Michael Phelps

U.S. swimmer Michael Phelps competes in the men's 4x100m medley relay final during the swimming event at the London 2012 Olympic Games on August 4, 2012 in London.

U.S. swimmer Michael Phelps poses on the podium with the gold medal after winning the men's 4x100 medley relay final at the London 2012 Olympic Games on August 4, 2012 in London. It marks Phelps' 18th gold medal, a new record.

From left: U.S. swimmers Matthews Grevers, Brendan Hansen, Michael Phelps and Adrian Nathan pose on the podium with the gold medal after winning the men's 4x100 medley relay final during the swimming event at the London 2012 Olympic Games on August 4, 2012 in London.

After receiving his gold medal for the men's 4x100m medley relay, Michael Phelps was called back to the podium by the governing body of swimming, FINA, to receive one more award - a silver trophy commemorating his standing as the most decorated Olympian ever, at the London 2012 Olympic Games on August 4, 2012 in London.

Michael Phelps of the United States competes in the Men?s 100m Butterfly Final on Day 7 of the London 2012 Olympic Games at the Aquatics Centre on August 3, 2012 in London, England. Phelps' victory in the butterfly gave the swimmer a record 17 Olympic gold medals.

Michael Phelps of the United States competes in the Men's 100m Butterfly heat 6 on Day 6 of the London 2012 Olympic Games at the Aquatics Centre on August 2, 2012 in London, England.

Michael Phelps of the United States competes in the Men's 200m Individual Medley on Day 5 of the London 2012 Olympic Games at the Aquatics Centre on August 1, 2012 in London, England.

Michael Phelps celebrates after winning the men's 4x200m freestyle relay final during the swimming event at the London 2012 Olympic Games on July 31, 2012 in London.

Silver medalist Michael Phelps poses on the podium after the men's 200m butterfly final during the swimming event at the London 2012 Olympic Games on July 31, 2012, in London.

Michael Phelps competes in the men's 200m butterfly final during the swimming event at the London 2012 Olympic Games on July 31, 2012, in London.

The United States' Michael Phelps celebrates after winning the men's 4x200m freestyle relay final of the swimming event at the London 2012 Olympic Games on July 31, 2012, in London.

From left to right, Conor Dwyer, Michael Phelps, Ryan Lochte, Ricky Berens poses on the podium with their gold medals after winning the men's 4x200m freestyle relay final during the swimming event at the London 2012 Olympic Games on July 31, 2012, in London.

United States' Conor Dwyer, left, Michael Phelps, center, Ryan Lochte, second right, and Ricky Berens, right, pose with their gold medals after their win in the men's 4 x 200-meter freestyle relay at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, Tuesday, July 31, 2012.

Gold Medallist Michaell Phelps of the United States poses on the podium during the medal ceremony for the Men's 4 x 200m Freestyle Relay final on Day 4 of the London 2012 Olympic Games at the Aquatics Centre on July 31, 2012, in London.

Michael Phelps poses on the podium with the gold medal after the US swimming team won the men's 4x200m freestyle relay final during the swimming event at the London 2012 Olympic Games on July 31, 2012, in London.

Michael Phelps of the USA explodes out of the water in his swimming heat in the Men's 200m Butterfly at the Olympic Games in London on July 30, 2012. — Frank Gunn ,AP Photo/The Canadian Press

Michael Phelps of the U.S. explodes out of the water in his swimming heat in the Men's 200m Butterfly at the Olympic Games in London on July 30, 2012.

Michael Phelps of the United States reacts after he competed in heat 5 of the Men's 200m Butterfly on Day 3 of the London 2012 Olympic Games at the Aquatics Center on July 30, 2012 in London, England.

U.S. swimmer Michael Phelps (left) dives in at the start of his Men's 200m butterfly heat swimming event at the London 2012 Olympic Games at the Olympic Park in London on July 30, 2012.

In a picture taken with an underwater camera U.S. swimmer Michael Phelps competes in the Men's 200m butterfly heats swimming event at the London 2012 Olympic Games at the Olympic Park in London on July 30, 2012.

Michael Phelps (front) and Nathan Adrian of the United States cheer on their teammates during the Men's 4 x 100m Freestyle Relay final on Day 2 of the London 2012 Olympic Games at the Aquatics Centre on July 29, 2012 in London, England.

American swimmer Michael Phelps reacts after the Men's 4 x 100m freestyle relay final swimming event at the London 2012 Olympic Games at the Olympic Park on July 29, 2012 in London. In an upset the French team beat the U.S. for the gold

From left: Adrian Nathan, Ryan Lochte, Cullen Jones and Michael Phelps of the United States pose with their silver medals won during the Men's 4 x 100m Freestyle Relay final on Day 2 of the London 2012 Olympic Games at the Aquatics Center on July 29, 2012 in London, England.

U.S. swimmer Michael Phelps competes in the men's 400m individual medley heats swimming event at the London 2012 Olympic Games on July 28, 2012, in London.

Michael Phelps of the United States dives off of the starting block as he competes in heat four of the men's 400m individual medley on day one of the London 2012 Olympic Games at the Aquatics Centre on July 28, 2012, in London.

U.S. swimmer Michael Phelps gives a press conference on July 26, 2012, on the eve of the London 2012 Olympic Games.

Michael Phelps smiles during a training session ahead of the London 2012 Olympic Games on July 24, 2012, in London, England.

U.S. Olympic swim team member Michael Phelps is interviewed by the media after practice at the University of Tennessee's Allan Jones Aquatic Center, July, 12, 2012, in Knoxville, Tenn.

