MIAMI (AP) Hark the unheralded running backs.
Fifth-round picks, an undrafted rookie and a player who was out of the NFL for three seasons are among those bursting into the clear this year, leaving defenses and fans wondering the same thing: Where did they come from?
The surprising success by such low-profile rushers as Jay Ajayi, Jordan Howard, Robert Kelley and Tim Hightower speaks to the challenge of evaluating running backs.
''It's not like it used to be,'' said Miami Dolphins offensive coordinator Clyde Christensen, an NFL assistant since 1996. ''When I first came in the league, you'd watch a tailback in college, and they all had a fullback in front of them, and they all got their 25 carries a game, and you compared them to each other. Now they're coming out of spread offenses, and there are a lot of good running backs out there who play for teams that don't run the ball as much. It's a lot harder comparing them now.''
Some of the most frantic scrambling this season has been by teams seeking someone to carry the ball, with the high injury rate at the position creating openings for under-the-radar runners.
Ezekiel Elliott, DeMarco Murray and Le'Veon Bell are having big seasons, as expected. But other Pro Bowl backs - Todd Gurley, Doug Martin, Adrian Peterson , Chris Ivory - have struggled or been hurt this year.
Four-time Pro Bowl rusher Arian Foster retired from Miami in October, the day after Ajayi's second consecutive 200-yard game for the Dolphins. Even coach Adam Gase was surprised by the way the job was seized by Ajayi, a fifth-round pick in 2015.
''When we were going into the season, we felt like we were going to get four guys involved in what we were doing,'' Gase said. ''So we didn't know if we'd really have enough touches to say a guy was going to have a 1,000-yard season.''
Ajayi is 44 yards from the 1,000 mark, and one of 11 players on pace to reach the milestone. Six did it last year.
Cowboys rookie Elliott leads the league with 1,392 yards.
''We've all seen how the league has gone as far as trends with the running backs,'' Gase said. ''It seems like rushing is coming back a little bit. You're seeing teams like Dallas, and obviously we're feeling pretty good about what we've got going on. There are other teams that are leaning on the running back a little more now.''
As a result, players like Kelley are obscure no more. Nicknamed ''Fat Rob,'' he has rushed for 587 yards while making six starts for the Washington Redskins, and is averaging 4.6 yards per carry.
In his best season at Tulane, he rushed for 420 yards - one reason he went undrafted.
''As a free agent, we didn't see a whole lot,'' coach Jay Gruden said. ''He didn't get many carries at Tulane.''
Kelley, listed at 6-foot and 220 pounds, has a rugged running style and rarely goes down on the first hit.
''I'm happy with how far I came,'' he said.
Ditto Howard, a rookie fifth-round pick from Indiana. He's one of the few bright spots this season for the Chicago Bears, rushing for 969 yards with a 5.0 average that ranks third in the league.
NFL teams often find their best running back in unlikely places, and tend not to devote high draft picks to the position. Elliott was the lone rusher taken in the first 44 selections of this year's draft.
In 2015 the Arizona Cardinals found a gem from Northern Iowa in the third round, and now David Johnson leads the league with 15 touchdowns and 1,830 yards from scrimmage.
''He's basically their team,'' Ajayi said.
In Green Bay, Eddie Lacy has been out since Week 6, which left the Packers shopping for replacements during the season. Lately they've been using James Starks, waivers acquisition Christine Michael and converted receiver Ty Montgomery, who has made three starts.
''We're a team that has gone through a lot of injuries,'' coach Mike McCarthy said. ''It's about creating opportunities for all of these guys.''
Hightower waited a long time for an opportunity after spending 2012-14 out of the NFL following a knee injury. He has become an effective backup to Mark Ingram with the New Orleans Saints, rushing for 453 yards and catching 18 passes for 178 yards.
Hightower said the nature of his position sometimes allows low-profile players to break out of a depth chart scrum.
''Running back I think is an easier position - just being honest - to come in and play right away,'' he said. ''As long as you understand the blocking scheme and how to protect the quarterback, the schemes are similar to college and even high school. The learning curve isn't as great, so a guy can come in from off the streets, or from anywhere.''
AP Pro Football Writer Howard Fendrich and Sports Writers Genaro C. Armas and Brett Martel contributed to this report.
For more NFL coverage: www.pro32.ap.org and http://twitter.com/AP-NFL
Follow Steven Wine on Twitter: http://twitter.com/Steve-Wine. His work can be found at http://bigstory.ap.org/content/steven-wine