Indianapolis Colts quarterback Andrew Luck stunned the sports world Saturday when he retired from the NFL after seven seasons at the age of 29.
Luck's retirement is shocking for many reasons, including his young age. The four-time Pro Bowler was the No. 2 betting favorite to win NFL MVP, per Caesars, behind Kansas City Chiefs star Patrick Mahomes. He threw for 4,593 yards and 39 touchdowns last season.
The Colts' expectations for 2019 were built on Luck leading the offense and a rising defense led by Darius Leonard, Tyquan Lewis and Malik Hooker.
Even though Luck's decision to leave the NFL while still in the prime of his career, it's not unheard of in a sport that takes a heavy physical toll on players.
Here are some of the other NFL stars who decided to walk away from the gridiron early.
Calvin Johnson, WR, Detroit Lions (2007-15)
If there is an early-retirement Hall of Fame for teams, the Detroit Lions would be in on the first ballot.
Calvin Johnson decided to hang up his cleats at the age of 30 in March 2016. The six-time Pro Bowler was in the midst of a historic run of six consecutive 1,000-yard seasons from 2010 to 2015.
Here's an excerpt from Johnson's retirement statement:
"After much prayer, thought and discussion with loved ones, I have made the difficult decision to retire from the Lions and pro football. I have played my last game of football.
"Let me assure you that this was not an easy or hasty decision. As I stated, I, along with those closest to me, have put a lot of time, deliberation and prayer into this decision and I truly am at peace with it."
Unlike Luck's decision, which seemed to come out of nowhere, there were at least whispers Johnson could walk away from the NFL.
Adam Schefter of ESPN reported in January 2016 that Johnson informed then-Lions head coach Jim Caldwell at the end of the 2015 season he was going to retire.
Megatron did deal with injuries that forced him to sit out three games in 2013 and 2014 combined. He holds the NFL record for most receiving yards in a single season (1,964 in 2012).
Barry Sanders, RB, Detroit Lions (1989-98)
Barry Sanders announced his decision to retire from the Lions at the age of 31 on July 27, 1999, the day before training camp started. He attempted to leave with as little fanfare as possible by faxing the announcement to his hometown newspaper, the Wichita Eagle.
Commemorating the 20th anniversary of Sanders' retirement, Dave Birkett of the Detroit Free Press wrote there were hints something was going on before it was announced:
"Sanders' retirement surprised some in the front office, but it wasn't totally unexpected. (Head coach Bobby) Ross said at the time he reached out to Sanders 13 times by phone call or letter after the 1998 season and never received a response—Ross says now he does not remember reaching out that often— and Sanders' late father, William, had been publicly critical of the coach and the organization."
Speaking at an NFL UK event in November 2015, Sanders said he was "young and foolish" when he retired:
Sanders left the NFL at the height of his powers. He was one year removed from being named co-NFL MVP with Brett Favre in 1997, when he ran for 2,053 yards. His 1998 campaign wasn't anything to scoff at, with 1,491 yards on 343 attempts.
Jim Brown, RB, Cleveland Browns (1957-65)
Decades before Sanders left in the prime of his career, Jim Brown pulled off a shocker of his own by retiring after nine seasons with the Cleveland Browns.
The three-time NFL MVP was the most dominant player of his era. He led the NFL in rushing yards eight times, rushing attempts six times and rushing touchdowns five times.
Brown informed Browns owner Art Modell of his choice to retire via a telephone call from London, where he was filming the movie The Dirty Dozen.
Per The Undefeated's Ryan Cortes, Brown had grown bored with football and was using the movie to stimulate his mind.
"It was July already, and Brown was about to enter the final year of a two-year deal that paid him a little over $60,000 per year," Cortes wrote. "But if he wanted to play the next season and collect his money, he'd have to ditch the film. Browns owner Art Modell issued a statement with an ultimatum in it."
In response to Modell's ultimatum, Brown issued a letter of his own:
"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 corney 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."
More than 50 years after retiring, Brown continues to have one of the most influential activist voices in this country.
Chris Borland, LB, San Francisco 49ers (2014)
Unlike the other players on this list, Borland's choice to retire was particularly stunning because we will never know how great he could have been.
In March 2015, Borland told ESPN's Outside the Lines his decision was motivated by long-term concerns about head trauma.
"I just honestly want to do what's best for my health," he said." "From what I've researched and what I've experienced, I don't think it's worth the risk."
Sanders and Brown have already been elected into the Pro Football Hall of Fame. Johnson has a strong case to get in when he's eligible in 2021. Luck may not have enough of a resume to get elected, but he still put together five excellent seasons.
Borland, on the other hand, only scratched the surface of what he could do in the NFL as a rookie with the San Francisco 49ers in 2014. The former Wisconsin star had 107 combined tackles, four passes defensed and 12 tackles for loss in 14 games.
Pro Football Focus' Ben Stockwell named Borland the second runner-up for NFL Defensive Rookie of the Year behind Aaron Donald and Khalil Mack:
"His 54 defensive stops were topped by only six other inside linebackers (all of whom played at least 997 snaps). When you factor in his playing time his run stop percentage (21.3%) dwarfs any other inside linebacker, including Rolando McClain's work as a two-down linebacker in Dallas. His work in coverage wasn't as strong as his work against the run, though it was by no means the weakness you occasionally see from rookie linebackers."
Borland was in line to be the next great 49ers middle linebacker, taking over the spot vacated when Patrick Willis retired earlier that same offseason.