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Later this week, Michael Phelps swims for more gold medals. And if you watch the races, keep an eye on his feet in those underwater shots.

You see, one of the main reasons Phelps is so much faster than the competition at the start of every race and on his turns, Michael Phelps kicks like a dolphin.

NPR's Howard Berkes explains.

HOWARD BERKES: Listen to how the underwater dolphin kick made a difference at the World Championships last year. Universalsports.com called the final flip turn of the 200 freestyle race.

(Soundbite of the World Championships)

Unidentified Man: Bringing it hard. And look at that turn that Phelps got. He is well in front. He's a length and a half in front of Van Den Hoogenband. The American is going to shatter the world record.

BERKES: The race was a lot closer until that last turn and Michael Phelps came out of it underwater, undulating his outstretched body like a whip.

Rowdy Gaines is an Olympic gold medalist and swimming analyst for NBC.

Mr. ROWDY GAINES (Olympic Gold Medalist; Swimming Analyst, NBC): He is one that can hold that kick longer than anybody else, especially in that last turn when everybody's popping up the surface, and he's still underwater taking that dolphin kick. That's his most dangerous weapon.

BERKES: That has actually been the subject of scientific study by an expert in fluid dynamics, no less.

Professor RAJAT MITTAL (Mechanical and Aerospace Engineering, George Washington University): My name is Rajat Mittal. I'm a professor of mechanical and aerospace engineering at the George Washington University.

BERKES: Mittal was studying dolphins for the U.S. Navy five years ago.

Prof. MITTAL: We were asked to understand and analyze how fish swim so efficiently. And it seemed like a natural extension to apply this to human swimming.

BERKES: So Mittal and his colleagues contacted U.S.A. Swimming and learned that the group had the same interest, especially when it came to the dolphin kick.

Mr. MITTAL: What we decided to do was essentially compare these swimmers to the dolphin, assuming that the dolphin is the ultimate swimmer. And the thing that we found was that Michael is able to use his body in a way that seems to be very, very different from the other athletes, and also seems to be much closer to dolphins than we have seen for any other swimmer.

BERKES: The dolphin kick first hit Olympic swimming big-time 20 years ago, after Harvard backstroker David Berkoff figured out something fundamental.

Mr. DAVID BERKOFF (Olympic Gold Medalist): It was pretty obvious to me that kicking underwater seemed to be a lot faster than swimming on the surface.

BERKES: That's because there's turbulence and air on the surface, and they create resistance. The Berkoff Blastoff, as it was called, was used at the start and after turns, with long stretches of that underwater undulating kick.

Mr. BERKOFF: I probably wouldn't have made the Olympic team. I probably would have been a good backstroker but not a great one. It was something that really kind of changed the way backstroke was swum.

BERKES: Berkoff won four Olympic medals with the kick, which others applied to other strokes. Swimming officials eventually imposed a limit of 15 meters underwater.

Many swimmers use the kick now, but none as efficiently as Michael Phelps, in part because of his size 14 feet, suggests Rajat Mittal of George Washington University.

Prof. MITTAL: Almost 90 percent of all the thrust is coming from the foot. And the flatter, bigger your foot is, essentially, the bigger paddle you have. And that's kind of one of the reasons why the fish always beat us is because they have extremely large flukes. Michael's foot size, I'm sure, plays a big role in his ability to swim very well.

BERKES: Mittal's studies also show that Phelps undulates his feet and body more — and with more flexibility — than 50 other swimmers he's observed. Phelps also was able to hold his breath longer, says NBC swimming analyst Rowdy Gaines.

Mr. GAINES: The best swimmers in the world can't go that far. They do the dolphin kick, but they just can't go far because they've got to take a breath. And Michael's lungs are so powerful that he's been able to use that as his final weapon off that last turn.

BERKES: It's a weapon credited in part for five gold medals and five world records here in Beijing. Phelps and his dolphin kick are entered in three more Olympic events.

Howard Berkes, NPR News, Beijing.

SIEGEL: And you can see videos of the dolphin kick and Michael Phelps at npr.org.

You're listening to ALL THINGS CONSIDERED from NPR News.

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