Michael Phelps owns 16 Olympic medals, 14 of them gold.

He won a record eight gold medals in these Beijing Olympics.

He might not be finished. He'll be just 27 years old at the next Olympics.

Some people are calling Phelps the greatest Olympian ever.

He's not.

The Greatest Olympian should not be determined simply by the number of medals he or she has won. For one thing, swimmers, along with gymnasts, have more opportunities to medal than athletes in other sports, because of the nature of the sport. The events are similar enough, and recovery time between them is much more manageable, allowing swimmers to compete in more races.

The stress that is placed on the body by a weight-bearing sport such as track is much greater than the stress of a non-weight bearing sport such as swimming. It's much more difficult for track athletes to win multiple races at the Olympic level. Only twice since 1956 has a man even managed to win both the 100- and 200-meter dashes in the same Olympics (Usain Bolt seems likely to be the third).

In swimming, it's fairly common to claim multiple victories. Why not? Three of the four strokes are fairly similar and use many of the same muscle groups. Phelps has won eight of his 16 medals (including seven gold) at one distance, 200 meters — the butterfly (twice), individual medley (twice), freestyle (twice), freestyle relay (twice).

The track and field equivalent would be the 200-meter freestyle dash, the 200-meter backward dash, the 200-meter sideways dash, and the 4x200 medley (frontward, backward, sideways) relay.

For a track athlete to match Phelps' feat, he'd have to run, say, the 100-, 200- and 400-meter dashes, the 110-meter high hurdles, the 400-meter intermediate hurdles, the 4x100 and 4x400 relays and contest the long jump.

That's impossible.

Ignoring the physical drain of the heats, quarterfinals and semifinals, and ignoring the lack of recovery time between races, no track athlete has that combination of talents to compete at the world-class level. For that matter, you can't even find an athlete who can win both hurdle races.

Swimmers also tend to have more longevity; track athletes usually have a shorter shelf life.

What all this adds up to is the potential for multiple medals. Consider how many swimmers are among the all-time Olympic medal winners: Phelps (16), Natalie Coughlin (12), Jenny Thompson (12), Dara Torres (12), Mark Spitz (11), Matt Biondi (11), Gary Hall Jr. (10), Franziska van Almsick (10). Of the 27 athletes who have won double-digit Olympic medals, eight are swimmers, 13 are gymnasts and two are track athletes.