Michael Phelps poses with his gold medal for the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31. He wins the 19th Olympic medal of his brilliant career, and the 15th gold (AP Photo/Matt Slocum).

United States' Michael Phelps competes in the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Mark J. Terrill).

United States' Michael Phelps, right, celebrates after winning gold in the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Mark J. Terrill).

From left, United States' Michael Phelps, United States' Conor Dwyer, United States' Ricky Berens and United States' Ryan Lochte pose with their gold medals for the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Mark J. Terrill).

United States' Michael Phelps competes in the men's 4x100-meter freestyle relay final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 29, (AP Photo/Mark J. Terrill).

United States' Michael Phelps, right, celebrates winning gold in the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31, (AP Photo/Mark J. Terrill).

United States' Michael Phelps, left, and France's Yannick Agnel speak after the men's 4 x 200-meter freestyle Relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31. The United States won gold, France silver (AP Photo/Mark J. Terrill).

Debbie Phelps, mother of United States swimmer Michael Phelps, and her daughter Hilary smile after Phelps' team won the gold medal un the men's 4 x 200-meter freestyle relay at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Lee Jin-man).

United States swim team members, from left, Ryan Lochte, Conor Dwyer, Ricky Berens and Michael Phelps walk with a U.S. flag after being awarded the gold medal for 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, July 31(AP Photo/Lee Jin-man).

United States' Michael Phelps dives in to start the last leg of the men's 4 X 200-meter freestyle relay before their gold medal win at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31, 2012 (AP Photo/Lee Jin-man).

Debbie Phelps, mother of United States' swimmer Michael Phelps, center, and his sisters Whiney, right, and Hilary, left, react as they watch swimming competitions at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Lee Jin-man).

United States' Michael Phelps reacts after receiving his gold medal for the men's 4x200-meter freestyle relay swimming final at the Aquatics Centre in the Olympic Park during the 2012 Summer Olympics in London, July 31 (AP Photo/Matt Slocum).