While Ken Shamrock hasn't called it quits, Randy Couture, Royce Gracie say what led to retirement

While Ken Shamrock hasn't called it quits, Randy Couture, Royce Gracie say what led to retirement,  If anyone can understand a retired fighter's internal struggles, it's Randy Couture.

After all, Couture set the template for the best-case scenario when a mixed martial artist returns to competition after thinking he was through. He provided one of the sport's seminal moments in 2007, when, at the age of 43, he came out of retirement and trounced Tim Sylvia to win the UFC heavyweight title.

So "The Natural" fully understands the desires which are driving his former rival, Tito Ortiz, and Stephan Bonnar, who meet in the main event of Bellator 131 on Sunday at San Diego's Valley View Casino Center. Ortiz retired in 2012 and had his return fight in May; Bonnar will have his first fight in two years.

"I know how hard that dream dies and letting go of that competitive spirit and moving on, having something else to move on to," Couture said.

While there is a wide swath of reasons why fighters return after calling it quits, from financial considerations, to an addiction to the spotlight, to simple competitive drive, the same drive which told Couture he could beat Sylvia.

"It really boils down to what's beating in your chest and whether you still have a passion for," Couture said. "I know as a fighter it was a tough decision for me to come to terms with hanging it up. I did it once, came out, doing it a second time I wanted to make it the final, I was almost 48 years old when I finally retired."

A recent event promoting Bellator 131 almost seemed an impromptu convention of UFC Hall of Famers, with five on hand, including Couture, Ortiz, Bonnar, Royce Gracie, and Ken Shamrock. The retired legends all understand what Ortiz and Bonnar have been through, especially when the two can still headline a show which has drawn quite a bit of attention.

For Gracie, the decision to retire didn't come overnight. MMA's first superstar had several long gaps in competition. He stayed on the sidelines five years after leaving the UFC in 1995. He also went three years between his second and third PRIDE appearances.

The legendary three-time UFC tournament winner's final bout was a decision win over Kazushi Sakuraba on a memorably bad K-1 Dynamite show at the Los Angeles Memorial Coliseum in 2007, a bout marred by an apathetic crowd and a DJ disrespectfully cracking jokes over the PA during the fight.

"You never say its impossible," said Gracie, who turns 48 in December, "But no, I'm done. You've got to know when to stop in this business, because that's when you get hurt. You've got to know when to stop. The body doesn't recover, even though I still run quite a bit, still train quite a bit, the next day the body is sore and I have to take a day off."

For his part, Shamrock, Gracie's foil in the early UFC days and Ortiz's chief rival during the nascent portion of the Zuffa era, last fought in 2010, an injury TKO loss on a card in South Africa.

Notably, though, the first UFC Superfight champion refuses to call himself retired.

"I can't say I'm retired because I'm not dead," Shamrock said. "That's the only day I'll call myself retired."

That said, Shamrock concedes it would take quite a set of circumstances to lure him back into the cage.

"I'm 50 years old," said Shamrock. "I have more aches and pains than most people will ever know. Not just from fighting, but also from pro wrestling. Just because that's not a ‘sport,' it's still, seven nights a week of a physical grind and it adds up fast. So to get me to go through a training camp again, to put my aches aside and go for one more fight, it would have to be the right offer at the right time with the right opponent."

That's something the 51-year-old Couture hasn't had to worry about. But he understands why the fighters like Ortiz and Bonnar continue on.

"I was fortunate to have acting and my other businesses to fall back on, so I think that competitor spirit in a lot of athletes," Couture said. "Especially unique athletes like combat sports athletes, that dies hard."