The soon-to-be former Manchester United boss has picked a good time to retire, completing 1,500 games as manager and going out at the top after winning yet another English league title.

Most people in the world of sport, though, don't get that chance. Their skills start to decline, or injury brings a premature end to a promising career, and the end is not nearly as glorious as was once imagined.

The examples are everywhere. Paul Gascoigne, once one of the best soccer players, saw his career descend into farce in the lower levels of Chinese and English soccer, and then into tragedy, spending his days in and out of rehab.

Or Manny Ramirez, who has hit the most post-season home runs in the history of baseball and once pulled in $22.5 million a year for his troubles, but was then suspended for 150 games for multiple violations of MLB's drug policy. He's now getting paid a pittance by a Taiwanese team called the Rhinos and last week dressed up as the Incredible Hulk on a train journey to a game.

Gascoigne and Ramirez may be extreme cases, but knowing when to call it a day helps protect an athlete's legacy. Eight-time NBA All-Star Yao Ming recognized that, after playing just five games in two years, it was time to start the next chapter.

It's a lesson that Liu Xiang would do well to learn. Liu's twin peaks came when he became Olympic champion in 2004 and world champion in 2007. After lining up at both the 2008 and 2012 Olympic Games - and then pulling out through injury - Liu has proved that he has the guts and determination to go with his undoubted talent. But there is nothing left to prove. The recent news that he will not run again this year was as depressing as it was predictable. Achilles injuries of the kind that Liu has had for more than 10 years do not get better with age.

There is pressure for Liu to continue, of course, from sponsors, race organizers and countless others, but if he limps off the big stage one more time - whether it's at the 2015 Worlds or the 2016 Olympic Games - he's in danger of tarnishing his legacy as Asia's greatest sprinter.

Every Chinese athlete since Yao has lived in his shadow, but in Liu's case there's no harm in following in Yao's footsteps.

The author is a Beijing-based freelance writer. dreyermark@gmail.com