Six Tour de France wins in a row. Just how hard is that?

The man from Austin, Texas, is now without peer on this planet - at least, in cycling's toughest race.

As Lance Armstrong cruised to an unprecedented sixth victory here Sunday, he made it look easy. But by winning the 2,119-mile, three-week bike race, he triumphs in what endurance experts say is the greatest test of human strength and stamina in sports today.

By winning six in a row, he scales the heights of athletic achievement enjoyed by other giants such as Michael Jordan and Pete Sampras. But he had to work a lot harder physically to get there.

"The tour is the toughest man-to-man sporting event in the world," says Steve Bauer, a Canadian who rode the race 11 times. "There is nothing like it. It's a monster."

Compared by competitors to running 20 marathons back to back, the world's premier bicycle race is more than just a supreme physical test of endurance.

With millions of fans lining the route, hundreds of thousands of dollars in prize money at stake, obsessive media attention, and persistent allegations of cheating, mental strength is as important as muscle.

A champion "has to have the physical gift from God," says Jonathan Boyer, the first American to ride the Tour. "But he also needs the mental stamina and drive and emotional stability" to win the race.

Armstrong, who defeated cancer to come back and win the Tour six times, has stamina and drive in spades. At an age when many cycling greats start to fade, Armstrong took this year's event decisively, winning five individual stages, the most he's ever done.

The single-mindedness of his pursuit of repeated victories has astonished fellow competitors and spectators alike. "I like to control things, like to win things, like to take things to the limit," he wrote simply in his recent book "Every Second Counts."

The Tour de France takes riders to the limit, and beyond. A 120-mile ride at an average speed of 28 mph is considered an easy stage. To sort the men from the boys, the Tour goes through the Pyrenees and the Alps, up 8-percent gradients mile after grueling mile, and many riders simply can't take it: 41 of the 188 riders who began this year's race dropped out.

"The Tour pushes your body further than it has ever gone," says Mr. Boyer, who has taken his own body to extraordinary lengths: In 1985 he won the 3,120-mile 'Race Across America' in nine days, two hours, and six seconds, sleeping for a couple of hours each night.

That sounds hard, but "the Tour is so much more intense," says Boyer. The Race Across America "is long and difficult, but you don't go fast enough to build up real fatigue in your muscles." Every evening on the Tour, though, the riders spend time in the hands of masseurs, dissipating the lactic acid in their legs.

Ironman triathletes, who swim 2.4 miles, cycle 112 miles, and then run a 26.2-mile marathon, can claim remarkable endurance skills. But their event "lasts only eight hours," points out James Raia, editor of Endurance Sports News, and competitors rest for a fortnight after a contest. Other extreme niche events, such as triple Ironman triathlons, "are more tests of sleep deprivation than athletic achievements," he adds.

"At the end of the first stage ...even your eyelashes ache," Greg LeMond, the first US rider to win the Tour once recalled. "And ahead of you lie another three weeks of Tour de France."

Skeptics insist that riders can make the pace under such challenging circumstances only with the aid of illegal drugs. Armstrong has come under particular suspicion since the publication last month of a book that strongly implies he has used EPO, a drug that boosts oxygen-carrying red blood corpuscles.

Armstrong fiercely denies doping, and points out that he has never tested positive. "Sometimes I've felt that if Lance can do something to stay one step ahead of the technology [to detect doping] he is making fools of us all," says Mr. Raia. "But I also know how focused he is as an athlete."

The US rider has other advantages, not least a powerful stable of teammates on the US Postal Service squad. "Cycling is a team sport and the USPS team has completely dominated this race," Raia points out.

"Having a strong team so that he can sit in the slipstream reduces his stress a great deal," explains Jeremy Whittle, editor of procycling.com, an online magazine. "Other riders have to depend more on their own resources."

Armstrong has also benefited from his team's attention to technology to reduce drag. "He leaves nothing to chance, from his helmet to the forks on his bike," said Richard Virenque, this year's "King of the Mountains,"(the best mountain climber on the tour - the seventh year in row)on French TV Saturday. "He is light years ahead of us" in technology.

At the same time, Armstrong puts himself through a rigorous training schedule geared to the Tour. He practices key stages of the Tour, and focuses almost exclusively on this one race, while other riders take part in other stage races in Italy and Spain.

If that single-mindedness has helped Armstrong win the Tour de France six times, it detracts from his overall status in the pantheon of cycling greats, say race experts. "Lance Armstrong is the greatest Tour de France rider ever, but he is not the greatest cyclist we have ever seen," argues Mr. Whittle. "That, indisputably, was Eddy Merckx, who won throughout the year at anything he turned his hand to."

Tour champs Mr. Merckx, Bernard Hinault, Jacques Anquetil and Miguel Indurain, each won five times. "They were all great champions in their own right," says Mr. Bauer. Armstrong is "very special, but I would avoid saying he is the greatest ever."

Still, Laurent Jalabert, a former racer who now commentates the Tour for French TV, says: "Nobody who has not done it can understand the scale of his exploit."