Jamaica's Usain Bolt crosses the finish line to win the gold in the men's 200-meter final during the 2008 Summer Olympics in Beijing. Anja Niedringhaus/AP hide caption

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Anja Niedringhaus/AP

Alexander Naddour, of the U.S. men's gymnastics team, bears the circular mark of cupping on his right arm as he prepares to compete on the pommel horse at the Rio de Janeiro Olympics on August 6. Alex Livesey/Getty Images hide caption

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Alex Livesey/Getty Images

Katie Ledecky set a new world record Friday in the women's 800-meter freestyle. In the pool, she celebrated with Team USA teammate Leah Smith. Christophe Simon/AFP/Getty Images hide caption

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Christophe Simon/AFP/Getty Images

Michael Phelps competes in the men's 200-meter individual medley final on Day 6 of the Olympic Games in Rio de Janeiro, Brazil. Phelps won the event by more than a length over swimmers from Japan and China. Mike Ehrmann/Getty Images hide caption

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Mike Ehrmann/Getty Images

Michael Phelps celebrates winning his 20th gold medal after a razor-thin victory in the men's 200-meter butterfly at Rio's Summer Olympics on Tuesday. Clive Rose/Getty Images hide caption

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Clive Rose/Getty Images

Michael Phelps waves after receiving his gold medal in the men's 200-meter butterfly at the U.S. Olympic swimming trials in Omaha on Wednesday. He described it as the hardest swim of his career. With the victory, he qualified for his fifth Olympic Games, something no other male American swimmer has ever done. Orlin Wagner/AP hide caption

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Orlin Wagner/AP

Researchers studying brains want to know what's happening in an area called the premotor cortex — the place in the brain that gears up for something the body is about to do, like swimming. Above, Michael Phelps dives off the starting blocks in the final heat of the men's 400-meter individual medley during the 2012 U.S. Olympic Swimming Team Trials in Omaha, Neb., on June 25. Jamie Squire/Getty Images hide caption

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Jamie Squire/Getty Images

How You Move Your Arm Says Something About Who You Are

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