Sanya Richards-Ross of USA celebrates after winning in women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. Sanya Richards-Ross of USA won gold medal. (Xinhua/Liao Yujie)

Sanya Richards-Ross of USA celebrates after winning in women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. Sanya Richards-Ross of USA won gold medal. (Xinhua/Guo Yong)

Sanya Richards-Ross of USA (C) competes during women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. Sanya Richards-Ross of USA won gold medal. (Xinhua/Yang Lei)

Sanya Richards-Ross of USA (C) competes during women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. Sanya Richards-Ross of USA won gold medal. (Xinhua/Guo Yong)

Sanya Richards-Ross of USA reacts after winning in women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. Sanya Richards-Ross of USA won gold medal. (Xinhua/Liao Yujie)

DeeDee Trotter of USA (C) reacts after competing in women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. DeeDee Trotter of USA won bronze medal. (Xinhua/Liao Yujie)

DeeDee Trotter of USA is pictured before competing in women's 400m final match, at London 2012 Olympic Games in London, Britain, on August 5, 2012. DeeDee Trotter of USA won bronze medal. (Xinhua/Liao Yujie)