As Lance Armstrong heads for a record seventh win in the Tour de France, Denis Campbell analyses the physical and emotional strengths that make him great

Soon after breakfast this morning, 161 of the world's top cyclists will mount their machines at Lezat-sur-Leze in the Pyrenees and embark on one of the toughest stages of the world's most gruelling sporting challenge, the Tour de France. They will spend the best part of seven hours in the saddle and, in draining heat, tackle 205.5km (128 miles) of hills, passes and climbs of unremitting cruelty at speeds which average 28-30mph but sometimes reach 65mph. No wonder the riders describe the Tour as three weeks of torture.

Whatever their best efforts, by tonight one man - Lance Armstrong - will almost certainly still be in the lead. Cycling's icon and figurehead is chasing an astonishing seventh consecutive win in an event which requires superhuman qualities. His tactics are simple: 'It's attack until they crack, or I do,' he says.

If he triumphs yet again when the Tour ends on the Champs-Elysees next Sunday, the American will have both rewritten the annals of sporting achievement and redefined the limits of human endeavour. Unhappily for his rivals, the 33-year-old Texan has more heart than them for the task ahead - quite literally.

'Genetically Lance is a freak,' says his friend Tony Doyle, the British former pursuit cycling world champion. 'His heart and lungs are bigger than most people's, and most other elite cyclists', so they make him more efficient as an athlete. He also generates far less lactic acid than the others, and he recovers quicker - vital in a race where you push yourself to the limit day after day after day.'

Most usefully, though, Armstrong is a survivor. 'Although the Tour is excruciatingly painful, for Lance it is still not as tough a battle as when he overcame testicular cancer,' explains Doyle. 'That means that he is mentally strong, very driven and has a lot higher pain threshold than the other guys because he's beaten a life-threatening disease. He can suffer that bit more.'

Cancer gave Armstrong the worst times of his life. Hearing he had a 60 per chance of dying. Having surgery to remove one testis. Chemotherapy to fight tumours that had spread to his lungs and brain. Five months in hospital as he defied doctors' expectations. But illness was also the making of him.

'In a strange way cancer did him a huge favour,' says Daniel Coyle, author of the recent biography Lance Armstrong: Tour de Force . 'It removed 15lbs of muscle and resculpted his body into the leaner shape. Before then he had been too big and too muscular, especially in the upper body. And it gave him the discipline that, allied to his talent, turned him into the sporting phenomenon he is today.

'He was already a world champion, but he had been an undisciplined kid: brash and uncontrollable. After he was diagnosed with cancer, he began displaying discipline at the millimetre level.'

When the doctor told him his body was riddled with tumours, Armstrong took in the news for a couple of minutes and then said: 'Let's get started. Let's kill this stuff.' Coyle says: 'He adopted a very scientific approach to fighting the cancer.' Out went coffee, red meat and dairy products. 'He went after the cancer in a very organised, methodical, aggressive way, and came away with an appreciation of what discipline could do for him.

He approaches cycling the same way. He weighs his food to ensure he does not go above his optimum weight, and constantly monitors his bodyfat ratio. 'After cancer, he turned the Tour into a problem he wanted to solve,' says Coyle.

'He pays attention to every last detail that contributes to the pursuit of excellence. That's what sets him apart,' believes Dave Brailsford, the performance director at British Cycling. 'He is always looking to make a tiny improvement to his nutrition, his position on the bike, or the science of how he trains.'

Thus he avoids ice cream, in case it causes indigestion, or carbonated water, lest it induce diarrhoea, or chocolate mousse - excessive sweating - and does not shave his legs the night before racing, in case the minimal energy required to regrow the hair makes a difference.

He has two key numbers: 74, his optimal weight in kilos, and 500, which is his maximum sustainable power in watts. If they are both right he will produce 6.7 watts per kilo. If he does that, he will almost certainly win. His key advantage is that he can sustain that power for an hour at a time, even up and down the Pyrenees. Germany's Jan Ullrich, his nearest rival, cannot quite match Armstrong's consistency.

'He is not happy until he has found "the shit" - the coolest helmet, or fastest bike or best teammate. He's about trying to make himself into "the shit", the thing nobody has seen before', says Coyle. His eight teammates in the Discovery Channel team, and their 30-strong back-up squad of specialists, all share one goal: for Armstrong to win.

His feats have inevitably prompted speculation thathis success might be down to performance-enhancing substances. Allegations to that effect in a book last year have led to ongoing legal action. Armstrong denies the accusations and has always tested negative.

Any conversation with one of cycling's cognoscenti about what makes Armstrong so good comes back to his cancer and the tough childhood which forged his iron will. He was born Lance Gunderson in Plano, Texas, in September 1971, when his mother, Linda Mooneyham, was 17. Her marriage to his father, Eddie Gunderson, broke down when he was just three. Armstrong has never spoken to, or about, his father since. 'Lance would undoubtedly not have turned out the way he has _ if I had stayed with his mother,' Gunderson admitted recently. 'I would probably have ballsed the whole thing up.'

Armstrong was named after Lance Retzel of the Dallas Cowboys gridiron team; his surname came from his mother's second husband, Terry Armstrong, his ex-stepfather. Times were hard for mother and son, financially and emotionally. 'When things got tough, I would always tell Lance that "this isn't a problem, this is an opportunity",' she has said. 'We weren't afraid of failing, only of giving up. Losing is worthwhile if you learn something worthwhile from it.'

But losing is the last thing on Armstrong's mind. Next Sunday, come what may, he intends to retire - and wants to bow out on a high, as a champion and a history-maker.

His place in posterity assured, he plans to unwind, spend more time with the three children from his first marriage, start a new family with his girlfriend, the rock singer Sheryl Crow, and devote even more energy to his cancer charity.

An adviser to George W Bush on the President's Cancer Panel, there is even talk of a career in politics, and following in Bush's steps by running for governor of his home state. Before then, though, there is one last week in the saddle - one final battle for the ultimate winner.

The Tour in figures

The Tour de France began in 1903 but was almost abandoned in 1904 because cheating was so common

There are 21 stages in 23 days over a total of 2,241 miles. The longest is 239.5km (150 miles)

Riders reach up to 70mph. At such speeds a burst tyre, skid or collision can be fatal

It is very demanding: 28 riders have dropped out already this year. Belgium's Georges Goffin began the race in 1909, 1911 and 1922 but never lasted beyond day one

The Alpe d'Huez has 21 'switchbacks', turns of at least 90 degrees

Riders often relieve themselves in the saddle to save time

Competitors consume up to 7,500 calories a day

Top riders earn about £1m a year, though Armstrong makes around £10m in race fees, prize money and sponsorship

Britain's Bradley Wiggins won a gold, silver and bronze at the 2004 Olympics but there are no Britons in the Tour de France