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American Kristin Armstrong defended her Olympic time trial crown with a storming ride.

Britain's Emma Pooley, a silver medallist in Beijing in 2008, finished sixth and Lizzie Armitstead, who won silver in Sunday's road race, 10th.

Germany's reigning world champion Judith Arndt claimed silver, while Russian Olga Zabelinskaya added a bronze to her bronze in the road race.

"I don't think I could have gone any faster," said Pooley, 29.

"You always have to push your hardest and I did. I'm really disappointed but if someone else is faster than you, you can't do anything."

Armstrong dominated, recording the quickest time through the two intermediate splits on her way to a time of 37 minutes 35 seconds over the 29km course - 15 seconds faster than Arndt.

Just 10 days shy of her 39th birthday, her performance is all the more impressive given she retired in 2009 to have a son, and returned to competition only last year.

As recently as May her chances of becoming only the second woman to defend an Olympic time trial title looked in doubt, as she broke her collarbone in a racing accident.

The former triathlete also crashed in Sunday's race, but her courage and determination are renowned in the sport and can she now call herself a double Olympic and double world champion.

At 38 years and 356 days, Armstrong becomes the oldest gold medallist in Olympic road cycling, beating by 40 days the record set by Alexandre Vinokourov in winning the men's road race on Saturday.

"When I came back, everyone asked me why in the world would I come back," Armstrong said.

"The reason I came back was because the feeling I got in Beijing, nothing could top that, but I couldn't imagine being on the top step of the podium with my son, Lucas, in my arms. I am now officially retired."

Armitstead, who was assisted by Pooley in the road race, said: "It didn't feel like a lap of honour. The whole thing was just painful.

"I'm disappointed for Emma after all the work she put in and help she gave me in the road race."

The time trial course, starting and finishing at Hampton Court Palace, was too flat and straight for 2010 world champion Pooley, who needs more corners and hills for her lighter frame and quick acceleration to come to the fore.

"I just wish we could have had Box Hill (part of the road race course) in the time trial," Pooley said.