In the video announcing his retirement on Tuesday, Luke Kuechly thanked his Carolina Panthers teammates and coaches, his parents and his two brothers, his girlfriend and all the fans who supported him as he became the best middle linebacker in the N.F.L. Left out of his speech were two men who almost certainly influenced his decision.

By retiring at 28, at the apex of a career defined by consistent excellence as much as repeated concussions, Kuechly joined the former Indianapolis Colts quarterback Andrew Luck and the former New England Patriots tight end Rob Gronkowski as under-30 stars who in the last 10 months chose long-term health over short-term success.

Before those announcements, players retiring in what seemed to be their athletic primes were regarded with singular shock. Calvin Johnson of the Detroit Lions, whose nickname is Megatron, retired in 2016 at the age of 30 and forced to repay the team $1 million of his signing bonus. The San Francisco 49ers linebacker Patrick Willis, a five-time, first-team All-Pro, was the same age when he retired in early 2015. These most recent retirements, taken together, represent a landmark shift in players’ priorities. Ever so slowly, the league’s warrior culture is eroding, dismantled by scientific research, individual financial security and an increasing awareness among players that continuing in the game they love may very well damage their quality of life.

Kuechly declared his intentions during the middle of a postseason teeming with upsets, tense games and memorable plays. But the most poignant moment of these playoffs might have come in an interview room, where, after losing at Green Bay on Sunday, Seattle Seahawks running back Marshawn Lynch launched into an unprompted soliloquy advising teammates to take care of their bodies, minds and finances.

The Hall of Fame defensive end Carl Eller runs the Retired Players Association, which tries to prepare players for life after the N.F.L. He said the current generation has learned from older players who have been vocal about their health problems.

“Players are definitely getting smarter,” Eller said on Tuesday. “They are gathering information to make some tough decisions. I don’t think we had some of that information.”

“For myself, it was all fuzzy ahead of me,” Eller added. “It was one step at a time. It was all hearsay. You were going blind into the dark. But today, there’s a lot more light at the end of the tunnel.”

Unlike Luck, who when he retired in late August acknowledged how wary he was of enduring more pain and rehabilitation, Kuechly did not mention any specific injuries, or the multiple concussions he sustained over his eight-year N.F.L. career. But he has spoken about struggling to recognize the difference between what he can tolerate and what he should.

The N.F.L. glorifies playing through pain, a doctrine propagated by fans, the news media, coaches and, especially, the players themselves, who have been incentivized by the zero-sum culture.

Players never feel as fresh as they do as when training camp begins, and then they spend the next five months — if they even last that long — pummeling their bodies. Those who play through a torn knee ligament (Philip Rivers) or a broken fibula (Jack Youngblood), or elect to have part of a broken finger amputated to avoid an extended post-surgery recovery (Ronnie Lott), are applauded for sacrificing, and ignoring, their well-being for the good of the team.

Just two weekends ago, Eagles quarterback Josh McCown played the second half of a loss to Seattle after tearing his hamstring off the bone. But McCown was playing only because the starter, Carson Wentz, removed himself from the game after absorbing a helmet-to-helmet hit, a real-time decision between his health and calcified notions about postseason glory.

Kuechly, who also had shoulder problems, missed seven games because of concussions between 2015 and 2017. In one frightening episode, during a game against New Orleans in November 2016, he was carted off the field, crying, his bewildered face a portrait of anguish.

Alex Boone, a former N.F.L. offensive lineman, thought about that scene after learning of Kuechly’s retirement on Tuesday night. Boone endured a dreadful concussion of his own in 2016, and likened the sensation to “hitting the reset button on your brain.” He cried, he said, for two days straight.

Crying excessively, as well as headaches, dizziness and vomiting, are some of the symptoms of a concussion.

“If you actually had to be like, ‘I can’t take this anymore,’ I can’t imagine how much worse they get,” Boone said. “This stupid game isn’t worth your kids watching you drool over yourself.”

Boone played for San Francisco in 2015, when two linebackers at opposite stages of their career — Patrick Willis and Chris Borland, a promising 24-year-old rookie — retired within a week of each other because of concerns about their safety. The news jolted the team, but Boone said teammates accepted it.

“It’s hard to get out of the league, because it’s addicting at times,” Boone said. “But if you knew your body couldn’t handle it, after working your whole life to get here, it’s admirable.”

An exquisite tackler, Kuechly approached the position of Ray Lewis and Dick Butkus with a modern sensibility, as stout against the pass as he was against the run. He was one of only four defensive players in the 2010s to make the All-Pro first team at least five times.

“There’s only one way to play this game, since I was a little kid, is to play fast and play physical and play strong,” Kuechly said in his video, which lasted more than three minutes. “And at this point, I don’t know if I’m able to do that anymore. And that’s the part that is most difficult. I still want to play, but I don’t think it’s the right decision.”

Chris Nowinski, the co-founder and chief executive of the Concussion Legacy Foundation, interpreted Kuechly’s comments to mean that he was thinking about his brain. He added later, in an email, that he thought concerns about C.T.E., or chronic traumatic encephalopathy, the degenerative brain disease linked to repeated head hits, played a role in some of the players’ decisions to retire at a relatively early age.

C.T.E. was diagnosed in 110 of the 111 brains examined by a neuropathologist, according to a study released in 2017. Researchers at Boston University announced in a 2019 study that tackle football players doubled their risk of developing the worst forms of C.T.E. for each 5.3 years they played.

“With his history of concussions, one concern would be developing permanent symptoms,” Nowinski said of Kuechly on Tuesday. “If he’s feeling healthy, with the career he’s had, he probably feels comfortable walking away and avoiding the chance that he stays too long.”

As a player, Kuechly demoralized opponents with his anticipation skills. He could decipher plays at the line of scrimmage and then call them out to his teammates, as if he knew every possible outcome. And as he announced his retirement at age 28, it was as if Kuechly were smart enough to understand that if he continued playing, he would not like what the larger outcome would be.

Ken Belson and Bill Pennington contributed reporting.

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