By Jane Elliott
BBC News, health reporter
This weekend sees the start of the biggest annual sporting event in the world - the Tour de France - and for the very first time it will be starting in London.
'Big Mig' had huge lungs
But what makes these cyclists stand out from the man in the street - is it their years of dedicated training?
Or are famous riders, like Miguel Indurain, Lance Armstrong, Eddy Merckx, Bernard Hinault and Jacques Anquetil, born with the genes that will give them the extraordinary physique required to be a top level cyclist?
Jonathan Folland, lecturer in exercise physiology at the school of sport and exercise sciences at Loughborough University, believes cycling greats do have to thank their parents for a good set of genes - but cannot rely on nature's largesse alone.
Fantastic lung capacity
"These cyclists are phenomenal endurance athletes with remarkable physical abilities," he said.
"I believe the physiology is half-innate and half-created."
One of the major attributes needed for a successful Tour de France cyclist is a fantastic lung capacity.
Spaniard Miguel Indurain, who took five successive titles, had lungs so big they displaced his stomach, leading to his trademark paunch.
Indurain's lung capacity was eight litres, compared to an average of six litres.
Doctors also assess lung performance using a measure called Vo2 max - the highest volume of oxygen a person can consume during exercise.
In this too Indurain was exceptional - his Vo2 max levels were 88 ml/kg/min - almost double that of an untrained man or woman.
Three time winner Greg Le Mond was thought to have had a Vo2 max of 92.5 at the height of his career - one of the highest ever recorded.
And Lance Armstrong, the American who won the race seven times, had a Vo2 max of 83 ml/kg/min.
Dr Folland explained: "A Vo2 max of anything over 70 is considered elite.
"These cyclists have the ability to turn oxygen into peak energy. Miguel Indurain was phenomenal at this."
Dr Keith Prowse, chairman of the British Lung Foundation agreed that these cyclists had respiratory systems that set them apart from ordinary mortals.
"The bigger the lung capacity the better it performs, although lung capacity can vary depending on height and gender.
"Generally the taller a person is the greater their lung capacity."
Dr Prowse said great cyclists usually reached their career peak between 23-35 when their body is mature enough to cope with the endurance demands a race like this placed on them.
Dr Folland said cyclists also needed strong hearts.
Lance Armstrong's heart, like that of many other athletes, is thought to be 30% larger than average.
He had a resting heart rate of 32-34 beats per minute (the average for males is 70 and 75 for women) - a trained athlete's resting heart rate is lower because it pumps more blood per beat than an untrained person's does.
Lance Armstrong won seven Tours
It is also thought that Armstrong's muscles produced lower levels of lactic acid during exercise.
This is a by-product of the body's energy production, and it is thought that the faster it can be cleared from the system, the less fatigue will kick in, and the faster the body can recover.
Another useful attribute for an elite endurance cyclist is a good composition of muscles - a larger proportion of their muscles are made up of what is called 'slow twitch' muscle fibres.
These contract slowly, but keep going for a long time - 'fast twitch' muscle fibres contract quickly, but rapidly get tired.
This makes 'slow twitch' muscle fibres ideal for endurance sports where the ability to keep at it for hour after hour is vital.
"Another way they are phenomenal is the way in which their bodies can metabolise fat (to release energy)," said Dr Folland.
"They also have a very good immune system, which prevents their bodies becoming ill despite the rigours of training."
But he said that although many of their attributes are innate, that intensive training, involving endurance work as well as repetitions, helps to hone their physique.
"They will be training for anything up to four or five hours a day.
"It is easy to see what makes the top cyclists different from the man in the street, what is more difficult is to see what makes these cyclists different from those other elite cyclists. It is such a mixture of things." he said.