For all the vaunted anatomical advantages that have propelled Michael Phelps in his bid to become the greatest Olympian of all time at the Beijing Games today, one curious statistic has gone largely unremarked. Out of water, the 23-year-old American had until recently one of the most fragile bodies ever recorded of any professional swimmer.

Although an all-round athlete at school, Phelps evolved by entirely aquatic means into the Baltimore Bullet who smashed world records, bypassing the regimen of weight training and running that most swimmers adopt. The result was that his highly flexible ankles and knees might have buckled under the strain of returning to the lacrosse field.

“On land, he’s one of the weakest swimmers we’ve ever measured,” said Genadijus Sokolovas, director of physiology for USA Swimming, the national governing body of competitive swimming, when Phelps was the sensation of the Athens Olympics in 2004.

One of the benefits of Phelps’s decision to begin weight training for last year’s world championships is that he risks little injury by hoisting his 13 Olympic gold medals - more bling than any athlete in the modern era - including seven won so far in Beijing. Having equalled his compatriot Mark Spitz’s record gold tally in a single Games, set in 1972, Phelps is on course to join the Olympian immortals by achieving the so-called “great eight” in today’s 4x100m medley relay.

Massacring world records in an effortless and imperious manner, his long strokes apparently slower than those of rivals thrashing in his wake, Phelps has challenged the hyperbole of sports writers. “Phelps can manipulate water like no human since Moses,” wrote one. “Phelps does not need testing for drugs but an outboard motor,” marvelled another.

Yet on dry land he remains something of a fish out of water. “I love being in water; it’s where I belong,” he declared. His nonchalant and quietly confident manner takes on a harder edge only when he talks about competing. His disconcerting ability to screen out all distractions has obliged Bob Bowman, his coach for 12 years, to “physically shake” the swimmer on occasion to dislodge him from his trance-like reverie just before a big race.

Bowman, who communicates with Phelps at the poolside in a language of hand signals and whistles, can live with that: “That’s why I’d never let him go to a sports psychologist. You don’t want anybody messing with that. He’s had the same approach since he was very young. There’s nothing on his mind. He’s able to block everything out.” He rates Phelps as “unbelievably kind-hearted” despite being “a solitary man” who is “incredibly invested in the success of people he cares about”.

What would a shrink detect beneath the Bullet’s double swimming caps? Phelps, who lists his interests as playing video games, watching television, sleeping and hip-hop, lives alone at Ann Arbor, Michigan, where he enjoys walking his bulldog Herman. He appreciates the fact that people leave him alone, but frets at “a total lack of recognition for swimming”.

The worst thing he has ever done was to be arrested for driving under the influence of drink in Salisbury, Maryland, in 2004, for which he was sentenced to 18 months’ probation and fined $250 (£135). Phelps confessed that he had “definitely let myself down and my family down. I think I let a lot of people in the country down”.

He seldom reads anything written about him. His coach, on the other hand, reads everything. Bowman calls his charge the “motivation machine” because “everything you throw at him - good, bad, critical - he’ll use it and feed on it to just keep trying harder, getting better”.

This shark-like singularity of purpose has led some to attribute the swimmer’s success to his hydrodynamic physique. Yet at 6ft 4in Phelps is not as tall as Tom Malchow (6ft 6in), the previous world record holder in the 200m butterfly, and his famous 6ft 7in “wingspan” does not approach the 7ft 5in reach of Michael Gross, who dominated the same event for much of the 1980s.

Phelps has several in-built advantages, notably an abnormally low build-up in the blood of lactic acid, which causes tiredness. Most swimmers require 20-30 minutes to recover from a race: Phelps can bounce back in 10 minutes, his energy level boosted by a 12,000-calorie daily diet, five times the intake of a normal adult male. He demonstrated this sensationally on Wednesday by winning the 200m butterfly in record time and then leading America’s 4x200m freestyle relay team to another record. He is also double-jointed in his elbows, knees and ankles, which extends his reach.

Few doubt that Phelps owes as much to nurture as nature. He has developed a “dolphin kick” that gives him an edge in the butterfly and accounts for his lightning turns in back-stroke and freestyle. He has also learnt to swim underwater without creating bubbles that drag around his hands. “The way Michael slips through the water, it’s ghostlike,” said Cecil Colwin, an Olympic coach.

His slick swimsuit, the Speedo LZR Racer, is frowned upon by those who do not think the most naked of sports should become a showcase for equipment. Speedo now owes Phelps the $1m bonus it pledged to him if his gold medal haul tallied with the seven won by Spitz. It is just the tip of a fortune that will now shower down on a boy from a broken home.

Born on June 30, 1985, Phelps grew up in the blue-collar mill town of Towson on the fringes of Baltimore in Maryland, the youngest child of Fred, a state trooper, and Debbie, a school administrator. According to Phelps’s mother, he was a “fidgety” boy who was diagnosed with attention deficit hyperactivity disorder and was bullied at school for his beanpole appearance. One of his teachers said he “will never be able to focus on a thing in his life”.

His parents’ acrimonious divorce when he was seven traumatised the boy and his sisters, Hilary and Whitney, who sought refuge from the “bickering” at the North Baltimore Aquatics Club. Phelps found the same remedy: “I feel most at home in water. I disappear.” Blossoming as a swimmer, by the age of 10 he held a national record for his age group and the following year he came under Bowman’s tuition at the pool. His rapid improvement led to him becoming, at 15, the youngest male Olympian since 1932. However, at Sydney in 2000 he finished fifth in the 200m butterfly. Unimpressed, his Baltimore classmates were merely curious to know if he had seen any “kangaroos hopping around”.

The story changed dramatically six months later, when at 15 years and nine months, Phelps became the youngest man to set a world swimming record, in the 200m butterfly. Over the next two years he set a pattern of setting and breaking his own records.

At the 2004 Athens Games he netted a haul of six golds and two bronzes, a feat achieved by only Alexander Dityatin, a gymnast, at the 1980 Olympics in Moscow. His dedication was absolute: “At 5.30 am, when I didn’t want to get up and train on some cold, dark morning, I’d make myself snap on the light and look inside my swimming cap. ‘Athens’ was stitched on the inside.” He had followed the same routine for 365 days, year after year.

After the Games, Phelps followed Bowman to the University of Michigan, where the latter became head swimming coach. Ostensibly Phelps was studying sports marketing and management, but his focus was on training and competing for Club Wolverine, affiliated with the university, while serving as a volunteer coach with the university’s official swimming team.

He is about to become very rich. Sponsorship by the likes of Visa, Omega and Speedo account for about £5m a year, but this is small beer compared with the estimated earnings of Tiger Woods, the world’s top golfer (£50m), and the tennis star Roger Federer (£16m).

However, Phelps now stands on the threshold of great wealth, provided he spruces himself up a bit. Lacking the handsome looks of Spitz, he will have to try harder. “He’s going to have to grow into his marketability,” said David Carter of the Sports Business Institute in California. “He needs to buff up his personality and really demonstrate his charisma.”

To a decent man whose mantra is “eat, sleep and swim”, that may prove a tougher proposition than becoming the greatest swimmer the world has ever seen.