Barry Sanders was one of the greatest running backs ever. He and Jim Brown are the only two tailbacks in NFL history to average at least five yards per carry on 2,300-plus career rushes.

Sanders is also known for the manner in which his career ended. In 1999, coming off yet another outstanding season, he floored the NFL by announcing his retirement.

Sanders' abrupt decision to retire was shocking, but it wasn't unprecedented.

Throughout NFL history, stars have walked away from the game in their prime. Injuries forced some out. Others were concerned about their long-term health or wanted to pursue other opportunities.

In at least one case, it was a bit of both. Only days after deciding to retire, an all-time talent suffered a career-ending injury.

Whatever the cause, some players' careers ended far earlier than fans wanted. Here are the most notable examples throughout NFL history.

Barry Sanders, RB, Detroit Lions

In the summer of 1999, Barry Sanders was still one of the NFL's most dangerous tailbacks.

Although he snapped his four-year streak of seasons with 1,500-plus rushing yards (including a 2,000-yard campaign in 1997), he came up nine yards short of that lofty mark in 1998. He also sat 1,457 yards behind Walter Payton's (then) all-time rushing record of 16,726 rushing yards. 

But on a late July afternoon, Sanders sent a fax to the Wichita Eagle (via the Oklahoman) announcing his retirement: 

"Shortly after the end of last season, I felt that I probably would not return for the 1999-2000 season. I also felt that I should take as much time as possible to sort through my feelings and make sure that my feelings were backed with conviction. Today, I officially declare my departure from the NFL.

"The reason I am retiring is simple: My desire to exit the game is greater than my desire to remain in it. I have searched my heart through and through and feel comfortable with this decision."

On the cusp of becoming the most prolific rusher in NFL history, Sanders walked away. As he said in a 2012 documentary (h/t Houston Mitchell of the Los Angeles Times), the record didn't matter all that much to him.

"I understood full well who Walter Payton was, what he accomplished," Sanders said. "Not just Walter Payton, with all the guys that had tried to do what Walter did. The record for me wasn't important enough to force myself to stay around to try to get the record."

Sanders' first-ballot induction into the Hall of Fame in 2004 was a reminder that while the 1997 NFL MVP didn't catch Payton, he still accomplished plenty.

Jim Brown, RB, Cleveland Browns

In his nine NFL seasons, Jim Brown won the rushing title eight times and MVP three times, including his final season in 1965. That year, he rushed for 1,544 yards and 17 touchdowns in 14 games. 

Brown was also an aspiring actor, and as training camp neared in 1966, he was in London filming The Dirty Dozen. When production was delayed, Cleveland Browns owner Art Modell made it clear that Brown's absence would not be excused. He began fining Brown $100 for every day he was late, according to Ryan Cortes of The Undefeated.

Brown retaliated with a statement of his own. First, he penned a letter to Modell that began (via Cortes):

"I am writing to inform you that in the next few days I will be announcing my retirement from football. This decision is final and is made only because of the future that I desire for myself, my family and, if not to sound corny, my race. I am very sorry that I did not have the information to give you at some earlier date, for one of my great concerns was to try in every way to work things out so that I could play an additional year."

In 2015, he told Tim Layden of The MMQB what he thought about Modell's fines: 

"You want the real story? I had no bargaining power. But the only thing the Browns had over me was that if I wanted to keep playing football, I had to play for the Browns. But they couldn't tell me I had to play football. Art was going to fine me for every day I stayed on the movie set? I said, 'Art, what are you talking about? You can't fine me if I don't show up. S--t, I'm gone now. You opened the door.'"

When Brown retired at the age of 30, he led the NFL in career rushing yards, single-season rushing yards, rushing touchdowns and total touchdowns.

The Browns haven't won a championship since.

Calvin Johnson, WR, Detroit Lions

Over his nine NFL seasons, Detroit Lions wide receiver Calvin Johnson topped 1,000 yards seven times, caught 12 or more touchdowns four times, averaged more than 86 yards per game and was named a Pro Bowler six times.

