London 2012 Olympics: Michael Phelps and Stars Who Must Walk out on High Note

Olympic athletes enter London's games at different junctures in their athletic careers. Some are on the downside of their physical competition, while others are just embarking on their journey.

Let's focus on the athletes who are probably gracing an Olympic stage for the final time. For these participants, this year's events take on a special meaning. 

It's their final chance to leave a lasting impression. These athletes will do their best to leave London on a high note.

Michael Phelps

Phelps needs three medals to become the all-time winningest athlete in Olympic history. He's an international icon who has dominated the past two sets of Summer Games. 

He will participate in seven events in London. He is expected to contend for a medal in all seven of them, including his two contests against fellow American Ryan Lochte.

Phelps has reached a status that few Olympic athletes do. He's produced a nearly unparalleled legacy, but he must make sure it stands alone after the London Games. He may not participate in the 2016 Rio Games, so he must take advantage of the definite opportunities.

All eyes, like always, will be on Phelps each time he takes to the pool.

Natalie Coughlin

Coughlin failed to qualify for any individual events, but she will participate in the 400-meter freestyle relay. This will be her only chance to win her 12th career Olympic medal.

She's won the American Swimmer of the Year award three times throughout her illustrious international career. According to her profile on USASwimming.org, she is the most decorated female athlete in World Championship history.

Coughlin is the Phelps of American female swimming. She's had a legendary run, but at 29 years old this could be her final run. She was disappointing in the US Olympic Swimming Trials, but she will try to bounce back in London.

No one wants to see a legend go out on a sour note. That goes for Coughlin as much as anyone.

Jeremy Wariner

Wariner won a gold medal in the 400-meter run at the 2004 Athens Games and followed that performance with a silver medal in the 2008 Beijing Games. 

Now at age 28, Wariner will try to catch lightning in a bottle one last time. He's a two-time champion, and he's definitely not someone the rest of the men's field should overlook.

Wariner isn't a major name, but he's one of Team USA's better 400 runners. He has a chance to medal, but he will certainly be in the running.

He will try to win his fifth career Olympic medal in London. If he can, his career resume will look very good in retrospect.

Allyson Felix

Felix's Olympic trial controversy with Jeneba Tarmoh adds to the intrigue surrounding her run in London, but she would be worth watching either way. She's a three-time world champion, but she's never won an individual Olympic gold.

She'd like nothing more than to remedy that situation this summer. She will be participating in the 100-meter dash as well as the 200 race. She's not expected to win gold in either, but it won't be for lack of trying. 

Felix is accomplished on the indoor and outdoor circuits. She's experienced, and she knows what's at stake. 

Expect her to race like her career depends on it two more times.