Not long after Miami Dolphins tight end Dustin Keller was carted off the field in Houston a year ago, struggling to manage the excruciating pain and disappointment derived from a blown-out knee, one question kept popping into his head.

"Why did that guy come right at my knees?"

Houston Texans rookie safety D.J. Swearinger had drilled Keller in the legs, just after the 6-foot-2, 255-pound tight end caught a short pass. The hit tore three of the four main ligaments in Keller's right knee and partially shredded a fourth. It also fractured his kneecap and damaged cartilage. In the locker room, a few minutes after one of the Dolphins' doctors had popped his kneecap back into place, Keller asked himself that question again:

"Why did that guy come right at my knees?"

After the game, Swearinger, then 21, had an answer at the ready:

"I'm sorry that this happened," he told reporters, "but, you know, with the rules, I had to go low. And that's something that I'm going to start doing now, you know, just to play within the rules."

The play brought to life an issue that had been largely theoretical to that point: With the NFL's aggressive crackdown against hits to the head and neck -- a response to the ongoing concussion crisis -- some players expressed concerns that defensive players would simply start to go low to avoid drawing penalties, fines and suspensions.

"When they set the rule, everyone knew what was going to happen," Cleveland's T.J. Ward told reporters shortly after his hit to the knee of New England's Rob Gronkowski in December knocked the All-Pro tight end out for the rest of the season and the playoffs. "It's pretty much inevitable, and they forced our hand with this one."

Some receivers make deals with opponents

"Outside the Lines" interviewed four safeties who have faced the wrath of the league for hits to the head: Washington's Ryan Clark and Brandon Meriweather, Cleveland's Donte Whitner, and Tennessee's Michael Griffin. All discussed trying to adjust to the changing rules while at the same time feeling increasingly hamstrung: hit high and worry about fines and suspensions, go low and worry about ending a guy's career.

Meriweather seems to be struggling to adjust. A repeat offender, he was suspended for the first two games of this season after what was deemed a helmet-to-helmet hit on Baltimore Ravens wide receiver Torrey Smith during a preseason game. In announcing his punishment, the NFL said it was Meriweather's "sixth violation of unnecessary-roughness rules relating to hits to defenseless players and impermissible use of the helmet."

AP Photo/Houston Chronicle/Brett Coomer

The hit on Dustin Keller that ended his season and put his career in jeopardy. AP Photo/Eric Gay

Speaking to "Outside the Lines" before that incident, Meriweather, Clark and Griffin each described scenarios that would have seemed unthinkable in the days of Jack Tatum or Ronnie Lott, or even more recently, John Lynch: Offensive players approaching them before games and pleading to be hit high rather than low.

"I've had a lot of guys say, 'Just hit me high, just knock me out. I don't care, as long as I'd be able to play next week, I'm perfectly fine, but don't go low,' " said Griffin, who was fined and suspended one game last season as a "repeat offender."

Meriweather and Clark both said offensive players have offered to pay their fines if they hit them high rather than low. Asked how often that has happened, Meriweather said: "All the time. All the time."

Said Keller: "Absolutely, 100 percent I'd rather be hit high . . . just like anybody else . . . you get hit high, say you get a concussion: That's tough to deal with, you may miss a game or two or something like that. But you still get to go home, walk home to your family."

Troy Vincent, the NFL's director of player operations and a former All-Pro defensive back who spent 15 years in the league, said that he hadn't heard about offensive players asking to be hit high instead of low and that he "can't speak for what discussion goes on between two opponents before a game." Asked what he thought about this happening, Vincent said: "I don't have an opinion on it."

The NFL provided data to "Outside the Lines" suggesting that the number of significant knee injuries, rather than increasing because of the rule changes, has actually declined slightly during the past three years -- from 198 in 2011 to 190 last season. The league also provided data showing 25 players fined for hits on defenseless receivers in 2013, compared to 40 in 2012 -- a reflection, the league says, that enforcement is working.

The league data also reflected a 13 percent drop in concussions, although the NFL declined to provide information about how those numbers broke down by position. The league said the number of concussions caused by helmet-to-helmet hits had dropped by 23 percent.