But in March 2016, the 30-year-old suddenly retired.

"After much prayer, thought and discussion with loved ones, I have made the difficult decision to retire from the Lions and pro football," Johnson said in a statement (via the Atlanta Journal Constitution). "I have played my last game of football."

While speaking to Dave Birkett of the Detroit Free Press a few months later, Johnson said he wasn't up for the wear-and-tear on his body anymore.

"My finger's jacked," Johnson said. "I mean, I'm beat up. After you play that long, you're going to be beat up, so it's just a time where you are content with what you did, and I'm content with what I did so far."

Just like with Barry Sanders, the Lions demanded Johnson repay of some of the signing bonus from the contract extension he signed in 2012. And just like it did with Sanders for many years, that created a frosty relationship between Johnson and the Lions.

Per Birkett, Johnson has made it clear what it will take to mend fences.

"They already know what they got to do," Johnson said. "The only way they're going to get me back is they put that money back in my pocket. Nah, you don't do that. I don't care what they say. They can put it back, then they can have me back. That's the bottom line."

If Johnson suited up today, he'd still be one of the NFL's 10 best receivers.

Robert Smith, RB, Minnesota Vikings

Over his first four years in the league, Minnesota Vikings tailback Robert Smith never had more than 700 rushing yards in a season. The second four were a much different story.

In 1997, Smith exploded for 1,266 yards on the ground and 5.5 yards per carry. It marked the first of four straight 1,000-yard seasons that culminated in 2000, when Smith led the NFC with 1,521 rushing yards and again gained more than five yards per carry.

At the age of 28, he appeared to be both in his prime as a runner and arguably the best running back in the NFC if not the entire NFL. But after his eighth season, he announced his retirement in a "brief statement" in the Cleveland Plain Dealer, according to the Associated Press (via ESPN).

Smith's agent told the AP that the two-time Pro Bowler "could easily play five more years without jeopardizing his health," but Smith wasn't so sure. Multiple surgeries were a big factor in his decision to quit football before his 30th birthday, as he told ESPN's Kevin Seifert in 2015.

"I've got a five-year-old and a three-year-old, and I can run with them and chase them without a problem," Smith told Seifert. "That's what I wanted to be able to do. It has been a blessing to have good health."

Life after football has had its bumps and bruises as well. Smith has been upfront in recent years about his alcohol addiction. But he's also carved out an accomplished and successful career as a broadcaster.

Patrick Willis, LB, San Francisco 49ers

In his NFL debut, San Francisco 49ers linebacker Patrick Willis piled up 11 total tackles and a forced fumble. By the end of his rookie season, the 11th overall pick from the 2007 draft had amassed a league-high 174 tackles, was named the Defensive Rookie of the Year and earned both Pro Bowl and first-team All-Pro honors.

Over the first seven seasons of his career, Willis topped 100 tackles six times and hit the 120-tackle mark five times. He was named a Pro Bowler each of those years and a first-team All-Pro five times.

Willis missed only six games across his first seven seasons, but a toe injury caused him to miss 10 in 2014. There was no reason to believe the 30-year-old wouldn't rebound the following year, however.

That is, until he stunned the league by announcing his retirement in March 2015.

Last year, Willis told the 49ers Insider Podcast (h/t Matt Maiocco of NBC Sports Bay Area) that he has no regrets about walking away from the game while he was still relatively healthy.

"For me, I felt like it was the perfect time—my body, everything. The stars aligned for me. I never set out to play this game for anyone else's expectations or what they thought I should do and how I should do it. I believed in myself before anyone else saw it. I never put that in anybody else's hands. So for me, it was the right time."

Willis wasn't the only surprise retirement that rocked the Niners that year, A week later, promising young linebacker Chris Borland announced his retirement after one season, citing concerns about head trauma.

