Now, a glance at the star's 2005 autobiography, Beneath The Surface, reveals that a rare disease and hyperactivity may be the reasons why Phelps is such a success in the pool.
In his late teens, already well advanced on the path to swimming greatness, Phelps attended a training session and felt his heart accelerate at an alarming pace. Bob Bowman, his coach, immediately consulted Phelps's mother, Debbie, and suggested he undergo tests.
His fear: Marfan syndrome, a disease which can lead to defects of the heart valve and aorta, and substantially reduce the life expectancy of those it afflicts.
"If you reach out your arms and form a T and your wingspan is longer than your height, you can be at risk," Smh.com.au quoted him, as stating in his book.
Phelps, indeed, displays the classic symptoms of the disease. His elongated frame is now 193cm - about six feet and four inches on the old scale. But his arm span is considerably more at 208cm.
"In my case, those measurements have always been very close. I didn't know at the time why the doctor decided to look into this. My mom and Bob didn't want me to freak out, so they told me that it was simply a good idea for young athletes to have an EKG [electrocardiogram] test in order to look at the heart," he wrote.
Phelps's measurements have prompted some to speculate that the very physical factors that have formed the fastest swimming stroke in history could be 23-year-old's fatal flaw.
"He grew unevenly," his mother once said.
"It was his ears, then he had very long arms, then he would catch up somewhere else," she added.
Tests cleared Phelps of Marfan syndrome at the time, but doctors have urged vigilance and the American star still undergoes annual check-ups for the disease.
Another bodily trait might also have helped transform Phelps into the perfect, indefatigable swimming machine. In sixth grade, the Maryland native was diagnosed with Attention Deficit and Hyperactivity Disorder and prescribed Ritalin to curb his near boundless energy.
After growing frustrated with the drowsiness caused by the drug, Phelps turned to sport, where the symptoms that so agitated teachers greatly impressed coaches.
"I had so many outlets for energy release. I'd go from a lacrosse game to a baseball game to swim practice," he wrote.