Part 1: Crash Course

Late on a sweltering August afternoon, the Titans were nearly finished with two hours of training camp drudgery. In the middle of a grass field next to a warm, muddy finger of the Cumberland River near downtown Nashville, Tennessee's first-string offense and first-string defense were locked in a monotonous stalemate, already sick of facing each other, both awaiting the air horn that signals the end of practice and brings them one day closer to games that count. Fans awaiting autographs stood behind a fence, staring vapidly at the scrimmage.

Rare is the action than can enliven a day like this. Then quarterback Vince Young stuck a handoff in running back Chris Johnson's belly, and Johnson darted through a small hole into the second level of the defense. Cornerback Cortland Finnegan bounced forward to confront Johnson; they were wearing helmets and shoulder pads, but only shorts, so Finnegan could induce a play-ending whistle just by thudding Johnson with his pads and wrapping him up to a standstill. But as Finnegan squared to make contact, Johnson dipped his left shoulder inside and then burst abruptly to the right, like a minnow startled in shallow water. Not only did Finnegan whiff on the thud, but he also would have whiffed if it had been touch football. The spectators gasped.

Johnson accelerated to the outside, sprinting toward the pylon 60 yards away. Free safety Michael Griffin caught him on the angle inside the five, but Johnson straightened Griffin with a vicious stiff-arm and rolled across the goal line with Griffin dangling from his forearm.

Ten minutes later, as Johnson walked off the field, it was suggested to him that maybe Finnegan was coasting. Thud scrimmage and all. "Nope," said Johnson. "I just dismissed him." Griffin? "Nuh-uh," said Johnson. "All real." He flicked his prodigious dreads from his face and nodded for emphasis.

Johnson, 24 years old and entering his third season, is a running back at the peak of his powers. In 2009 he became only the sixth player in league history to rush for more than 2,000 yards, and he set an NFL record with 2,509 yards from scrimmage. Johnson rushes with a palpable urgency befitting a man who runs a 4.24 in the 40, a speed all but unheard of in a running back. And he's urgent away from the field as well. Despite having three years remaining on the rookie contract he signed out of East Carolina in July 2008, he had his deal restructured just before the start of training camp, reportedly quadrupling in his salary for 2010. Because Johnson knows he is also this: endangered.

The Big Back ("big" as in "Give me the ball every down") is central to NFL lore. Jim Brown was a Big Back. Walter Payton too. Eric Dickerson. Emmitt Smith. The Big Back carries the ball on nearly every running play, supporting half the offense on long, punishing drives that often conclude in the end zone—and lead the way to Canton. Yet subtly, the NFL is shifting, devaluing the position that once defined the sport.

"One day you're a real good back," says Johnson. "And the way I look at it, right now I'm better than all of them. Then all of sudden something happens to you, and you never did get paid. So why shouldn't I get paid now?"

Terrell Davis, retired in 2002

Status: banged-up

• "How am I doing, physically?" Davis says in response to a question. "Horrible." In his first four years Davis averaged more than 330 carries and 1,600 yards a season and won two Super Bowls with the Broncos. "He was off the charts in big games," says Mike Shanahan, who coached Davis in Denver and brought him to Washington this summer to work with the Redskins.