Tiki Barber, RB, New York Giants

For a good portion of his NFL career, Tiki Barber was known more for fumbling and being a twin than for being a high-end tailback.

Barber gained only 250 rushing yards over his first three seasons combined. From 2000 to 2003, he gained 1,000 rushing yards in a season three times, but he also had a staggering 35 fumbles.

In 2004, the New York Giants hired a new head coach in Tom Coughlin, who threatened to bench Barber if he didn't stop fumbling. Sure enough, Barber started toting the rock differently and took off like a rocket.

From 2004 to 2006, Barber peeled off three straight seasons with more than 1,500 rushing yards, including an 1,860-yard campaign in 2005. He topped 2,000 total yards in all three of those seasons and made three straight Pro Bowls.

But by October 2006, Barber had already made it clear that season would be his last. He told the Talk of Fame Network in 2018 that an early-season game against the rival Eagles sealed the deal.

"To put it into clarity, my first game against Philadelphia (in) my last season, we played at Philly. It was at the new 'Linc' (Lincoln Financial Field), I got the crap beat out of me by Jeremiah Trotter, and I walked out that game saying, 'I'm done.'

"It was like the second or third week of the season (it was the second), and I knew I was done because I didn't feel anything anymore. And I didn't want to do it anymore. I told my fullback, 'Finny' (Jim Finn), and he was like, 'What the hell are you talking about?' I said, 'Finny,' I just don't feel it anymore. Don't get me wrong. I'm going to have a fantastic season, but I'm done.'"

Barber's timing wasn't ideal. In his first year of retirement, the Giants shocked the undefeated New England Patriots in Super Bowl XLII. His broadcasting career fizzled quickly, and his comeback attempt in 2011 never got off the ground.

The Giants won the Super Bowl that year as well, which feels like rubbing it in.

Earl Campbell, RB, Houston Oilers

Earl Campbell barreled his way into the NFL the same way he plowed into the line of scrimmage: so hard that folks didn't know what hit them.

The first overall pick in the 1978 draft led the NFL with 1,450 rushing yards as a rookie and was named both the Offensive Rookie of the Year and Offensive Player of the Year. The following season, Campbell gained 1,697 yards and won both OPOY and Most Valuable Player. In 1980, Campbell led the league in rushing and was named OPOY again—this time with a jaw-dropping 1,934 yards on the ground.

However, Campbell's hard-charging running style took its toll. After topping 1,300 rushing yards in 1981, Campbell missed almost half the 1982 season. He rebounded with 1,301 rushing yards in 1983, but after he averaged fewer than three yards per carry over the first six games of the 1983 season, the Houston Oilers traded him to the New Orleans Saints.

Mired in a timeshare with Wayne Wilson, Campbell averaged a respectable 4.1 yards per carry in 1985. But by August 1986, the 31-year-old had had enough.

"I'm a man; I'm not a little boy," Campbell said, via the Los Angeles Times. "I believe this is the best thing—not only for myself, but for the Saints."

Failing to reach 10,000 career rushing yards didn't keep Campbell from being inducted into the Hall of Fame. And given the physical problems that have dogged him since his retirement, he perhaps should have retired even sooner.

But at the time, the news that Campbell was retiring stunned both fans and his head coach.

"(It) was a complete surprise to me," Jim Mora said, via the Times. "He was still our No. 1 tailback."

Al Toon, WR, New York Jets

When Al Toon called it a career, the three-time Pro Bowler didn't have much choice.

It didn't take Toon long to become a centerpiece of some potent New York Jets offenses of the late 80s and early 90s. In his second season, topped 1,000 receiving yards. He would do so again in 1988 while catching a career-high 93 passes.

But in each of his final six seasons, Toon missed time due to injuries. And in November 1992, Toon suffered the ninth diagnosed concussion of his career.

In 2016, he told Fox Sports' Sam Gardner that he had no regrets about walking away when he did.