"There is a choice," Vincent said, "and we see many of our players, the majority of our players, are adjusting and tackling properly and doing the right thing."

Vincent said the league studies "everything. We look at every aspect of the game at every position. Low hits, high hits." But he said there is no specific discussion of rule changes to deter players from going low, largely because the NFL hasn't seen a spike in that problem despite the high-profile injuries to Keller and Gronkowski.

Griffin said the Titans' tackling drills now put attention on going low.

"We're taught even now to go for the legs, to shoot for the legs," he said. "We're taking practice dummies and a mat and we're hitting a dummy low and trying to hit and roll and different things on how to go for the legs."

Speed, angles make adjustments harder

Last year, after he suffered a broken leg on a low tackle by Ravens safety Matt Elam, Green Bay Packers wide receiver Randall Cobb suggested the creation of a hit zone for defenders from mid-thigh to upper shoulders. Keller agrees with this notion, saying defensive players should be able to find a spot so there aren't injuries like his.

"There was definitely some spots you could have hit to make it a clean hit, a good tackle, could have got a real good lick on me, too, that wouldn't have caused a season-ending injury," Keller said of Swearinger's hit on him.

Yet the defensive backs "Outside the Lines" interviewed all lamented the shrinking strike zone, both as unrealistic given the speed of the game and potentially more dangerous.

Washington safety Ryan Clark says offensive players have offered to pay any fines he would be assessed for hitting them up "high" as opposed to the legs, which could end their careers. Geoff Burke/USA TODAY Sports

Clark, who was fined $55,000 in a two-week span in 2011 but has been fine-free since, said he tries to aim for the thigh when possible but knows the knee isn't far away. And the knee can go both ways -- from a potential brutal injury to the offensive player to a knockout blow to the head of the defensive player. Clark, entering his 13th season, said the only time he was ever knocked out was when he took a knee to the helmet as he tried to make a low tackle. And he said aiming for the middle isn't necessarily a better option.

"You're opening yourself up for shoulder injuries and neck injuries because you're trying to hit guys bigger than you running full speed in their midsection, in their hips, in their thighs," Clark said. " . . . If you're going to protect offensive players in that matter so heavily, so staunchly, there's no way to protect us on the other side of it.

"It's just the way you have to play the game now, it doesn't allow you to be full speed, it doesn't allow you to be as explosive as you usually are, it doesn't allow you to hit people in places that were less harmful to you . . . I can hit you in your hips or in your ribs and hope that my force is enough to bring you down. Other than that I just have to wrap you up and tackle you, which then takes away the part [of] the intimidation in between the hashes because you know I'm going to stop my feet, grab you and drag you down. So I think it puts you at risk for injury, and it puts you at risk for being a bad football player."

Asked about Clark's comments, Vincent said: "Ryan has done a phenomenal job. He's a phenomenal tackler, he has been able to adapt. He's a perfect example of someone who has adjusted his play."

Asked again, though, to comment specifically on Clark's concern that the game was being made more dangerous for defensive players, Vincent said only that "I think Ryan should share with people how he does it, how he's successful, because he's done it, so he knows it's possible to tackle properly."

But Clark and the other defensive backs interviewed for this story emphasized the challenges of hitting a limited strike zone are compounded by how fast the game is unfolding.

"You start your initial aiming point somewhere say between the chest and the waist and by the time the guy's crouched and ducked, which all happens in probably like point-a-whole-bunch-zeros-of-one second," Clark said. "You don't have the time to do that, and what the NFL is saying though is that the onus is on the defender and the defensive player to know that, to adjust to that and you just can't. It's impossible."

Said Meriweather: "It's like playing chicken in a car. Once you get up to somebody's speed, you never know what the other person is gonna do. So, you're kind of like running into each other. If he goes the wrong way, it's going to be a head-on-head collision."

During an interview several months ago, Meriweather said: "I'm trying a little bit of everything. And hopefully, I find my knack in doing something different so I don't get suspended again."

