “hesitant” following a loss to San Francisco in Week 1.

It quickly became clear to Vikings offensive coordinator Norv Turner that Peterson ran better in a traditional formation. Peterson averaged just 1.7 yards per carry in the shotgun and 5.3 yards under center through the first 12 games.

“Our bread and better has been my bread and butter from 8 yards back,” Peterson said last month. “Coach Turner, he’s noticed that, and we just kind of put our team and the offense in position to be more productive.”

Catering the offense to Peterson’s skills helped the Vikings start 8-2 before three straight losses, including Thursday night at Arizona. Peterson leads the league with 1,251 yards rushing, most coming with Bridgewater under center.

In Philadelphia, coach Chip Kelly takes plenty of heat for not tailoring his offense to suit Murray. The 2014 NFL rushing champion hasn’t lived up to expectations after leaving Dallas to join the Eagles. He played just 14 snaps in a win at New England last week.

Some blame Murray’s struggles on his inability to adapt to Kelly’s up-tempo offense run mostly out of the shotgun. Murray was a downhill runner playing behind an excellent offensive line on the Cowboys. He ran straight ahead and found gaping holes.

Murray isn’t hitting the open lanes quickly this season and he seems uncomfortable on sweeps and zone-read plays.

Asked if the offense would be better if Kelly adapted the plays to his strengths, Murray said: “I don’t call the plays.”

His terse response indicates his displeasure with the scheme.

Most running backs prefer lining up deep in the backfield because it allows them to get a running start and gives them better field vision. The shotgun is more advantageous for the quarterback in what has become a passing league.

Murray knew Kelly’s philosophy, but still signed a $40 million, five-year contract in March.

“In college, he ran from the (shotgun) every single snap,” Kelly said. “He knew coming in what we were going to do. We never had any discussion that we were going to change our system.”

The numbers don’t support the theory that Murray is better running from under center. He’s averaging 3.7 yards per carry (518 yards on 139 carries) out of the shotgun and only 2.1 yards (51 yards on 24 carries) when he’s behind the quarterback. In four seasons with the Cowboys, Murray averaged 5.3 yards in the shotgun.

“I think a player may like something more than another, but that doesn’t mean that they are not good at the thing that they may not (like),” Eagles offensive coordinator Pat Shumur said.

Eight teams entered the season with no fullback on their roster. But some of the league’s top rushers are power backs, including Chris Ivory of the Jets, Tampa’s Doug Martin and Carolina’s Jonathan Stewart.