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James Cracknell keeps on setting himself astonishing physical challenges.

One of Britain's most famous athletes, James won two Olympic gold medals for rowing and six World Championship titles.

Since retiring from competitive rowing in 2006, he has set himself a series of epic endurance tests. He rowed across the Atlantic and last year took on the Marathon des Sables - seven marathons in six days in the searing heat of the Sahara Desert.

After that he set off for the United States to attempt a new world record - crossing from Los Angeles to New York in eighteen days, by running, cycling, rowing and swimming.

But while he was cycling along Route 66 in Arizona, disaster struck. He was hit from behind by the wing mirror of an oil tanker, leaving him with a fractured skull and brain damage.

He had twenty five staples put in the back of his head and was unconscious for ten days, but then began the long process of recovery. James tells Outlook's Matthew Bannister how the accident has affected his memory and his self confidence.

He also explains why it was so important to set himself another challenge - the Yukon Arctic Ultra, cycling 430 miles across northern Canada. A challenge he completed in February this year.

His personal journey is the subject of click "Unstoppable? The James Cracknell Trilogy" on Discovery Channel.

First broadcast on 29 March 2011