Dolphins have a kick that would make Michael Phelps jealous - 212 pounds worth.

How dolphins are able to swim so fast first preoccupied researchers back in 1936, when zoologist James Gray calculated the drag dolphins must overcome to swim faster than 20 miles an hour. Gray said dolphins lacked the muscles to swim so fast, and yet they did. This became known as Gray's Paradox.

Gray theorised that their speed possibly had something to do with their skin. Over the decades, scientists found flaws in Gray's work, and most biologists have rejected his theory.

Now a team of scientists has used sophisticated underwater video to measure the power of a dolphin's tail.

They calculate 212 pounds of thrust - more than triple what a top Olympian like Phelps can produce and enough to swim with the zip (vigour) that confounded (astonished) Gray seven decades ago.

"There is no paradox(not a strange situation). The dolphins always had the muscles to do this," said Frank Fish, professor of biology at West Chester University in Pennsylvania. "Gray was wrong."

Fish worked with Rensselaer Polytechnic Institute engineering professor Tim Wei, who uses digital video and millions of tiny bubbles to study the complex movement of water roiled (disturbed) by swimmers. Computers track the bubbles' movement, making the invisible flow of water visible. He has used the technique to help U.S. Olympic swimmers get the most from their stroke, and now on dolphins, too.

Researchers taped former Navy research dolphins swimming through bubble clouds in a tank at Long Marine Lab at the University of California Santa Cruz.

The tank was too small to capture videos of the dolphins at full speed, so they also videotaped them performing tail stands on the water. The thrust was calculated based on the dolphins' weight and measurements of the wake created by their tails. Wei presented the findings in San Antonio at an American Physical Society conference.

Harvard University biology professor George Lauder called the research helpful, saying that while few biologists still put stock in Gray's Paradox, some engineers do. "The door was already pretty well closed," Lauder said, "but no one has ever measured the thrust directly."

- AP