The Science Behind Swimmers' Dolphin Kick

Dolphin Kick In The Olympics

Swimmer plots i i

hide captionA computer simulation shows how water flows over the swimmer's body.

Rajat Mittal/Center for Biomimetrics and Bioinspired Engineering at The George Washington University
Swimmer plots

A computer simulation shows how water flows over the swimmer's body.

Rajat Mittal/Center for Biomimetrics and Bioinspired Engineering at The George Washington University

One of the many contributing factors to Michael Phelps' success in the Olympic pool this week is his use of the dolphin kick, the undulating, wavelike motion he makes underwater.

Researchers at The George Washington University have been studying the mechanics of the kick in creatures such as fish and dolphins since 2003, and they have been applying that knowledge to human swimming.

Dr. Rajat Mittal, a professor of mechanical and aerospace engineering at GWU whose specialty is in fluid mechanics, says it seemed like a logical extension to apply his research to human swimming. In collaboration with USA Swimming, Mittal and his graduate students created models and simulations of elite swimmers to study how and why the kick works so effectively.

At a fundamental level, Mittal says, an object will experience different types of drag as it moves through the water.

Waves Create Drag

"If the body is moving on the surface, it creates waves," Mittal says. "Those waves actually create an extra component of drag. So if you can move underwater instead of on the surface of the water, you can actually eliminate this component of drag."

Mittal says swimming underwater is the fundamental secret of why the dolphin kick is so effective: The more a swimmer can swim below the surface, the more efficient he or she can be.

A swimmer can control various factors, Mittal says, including the frequency of the kick and its amplitude, or height.

But how the athlete moves his or her feet in the water is most important, Mittal says. "Our simulations and animations have shown that almost 90 percent of all the thrust — the propulsion — for the swimmers is coming from the part of the foot beyond the ankle."

Big Feet Yield Power

A flatter and bigger foot provides more thrust, and Phelps' size 14 feet certainly help him. Mittal has studied underwater video that shows Phelps is able to hyperextend his ankle beyond the point of a ballet dancer. This flexibility provides Phelps with a tremendous advantage, because a swimmer wants to whiplash his feet for maximum thrust.

Mittal and a team of graduate students obtained digital full-body scans of two elite swimmers, Gabrielle Rose and Lenny Krayzelburg, and animated the models with software (the same kind frequently used for animating characters in feature films such as Shrek).

After a two-day process of superimposing the computer model frame-by-frame onto video of the real swimmer, the animated swimmer data is put into a fluid dynamics simulation. The computations are so complex, Mittal says, they run on a supercomputer for 1.5 months.

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