President Bush (search) charged up punishing climbs and down steep dirt paths on his high-performance bike Monday, winding up flat on his back.

The president dusted himself off, waved his medics away and kept rolling, a small cut on his knee and dirt on his back the only signs he had wrecked. He allowed that he was a bit shaken up.

Bush's new hobby is a way to get his heart rate up and spend time outdoors without aggravating his achy knees. With an Associated Press reporter riding with him, Bush pedaled to remote corners of his 1,600-acre ranch.

Bush has been riding the knobby-tired bikes since February, and he rides with abandon.

He takes on dangerous sections that would give veterans pause. He keeps a cramp-inducing pace on long uphill sections, panting hard by the time he reaches each peak, backing off a little to recover and then attacking the next hill. He pants hard, emitting low "hrrr, hrrr, hrrr" grunts with each stroke of the pedals, his shoulders bobbing up and down.

Over an 18-mile ride that lasted an hour and 20 minutes, he burns about 1,200 calories and his heart rate reaches 168 beats per minute. That's about four times his resting rate and in the same range as Lance Armstrong's (search) when the six-time Tour de France (search) winner is pedaling hard.

"At my age, you're more concerned about the cardiovascular" benefits of a workout, the 58-year-old president said. Mountain biking, he said, has a certain "mind-clearing" effect on him, as well.

His bike is one of the best in the business: a Trek Fuel 98 made of space-age carbon fiber. The frame is adorned with high-tech components that Bush professes to know little about, including a motorcycle-style front and rear suspension that soaks up big bumps.

List price: about $3,100. He had it specially fitted by a Washington bicycle retailer.

"My right knee has finally had it," Bush said. "Running is really a painful experience for me now."

"I was looking for a different way to get outside and get exercise," Bush said. "Swimming is outside exercise, but you don't get the feeling of the wind rushing past you, nor can you swim your favorite piece of property."

Swimming does not offer countless ways to get injured either. Crashes are routine in mountain biking, and Bush has been baptized with a few wrecks.

On May 22, he lost traction on a dirt road, scraping his chin, upper lip, nose, right hand and both knees. The next day, a Secret Service (search) agent riding behind him slammed onto the ground at high speed on a paved section, breaking his collarbone and three ribs.

Bush approaches steep downhills warily.

In the moments before Monday's crash, he warns his riding party of a sharp drop and a hard left turn ahead.

"I'm gonna show you a hill that would choke a mule," he says.

He hits the brakes and is steadily advancing downhill when his front tire loses its grip amid the loose rocks. His foot gets stuck in a strap that keeps it on the pedal.

In the blink of an eye, his rear wheel is in the air, and Bush is flying high over the handlebars, landing on his back with the bike on top of him.

He lies motionless for a few moments. The reporter hoists the bike off him just as his medics arrive to attend to him.

There are trees and a drop-off nearby, and the road is littered with rocks, but Bush is uninjured.

A reflector has snapped off the bike. He leaves it as a warning marker for next time. Bush straightens out his handlebars, throws a leg over the bike and keeps rolling.

"We've got thrills, spills -- you name it," he says.

But he is tentative descending the remainder of the downhill section, dabbing a foot on the ground as he goes. Crashes often leave riders mentally rattled, and Bush acknowledges the effect.

"I was trying to make sure I didn't get going so fast, because that is a very steep left turn," Bush says.

He jokes that he was leading the "peleton," the rolling swarm of bicyclists in races like the Tour de France -- a race he watched regularly this month before Armstrong's victory Sunday.

"I was cautious of my fellow bikemen, I didn't want to cut anybody off and drive them into the canyon," Bush says with a smile. "So I slowed down and because I slowed down, I lost inertia and tumbled."

Bush loves showing off his ranch, and he takes his guests -- and the Secret Service agents who ride with him, pistols bulging through their shirts -- to rarely visited corners of it.

Monday's ride takes his entourage past the new office that contractors are close to finishing, a 2,500-square-foot structure with a stone facade and lots of windows where he says he will probably practice his convention speech next month. He slips at first, saying he will practice his inauguration speech there.

A 50-acre patch of newly turned black earth will serve as the field where Laura Bush cultivates blue stem flowers that she plans to distribute.

In one remote section, cattle stare back at him as he rides a path littered with cow dung.

Bush is here unwinding during the Democratic National Convention and before the home stretch of his re-election campaign, and he has spent the morning in meetings, some of them concerning the recommendations of the independent Sept. 11 commission.

The ride Monday is officially a politics-free zone, and Bush doesn't want to talk business. He swats away questions about what his ad man, Mark McKinnon, is doing on the ranch. He declines to talk about the Sept. 11 commission.

When the reporter points out that Democrat John Kerry (search) has a $8,000 road bicycle, Bush says, "Who?"