London Olympics | Relay races

United States’ Sanya Richards-Ross, front, leads teammates Allyson Felix, DeeDee Trotter and Francena McCorory down the track after winning gold in the women’s 4x400-meter relay. Russia won silver and Jamaica bronze.  (David J. Phillip / Associated Press)

United States runners, from left, Allyson Felix, Francena McCorory, Sanya Richards-Ross and DeeDee Trotter celebrate after winning gold in the women’s 4x400 relay.  (Wally Skalij / Los Angeles Times)

Allyson Felix takes the baton from DeeDee Trotter. Felix ran her leg in 47.8 seconds to give the U.S. team a big lead.   (Wally Skalij / Los Angeles Times)

DeeDee Trotter of the United States passes the baton to Allyson Felix.   (Wally Skalij / Los Angeles Times)

With a huge lead, Francena McCorory of the United States takes the baton from Allyson Felix. The U.S. relay team won by 3.36 seconds.   (Wally Skalij / Los Angeles Times)

Jamaica’s Usain Bolt pulls away from the United States’ Ryan Bailey to win the 4x100 men’s relay.  (Wally Skalij / Los Angeles Times)

Jamaica’s Usain Bolt celebrates the gold in the men’s 4x100 relay. The Jamaican team set a world record with a time of 36.84 seconds.  (Wally Skalij / Los Angeles Times)

Jamaica’s Usain Bolt celebrates winning gold in the men’s 4x100 relay. The United States was second and Trinidad third.   (Wally Skalij / Los Angeles Times)

Jamaica’s Usain Bolt, center, outpaces Ryan Bailey of the United States in the anchor leg of the men’s 4x100-meter relay final.  (Gabriel Bouys / AFP / Getty Images)

Usain Bolt of Jamaica celebrates on the medal stand after the men’s 4x100-meter relay in which he and his teammates set a world record.  (Kerim Okten / EPA)

Jamaicans celebrate in the Half Way Tree neighborhood of Kingston, Jamaica after Usain Bolt and teammates won the men’s 4x100-meter relay gold medal and broke the world record.  ( AFP / Getty Images)