And then there are his size 14 feet and hyperflexibility that gives him a range of motion that provides tremendous propulsive efficiency. Finally, he has demonstrated, virtually from his first laps at age 7, a preternatural relationship with the water. While “feel for the water” – a natural instinct for finding just the right positions in which to slink through the water and work with it effectively – cannot be measured, Phelps just has it. His one seeming deficit is in physical strength. He has never lifted weights and has scored as one of the weakest elite swimmers ever tested by physiologists at U.S. Swimming – which speaks volumes about the relative importance of movement economy vs. power.
http://www.totalimmersion.net/2004%20articles/august/phelps.html