Members of second placed team USA Sanya Richards-Ross, Francena McCorory, Natasha Hastings and Allyson Felix pose with medals after the women's 4x400metres relay at the 15th IAAF World Championships at the National Stadium in Beijing, China, August 30, 2015. REUTERS/Damir Sagolj

Second placed U.S. team Allyson Felix, Natasha Hastings, Francena McCorory, and Sanya Richards-Ross (R-L) present their silver medal as they pose on the podium after the women's 4 x 400 metres relay event during the 15th IAAF World Championships at the National Stadium in Beijing, China, August 30, 2015. REUTERS/Kim Kyung-Hoon 

(L-R) Francena McCorory of the U.S. poses with her teammates Natasha Hastings, Sanya Richards-Ross and Allyson Felix after placing second in the women's 4 x 400 metres relay final at the 15th IAAF Championships at the National Stadium in Beijing, China August 30, 2015. REUTERS/Kai Pfaffenbach 

Second placed U.S. team (L-R) Allyson Felix, Francena McCorory, Natasha Hastings and Sanya Richards-Ross hold their national flag as they celebrate after the women's 4 x 400 metres relay final during the 15th IAAF World Championships at the National Stadium in Beijing, China, August 30, 2015. REUTERS/Damir Sagolj 

Second placed U.S. team (L-R) Allyson Felix, Francena McCorory, Natasha Hastings and Sanya Richards-Ross hold their national flag as they celebrate after the women's 4 x 400 metres relay final during the 15th IAAF World Championships at the National Stadium in Beijing, China, August 30, 2015. REUTERS/David Gray 

Allyson Felix, Francesca McCorory, Natasha Hastings and Sanya Richards-Ross from the U.S. react after placing second in the women's 4x400m event during the 15th IAAF World Championships at the National Stadium in Beijing, China August 30, 2015. REUTERS/Lucy Nicholson

Allyson Felix, Francesca McCorory, Natasha Hastings and Sanya Richards-Ross from the U.S. react after placing second in the women's 4x400m event during the 15th IAAF World Championships at the National Stadium in Beijing, China August 30, 2015. REUTERS/Lucy Nicholson

Francesca McCorory, Natasha Hastings and Sanya Richards-Ross from the U.S. react after placing second in the women's 4x400m event during the 15th IAAF World Championships at the National Stadium in Beijing, China August 30, 2015. REUTERS/Lucy Nicholson

Jessica Beard of the U.S. (R) and Anyika Onuora of Britain (3rd R) pass the baton to their teammates Sanya Richards-Ross (2nd R) and Kirsten McAslan (C) during their women's 4 x 400 metres relay heat at the 15th IAAF Championships at the National Stadium in Beijing, China August 29, 2015. REUTERS/David Gray

Sanya Richards-Ross of the U.S. (R) passes the baton to teammate Francena McCorory during the final handover of their women's 4 x 400 metres relay heat at the 15th IAAF Championships at the National Stadium in Beijing, China August 29, 2015. REUTERS/David Gray

Jessica Beard of the U.S. (R) and Anyika Onuora of Britain (2nd L) pass the baton to their teammates Sanya Richards-Ross (2nd R) and Kirsten McAslan during their women's 4 x 400 metres relay heat at the 15th IAAF Championships at the National Stadium in Beijing, China August 29, 2015. REUTERS/Lucy Nicholson 

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross during the press conference
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross during the press conference
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross during the press conference
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross during the press conference
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross poses for a photo
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross (L) and Aries Merritt (R) pose for a photo
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
USA's Sanya Richards Ross (L) and Aries Merritt (R) pose for a photo
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
(L-R) France's Renaud Lavillenie, USA's Aries Merritt, Sanya Richards Ross and New Zealand's Valerie Adams pose for a photo
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
(L-R) France's Renaud Lavillenie, USA's Aries Merritt, USA's Sanya Richards Ross and New Zealand's Valerie Adams share a joke
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
(L-R) France's Renaud Lavillenie, USA's Aries Merritt, USA's Sanya Richards Ross and New Zealand's Valerie Adams share a joke
Action Images via Reuters / Matthew Childs
Livepic

