Unlike most people, he also has an extraordinarily long arm reach, with his arm span some 3 inches longer than his 6 ft 4 inch height. This effectively means he has extra longer "oars" to propel a shorter body through the water.
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Perfectly formed for the pool, he looks less assured on dry land, occasionally stumbling and knocking into things. Indeed last year, he broke his wrist trying to stop himself falling, briefly putting his Olympic adventure in jeopardy.
"He was always a big string bean and he was very awkward," his mother, Debbie, told reporters in Beijing this week.
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Did you know? When Australian swimmer Ian Thorpe said he did not think anyone could win eight races at a single Olympics, Phelps memorised the quote to motivate him for Beijing.