"I had three kids and was happily married and wanted to make sure I was doing the right thing for my family,. So the decision wasn't difficult when I had all the information. I just felt blessed to have the opportunity to play the game for as long as I did.

"... I was disappointed that I wasn't able to make the decision based on other factors—primarily just, 'OK, I'm done with this. I've satisfied my need to play and it's time to move on.' But there was never one moment of regret."

Toon also said that despite what's now known about the long-term effects of concussions, he doesn't regret playing the game.

"It was a wonderful experience, and I probably wouldn't change it even if I had the opportunity to. It opened a lot of doors for me, I learned a lot about who I am and it gave me a leg up financially going forward. So I was extremely blessed, and I'm very appreciative of the opportunity that the Hess family gave me."

Had his career not been cut short, Toon likely would be mentioned along with Art Monk, Andre Reed and some of the other great wideouts of his era.

Bo Jackson, RB, Los Angeles Raiders

Bo Jackson wasn't just a great running back. He was a phenomenon.

Whether it was with his "Bo Knows" commercials or by single-handedly breaking a video game, Jackson captured the imagination of sports fans everywhere.

However, his NFL career came to an abrupt, tragic end.

During a playoff game against the Cincinnati Bengals in January 1991, Jackson suffered a career-ending hip injury. After four truncated seasons split between the Los Angeles Raiders and Kansas City Royals, Jackson was done with football.

But before Jackson suffered that injury, he was already planning to quit the game. In 2012, he told ESPN that the 1990 season was going to be his last regardless (h/t Levi Damien of SB Nation).

"Four days before I had the hip injury, my wife and I sat down and talked about my sports career and I was planning on announcing my retirement from football that season ... I swear to you ... I didn't lose the love for the game, I've never lost the love for the game, but it was getting hard to ... because my oldest was getting ready to start school.

"I didn't want to take him out of school in Kansas City, then take him out of school, put him in school in Los Angeles, and when the season was over, we go back to Alabama. I didn't want him being moved around and shuffled around like that.

"It was more because my family and my kids. I was willing to go and do something else and to make sure my kids didn't grow up and have the childhood that I did."

Had Jackson not gotten hurt and had instead just walked away from the NFL, every other sudden retirement featured here—from Sanders to Brown—would pale by comparison in terms of sheer shock value.

Gale Sayers, RB, Chicago Bears

For decades, Gale Sayers has been the poster boy for a promising career cut short by injury.

As a rookie in 1965, he finished second in the NFL in rushing and won Rookie of the Year. In his second season, he won the rushing title, picking up more than 1,200 yards on the ground and averaging 5.4 yards per carry.

On a per-touch basis, Sayers was having an even better year in 1968, averaging a gaudy 6.2 yards per carry. But one week after he tallied a career-high 205 rushing yards against the rival Packers, Sayers tore his ACL, MCL and meniscus in his right knee during a game against the 49ers.

When Sayers returned to the field in 1969, it was clear that he had lost a step. But he still led the league in rushing and was the only 1,000-yard rusher that season.

During the 1970 preseason, Sayers suffered an injury to his other knee. The then-27-year-old tried to play through it, but he ultimately had surgery and was ruled out for the remainder of the season.

He would play two more games and gain 38 yards on 13 carries before announcing his retirement in 1971.

That Sayers was a no-brainer inductee into the Hall of Fame is a testament to just how great he was when healthy. Former teammate Johnny Morris said Sayers was as dangerous a runner as any man who ever played, according to Mark Potash of the Chicago Sun-Times.

"I played with Gale. I covered Payton [as a sportscaster/announcer], and I've covered a lot of guys over the years. If I wanted one player for a season, I'd take Walter Payton. But if I wanted a player for one play, I'll take Gale Sayers—above every running back I've seen, whether it be Jimmy Brown or O.J. Simpson."

It's fascinating to wonder what Sayers might have been capable of in the modern era.