In the unmistakable rasp of a drill sergeant, special teams coach Ben Kotwica barked, “I want to hear something!”

Thus began the Redskinsfirst day in pads at training camp, with coaches demanding the audible smack of all-out collisions as players cycled through two hours of early-morning contact drills.

Among those hurling his body into the fray with total abandon Saturday was second-year running back Chris Thompson, the tiniest among them, listed at a generous 5 feet 8.

In the first special teams session, Thompson fielded a punt, secured the ball, crouched low and streaked downfield like a jet-powered water bug. Later, lining up as a running back, he grabbed a pass from quarterback Kirk Cousins, plowed through two defenders and kept on running well after everyone else had stopped.

Counting Saturday, Thompson has just 18 days of training camp left to impress first-year Redskins Coach Jay Gruden with his speed, sure hands, quickness and determination.

The Post Sports Live crew looks at the biggest story lines ahead of Redskins training camp, from the struggling secondary to keeping all of the offensive stars happy. (Post Sports Live/The Washington Post)

Far trickier is the task of convincing Gruden that he can stay healthy. With his last three seasons cut short by injury, Thompson must prove he’s sturdy enough to be relied upon as a third-down back and return specialist for a 20-week NFL season.

“At this point, I just have to go hard in practice every single day,” Thompson, 23, said after the session, oblivious to the two blood smears on his jersey. “I know I’m going to have some nicks and bruises here and there, but as long as I’m not injured, I want to practice. I can practice hurt; I can practice in a little pain. I just have to show them I can make it through training camp and, then, when those games come around, just show ’em that I can be there.”

It’s among the more competitive battles of Redskins training camp, with six running backs vying for three or four slots.

Alfred Morris, the 5-10, 224-pound bulldozer of a back, is a lock to retain starting honors. Behind him is a scramble for the third-down role with Roy Helu Jr. holding the inside track. The others in the mix: Evan Royster, the former Penn State standout who’s currently sidelined by a strained hamstring; Thompson, who struggled with return duties as a rookie and ended up placed on injured reserve to undergo surgery on a torn left shoulder muscle; and two rookies who’ve shown promise — Southern Cal’s Silas Redd and Baylor’s Lache Seastrunk.

Although two or three among them will be cut, they’re a tight-knit group. They urge one another on during blocking drills, they cheer when one among them makes a big gain and they form a circle at the end of practice, arms draped over shoulders, and pray.

“It’s like we’re brothers,” Thompson said. “We look out for each other. It’s no selfishness. We’re all helping each other out and, hopefully, making it be a hard decision for our coach.”

There’s a fresh, new vibe in the air at Redskins training camp under Gruden, players say.

On one hand, he seems like one of the guys. A boyish 47, he’s quick to laugh and moves easily among players. On the other hand, he’s a tough task-master, demanding quick tempo, tenacity and hard hits during practice.

Above all, Gruden has stressed that every job — with the exception of quarterback Robert Griffin III’s — is up for grabs. And unlike many coaches who pit their first-team offense against first-team defense, and second-team offense against second-team defense, Gruden prefers to mix it up, giving presumptive backups a chance to show their mettle against the elites. Otherwise, he recently explained, how else can he know whether he’s overlooking someone deserving?

The implicit message is one of opportunity.

And there should be plenty of it for backs in Gruden’s offense, despite the preseason fixation on the wide receiver corps. Gruden’s passing attack will lean on tight ends and running backs to give Griffin other options if Pierre Garcon and DeSean Jackson are double teamed.

“We have Jordan Reed; we have [Logan] Paulsen, but we need another back to emerge as that third down-type back, pass-catching type back,” Gruden said this week.

So far, Gruden has liked the speed and catching ability he has seen from Thompson. And the coach said he plans to add some plays for him out of the backfield in the coming days to see how he fares.

That’s all Thompson has wanted from age 6 — opportunity — when he told his stepfather he wanted to be an NFL player when he grew up.

“He just told me, ‘Son, if that’s what you want to do, we’re going to make sure you get there,’ ” Thompson said.

But his football career nearly ended his junior year at Florida State.

It was a toss play on third and two. Thompson was so focused on getting the first down that he hurled himself head-first into a safety lurking on the line of scrimmage.

“I just went head first into his chest,” Thompson said.

The impact broke a vertebra, and the pain was unspeakable.

He spent the next four months in a back brace. For the first month, he couldn’t do anything except go to class, come back home and get back in bed.

Most of his friends and relatives said he was crazy to try to play football again, having come so close to being paralyzed. His mother and stepfather kept faith.

“I’ve grown up in church all my life,” Thompson said. “It’s always, ‘God has a plan.’ They were letting me know all the time that God has a plan, that it all happened for a reason and that if I wanted to come back and play ball, I was going to come back stronger.”

Thompson returned for his senior year, only to have it cut short by a torn anterior cruciate ligament.

Nonetheless, the Redskins used a fifth-round pick to acquire him, with Coach Mike Shanahan saying had it not been for the injuries, Thompson would have been scooped up in the first or second round.

Only rarely does Thompson let his mind wander to that day when his football career does end. As quickly as he can, he brushes it off like an onrushing tackler.

“I feel like I’m here for a reason, and I feel like it’s not for me to just be here two or three years and be done,” Thompson said. “I feel like I can leave on my own terms. That’s always kind of been my mind-set.”

Dan Steinberg contributed to this report in Richmond.