CRAWFORD, Texas — While the Democrats were at their convention Monday, working to wrest his job away, President Bush was charging up punishing climbs and down steep dirt paths on his high-performance bike. At one point he went over the handlebars, landing flat on his back.

The president dusted himself off from his fall on a treacherous descent, 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.

Bush sees his new hobby as 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 mountain 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, 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 nearly four times his resting rate and in the same range as Lance Armstrong's when the six-time Tour de France winner is pedaling hard.

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

His bike is one of the best: a Trek Fuel 98 made of space-age carbon fiber with 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."

"Swimming is outside exercise, but you don't get the feeling of the wind rushing by you, nor can you swim your favorite piece of property," he said.

Swimming does not offer countless ways to get injured either. Crashes are routine in mountain biking.

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 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.