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A Secret Tool for the U.S. Swim Team

High-speed tracking techniques to measure fluid dynamics improve swimmers’ strokes.
August 15, 2008

Around the time that the swimwear company Speedo was calling on NASA scientists to help create the now famous LZR Racer suit–an enhanced skin that many people credit for more than a dozen world records broken by swimmers so far this week in Beijing–a scientist in New York began working on a different tool for the swimmer’s armory. Over the past five years, Tim Wei, a mechanical and aerospace engineer at Rensselaer Polytechnic Institute, has revamped an established technique in fluid dynamics to study human movement for the first time. The method allows scientists and coaches to study how fast and hard a swimmer pushes the water as she moves through it. Swim coach Sean Hutchison, who put two athletes on the Olympic swim team, says that he used Wei’s insights as the basis for every technical change he made with swimmers leading up to the Olympic trials and games this year.

Kick start: A special device built to analyze a swimmer’s thrust (triangular structure, right) can help her stroke. Here, the red vertical line shows how much force the swimmer generates as she kicks.

Wei uses a tracking technique called digital particle image velocimetry, commonly used to measure the flow of small particles around an airplane or small fish or crustaceans in water. For water-based flow experiments, researchers pour minute silver-coated beads into water and illuminate them with a laser. A high-speed digital video camera tracks the downstream flow of beads over the creature. “But ramping up to large scales is hard,” says biologist Frank Fish, who studies the propulsion of aquatic mammals at West Chester University and has collaborated with Wei on dolphin studies. “Shining lasers on swimmers and immersing them in water full of glass beads may be asking them to go above and beyond in the name of science.”

Wei devised a novel solution: instead of glass beads, he filtered compressed air in a scuba tank through a porous hose to create bubbles about a tenth of a millimeter in diameter. An athlete swims through a sheet of bubbles that rises from the pool floor, and a camera captures their flow around the swimmer’s body. Images show the direction and speed of the bubbles, which Wei then translates into the swimmer’s thrust using software that he wrote. “More force equals faster swimming,” he says.

In collaboration with Hutchison, who coaches elite athletes outside Seattle, Wei filmed Olympic gold medalist Megan Jendrick and more junior swimmer Ariana Kukors in a flume swimming breaststroke, which has a froglike kick. Jendrick’s velocity vectors signaled a fast speed, and they pointed straight out from the bottom of her feet. This meant that her feet threw water behind her, thrusting her forward, much the way that an ice skater who throws a ball will shoot herself in the opposite direction. By comparison, Kukors, a less experienced elite swimmer, had slower vectors that ran parallel to her feet, which meant that she slid through the water.

“[Hutchison] took that and modified the breaststroke kick of all his elite athletes,” says Wei, who presented his work to USA Swimming, the sport’s governing body, in 2007. In a sport in which shaving tenths of a second can be cause for celebration, Hutchison reported that by adapting her kick, Kukors dropped several seconds in a breaststroke event, although she just missed the Olympic team. Jendrick and another of Hutchison’s swimmers, Margaret Hoelzer, are competing this week at the games, where Jendrick placed fifth in the 100-meter breaststroke and Hoelzer, who won a bronze in the 100-meter backstroke, hopes to win gold in the 200 back. She broke the world record in the event in July.

More recently, Wei has turned his attention to a swimmer’s thrust. With funding from USA Swimming, Wei built a force balance, an upside-down triangular frame that acts like a bathroom scale. Swimmers lie outstretched in the water and kick into the frame, and it measures their propulsion over time. The output, which for an elite swimmer like Kukors showed a sinusoidal, repetitive wave, can help coaches determine whether an athlete should try to generate more force with a harder, bigger kick rather than a shallower, quicker one. “It depends on the individual swimmer,” says Wei, who hopes to combine flow and thrust measuring tools into one image. He also wants to make more measurements of athletes swimming freely, rather than pushing against a wall or in a flume.

Wei will meet with USA Swimming biomechanics coordinator Russell Mark in the fall to talk about what to do next. “Russell’s job is providing coaches with a sound physics base for whatever they tell swimmers to do,” Wei says. USA Swimming also relies on computer-based flow analysis using whole-body scans of swimmers; these could be combined to determine how one validates the other.

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