Athletics - IAAF Diamond League 2015 - Sainsbury's Anniversary Games Preview Press Conferences - Grange Tower Bridge Hotel, London - 23/7/15
(L-R) France's Renaud Lavillenie, USA's Aries Merritt, Sanya Richards Ross and New Zealand's Valerie Adams pose for a photo
Action Images via Reuters / Matthew Childs
Livepic

The hand of Sanya Richards-Ross of the U.S. is pictured at the start of the 400m women event at the IAAF Diamond League Athletissima athletics meeting at the Pontaise Stadium in Lausanne, Switzerland, July 9, 2015. REUTERS/Denis Balibouse

The U.S. 4x800 relay team stand on the medal podium with their gold medals at the IAAF World Relays Championships in Nassau, Bahamas May 3, 2015. From left are Phyllis Francis, Natasha Hastings, Sanya Richards-Ross and Francena McCorory. REUTERS/Mike Segar 

Sanya Richards-Ross, Ajee Wilson, Shannon Rowbury and Treniere Moser (L-R) of the U.S. team celebrate on a victory lap with U.S. national flags after setting a new world record in the distance medley relay race at the IAAF World Relays Championships in Nassau, Bahamas, May 2, 2015.

Treniere Moser, Shannon Rowbury, Ajee Wilson and Sanya Richards-Ross (L-R) of the U.S. team celebrate on the medals podium after setting a new world record in the distance medley relay race at the IAAF World Relays Championships in Nassau, Bahamas, May 2, 2015. REUTERS/Mike Segar 

Sanya Richards-Ross (standing on L), Treniere Moser, Ajee Wilson (standing on R) and Shannon Rowbury of the U.S. team pose with the clock after setting a new world record in the distance medley relay race at the IAAF World Relays Championships in Nassau, Bahamas, May 2, 2015. REUTERS/Mike Segar 

Sanya Richards-Ross of the U.S. celebrates after winning the women's 400 metres at the IAAF Diamond League athletics meet, also known as Memorial Van Damme, in Brussels September 5, 2014. REUTERS/Eric Vidal (BELGIUM - Tags: SPORT ATHLETICS) 

Sanya Richards-Ross of the U.S. wins the women's 400 metres event during the IAAF Diamond League athletics meet, also known as Memorial Van Damme, in Brussels September 5, 2014. REUTERS/Eric Vidal (BELGIUM - Tags: SPORT ATHLETICS) 

Sanya Richards-Ross of the U.S. reacts after winning the women's 400m event at the Paris Diamond League meeting at the Stade de France Stadium in Saint-Denis, near Paris, July 5, 2014. REUTERS/Gonzalo Fuentes (FRANCE - Tags: SPORT ATHLETICS) 

Sanya Richards-Ross of the U.S. (R) reacts after crossing the finish line of the women's 400m event at the Paris Diamond League meeting at the Stade de France Stadium in Saint-Denis, near Paris, July 5, 2014. REUTERS/Gonzalo Fuentes (FRANCE - Tags: SPORT ATHLETICS) 

Sanya Richards-Ross of the U.S. (R) celebrates on the track with her teammates after winning the women's 4x400 metres relay event during the IAAF World Relays Championships in Nassau, Bahamas, May 25, 2014. Seen at left is DeeDee Trotter and at back centre is Joanna Atkins. REUTERS/Mike Segar (BAHAMAS - Tags: SPORT ATHLETICS) 

The U.S. women's 4x400 metres relay team takes a victory lap after winning their event during the IAAF World Relays Championships in Nassau, Bahamas, May 25, 2014. Seen (L to R) are Joanna Atkins, Natasha Hastings, DeeDee Trotter and Sanya Richards-Ross. REUTERS/Mike Segar (BAHAMAS - Tags: SPORT ATHLETICS) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS TPX IMAGES OF THE DAY) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross (L) takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross (L) takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS TPX IMAGES OF THE DAY) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross (L) takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross (L) takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

U.S. President Barack Obama greets members of the 2012 U.S. Olympic and Paralympic teams, including 400 meter gold medalist Sanya Richards-Ross (C) and 800 meter freestyle swimming gold medalist Katie Ledecky (bottom R) during a reception at the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross (L) takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