But then came his hit on Smith and the two-game suspension that was upheld after an appeal. Meriweather said he didn't believe the hit was helmet-to-helmet, telling reporters: "I tried to aim at his numbers . . . I went in and aimed low, and I hit him with my shoulder. I did everything my coaches taught me to do, and I got the flag."

Vincent issued a statement that Meriweather "delivered a forceful blow to the head and neck area of a defenseless receiver with no attempt to wrap up or make a conventional tackle."

Players face a set schedule of fines as agreed to in the collective bargaining agreement, but heightened discipline such as suspensions is determined by the league office on a play-by-play basis and impacted by the player's history as a repeat offender.

Griffin said he believes appearances are more important to the league than anything at this point -- and that he was told as much by the NFL's competition committee. He said he was directed to attempt to use his arms in a tackle if for no other reason than to make the tackle seem less dangerous.

"[They said] it wouldn't look as violent and it would look more as if you're following the rules rather than launching yourself or whatever violent intent that it looks like it has if you just bring your arms," Griffin said.

Said Vincent: "I hope that we're not judging someone or making a call off perception or appearance. Make the call, the rules are in place, they're clear, we need to be consistent but I hope that's not taking place."

The Titans' Michael Griffin was suspended one game in 2013 for this hit on Mychal Rivera that knocked the Raiders tight end's helmet off. AP Photo/Ben Margot

Griffin also described an exchange he said he had during the appeal process:

"One thing they told me during the whole appeal hearing is sometimes you got to realize you got to just let the man catch the ball. And the question I asked them back is, 'Are you going to pay me ... because I'm getting paid by the Tennessee Titans, and you're just telling me whether or not I get to play next Sunday or how much money is coming out of my pocket. That's not going to let me keep my job by saying you have to sometimes realize you have to let people [catch the ball]."

When Griffin said he relayed the message to one of his coaches, the response was: "That's crazy."

Vincent, who is not part of the appeal process, said he found Griffin's story "very hard to believe."

Whitner said he actually has had considerable success with fighting off punishment, receiving at least a half-dozen flags for unnecessary roughness but paying "zero dollars in fines." He prides himself on tackling "the right way" at whatever cost, even if it means risking injury.

But Whitner's beef is that while the fines and suspensions can be rescinded upon review, the flags cannot.

"Fifteen yards in the National Football League, that's a big deal," Whitner said. "The way that they're calling it is really affecting wins and losses and the outcomes of football games."

To that point, the NFL provided data to "Outside the Lines" showing that of 61 penalties called for hits on defenseless receivers last season, 14 were deemed incorrect calls -- a miss rate of 23 percent. On the flip side, the league said 25 players were fined for hits on defenseless receivers, with 12 of those not flagged for penalties during a game.

Players say fans can expect more offense

So, where is this all headed for a league that once marketed itself almost exclusively around violence but is now trying to sell a more nuanced package of safe aggression? Vincent, of course, has bridged both worlds. He played alongside some of the game's biggest hitters in Brian Dawkins and Lawyer Milloy. He also has found himself in the heart of the discussions around safety, first as the president of the NFL Players Association and now in his role as a league executive.

To him, it's all just about change and evolution.

"I think we're just at a different place," he said. "I think with equipment, the way our game is viewed, I think safety measures -- with our safety committees -- we just all see the game differently."

But Clark and Meriweather say the result is not merely a game more protective of offensive players, but one that will change the look of the game entirely.

"It used to be if a guy's good in shorts and good at the [NFL] combine, you still didn't know if he's going to be a good player because the game was so physical, the game was played with pads," Clark said. "Now, that's not the truth. If you see a guy who's really good at the combine, he's probably going to be really good at football because of the way the game is changing."

Meriweather, entering his eighth season, said the defensive backs who defined the game for years wouldn't be the same in today's game.

"The name Ronnie Lott would mean nothing," he said, adding that he thinks the league ultimately will succeed in getting players to dial it back as they try to tackle in a shrinking zone. "I think eventually you'll see a lot more people playing the way the NFL likes."

What will that game look like?

"I have no idea," Meriweather said, laughing. "I have no idea. I think it will be a lot of points scored."

Simon Baumgart is a producer in ESPN's Enterprise and Investigative Unit.