Michael Phelps signs autographs for fans during Day Seven of the 2012 U.S. Olympic Swimming Team Trials at CenturyLink Center on July 1, 2012 in Omaha, Nebraska.

U.S. Olympic swim team member Michael Phelps practices at the University of Tennessee's Allan Jones Aquatic Center, July, 12, 2012, in Knoxville, Tenn.

Davis Tarwater, left, and Michael Phelps dive at the start of men's 100-meter butterfly semifinal at the U.S. Olympic swimming trials, June 30, 2012, in Omaha, Neb.

Michael Phelps swims to victory in the men's 100-meter butterfly final at the U.S. Olympic swimming trials, July 1, 2012, in Omaha, Neb.

Ryan Lochte, left, and Michael Phelps prepare to swim in the men's 200-meter freestyle final at the U.S. Olympic swimming trials, June 27, 2012, in Omaha, Neb.

US swimmer Michael Phelps competes in the Men's 400m Individual Medley event during the 4th meet of the Short Course Swimming World Cup in Berlin October 22, 2011. Phelps took gold.

Michael Phelps speaks to the media during a press conference to debut the new Speedo FASTSKIN 3 at Skylight Studios on November 30, 2011 in New York City.

U.S. Michael Phelps competes in his men's 200m Individual Medley heat at the FINA Swimming World Championships in Shanghai, China, July 27, 2011.

U.S. Michael Phelps competes in a men's 200m Butterfly heat at the FINA Swimming World Championships in Shanghai, China, July 26, 2011.

US swimmer Michael Phelps prepares for a men's 200 meters Butterfly heat at the FINA short course swimming World Cup in Berlin, Nov. 14, 2009.

Michael Phelps shows his gold medal — AP Photo/Andrew Medichini

Michael Phelps of the United State shows his gold medal after winning the Men's 100m Butterfly final, at the FINA Swimming World Championships in Rome, Aug. 1, 2009.

Michael Phelps competes in the 200-meter butterfly final during the Santa Clara International swim meet in Santa Clara, Calif., June 12, 2009. Phelps touched first in 1 minute, 54.37 seconds, beating the field by 5.61 seconds.

This combination of photos shows United States' Michael Phelps holding each of his eight gold medals after (top, left to right) the 400m individual medley, 4x100m freestyle relay, 200m freestyle, 200m butterfly, (bottom, left to right) 4x200m freestyle relay, 200m individual medley, 100m butterfly and the 4x100-meter medley relay at the Beijing 2008 Olympics in Beijing.

Michael Phelps celebrates winning the gold in the men's 100-meter butterfly final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 16, 2008.

Michael Phelps swims the butterfly leg of the men's 4x100-meter medley relay final to win gold during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 17, 2008.

Photographers surround Michael Phelps as he is greeted by his mother Debbie and sisters Whitney Flickinger, to the left and Hilary Phelps, after the medal ceremony for the men's 4x100-meter medley relay final, during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 17, 2008.

Michael Phelps reacts as he wins gold in the final of the men's 100-meter butterfly during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 16, 2008.

Michael Phelps swims to the gold medal in the men's 200-meter individual medley final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 15, 2008. In background is United States' Ryan Lochte, who took bronze.

Michael Phelps holds up his gold medal for the men's 200-meter individual medley final at the Beijing 2008 Olympics in Beijing, Aug. 15, 2008.

Michael Phelps swims in a men's 200-meter individual medley semifinal during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 14, 2008.

Michael Phelps celebrates with his mother, Debbie, after the men's 4x200-meter freestyle relay at the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 13, 2008. The U.S. team won the event in world record time.

The U.S. team, from left, Ricky Berens, Ryan Lochte and Michael Phelps celebrate winning the gold medal as they set a new world record in the men's 4x200-meter freestyle relay final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 13, 2008.

Michael Phelps swims on his way to winning the men's 200-meter butterfly — AP Photo/Greg Baker

Michael Phelps of the United States swims on his way to winning the men's 200-meter butterfly semifinal during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 12, 2008.

Michael Phelps of the United States, left, swims with Takeshi Matsuda of Japan during the men's 200-meter butterfly final in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 13, 2008. Phelps won the gold medal in world record time and Matsuda won the bronze.

From left, Garret Weber-Gale, Jason Lezak, Michael Phelps and Cullen Jones of the U.S. celebrate winning the gold in the men's 4x100-meter freestyle relay final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 11, 2008.

Michael Phelps and his teammate Garret Weber-Gale, right, celebrate as their team wins the gold in the men's 4x100-meter freestyle relay final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 11, 2008.

Michael Phelps swims in a men's 200-meter butterfly heat during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 11, 2008.

Michael Phelps swims on his way to winning the goal medal in the men's 400-meter individual medley final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 10, 2008.

Michael Phelps swims to a new world record and the gold medal in the final of the men's 400-meter individual medley during the swimming competitions at the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 10, 2008.

"How to Train with a T. Rex and Win 8 Gold Medals" — AP Photo/David J. Phillip

Michael Phelps laughs as he listens to the national anthem after winning the gold in the men's 400-meter individual medley final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 10, 2008.

Michael Phelps swims in a men's 400-meter individual medley heat during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 9, 2008.

Michael Phelps sticks out his tongue as he prepares for the men's 400-meter individual medley final during the swimming competitions in the National Aquatics Center at the Beijing 2008 Olympics in Beijing, Aug. 10, 2008. Phelps won the gold medal.