U.S. Olympic 400m sprint gold medalist Sanya Richards-Ross takes a picture with U.S. President Barack Obama as the 2012 U.S. Olympic and Paralympic teams visit the White House in Washington, September 14, 2012. REUTERS/Jonathan Ernst (UNITED STATES - Tags: POLITICS SPORT OLYMPICS) 

Jamaica's Usain Bolt poses with Sanya Richards-Ross of the U.S. after he won the men's 200m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Michael Buholzer (SWITZERLAND - Tags: SPORT ATHLETICS)

Sanya Richards-Ross of the U.S. celebrates as she won the women's 400m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Michael Buholzer (SWITZERLAND - Tags: SPORT ATHLETICS)

Sanya Richards-Ross of the U.S. celebrates as she won the women's 400m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Michael Buholzer (SWITZERLAND - Tags: SPORT ATHLETICS)

Sanya Richards-Ross of the U.S. celebrates as she won the women's 400m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Ruben Sprich (SWITZERLAND - Tags: SPORT ATHLETICS)

Sanya Richards-Ross of the U.S. crosses the finish line in front of Botswana's Amantle Montsho (L) to win the women's 400m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Ruben Sprich (SWITZERLAND - Tags: SPORT ATHLETICS)

Sanya Richards-Ross of the U.S. celebrates as she won the women's 400m race during the Weltklasse Diamond League meeting in Zurich August 30, 2012. REUTERS/Ruben Sprich (SWITZERLAND - Tags: SPORT ATHLETICS)

U.S. relay team (L-R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross stand on the podium with their gold medals for the women's 4x400m relay at the victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT OLYMPICS SPORT ATHLETICS)

U.S. relay team (L-R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross, who won gold medals, react on the podium during the women's 4x400m relay victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Gary Hershorn (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

U.S. relay team (L-R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross, who won gold medals, stand on the podium during the women's 4x400m relay victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT OLYMPICS SPORT ATHLETICS)

(L to R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross of the U.S. wave with their gold medals from the women's 4x400m relay during the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Gary Hershorn (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

U.S. relay team (L-R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross, who won gold medals, listen to the national anthem during the women's 4x400m relay victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT OLYMPICS SPORT ATHLETICS)

U.S. relay team (L-R) DeeDee Trotter, Allyson Felix, Francena McCorory and Sanya Richards-Ross, who won gold medals, react during the women's 4x400m relay victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT OLYMPICS SPORT ATHLETICS)

(From L to R) Allyson Felix, Sanya Richards-Ross, DeeDee Trotter and Francena McCorory of the U.S. pose for photos after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Kai Pfaffenbach (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

(From L to R) Sanya Richards-Ross, DeeDee Trotter, Allyson Felix and Francena McCorory of the U.S. hold their national flags after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

(From L to R) Sanya Richards-Ross, Francena McCorory, Allyson Felix and DeeDee Trotter of the U.S. hold their national flags after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Lucy Nicholson (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

Gold medal winners (L-R) Sanya Richards-Ross, Francena McCorory, Allyson Felix, and DeeDee Trotter of the U.S hold their medals during the women's 4x400m relay victory ceremony at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Eddie Keogh (BRITAIN - Tags: SPORT OLYMPICS SPORT ATHLETICS)

(From L to R) Sanya Richards-Ross, Francena McCorory, Allyson Felix and DeeDee Trotter of the U.S. hold their national flags after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Lucy Nicholson (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

(From L to R) Allyson Felix, Sanya Richards-Ross, DeeDee Trotter and Francena McCorory of the U.S. pose for photos after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Kai Pfaffenbach (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)

(From L to R) Francena McCorory, Allyson Felix, DeeDee Trotter and Sanya Richards-Ross of the U.S. hold their national flags as they pose for photos after they won the women's 4x400m relay final at the London 2012 Olympic Games at the Olympic Stadium August 11, 2012. REUTERS/Kai Pfaffenbach (BRITAIN - Tags: SPORT ATHLETICS OLYMPICS)