Running hurt his knees. Swimming cramped his style. So President Bush (search) took up mountain biking, a sport that gets him outdoors, pumps up his heart rate and, every so often, sends him crashing to the ground.

Bush offered a glimpse of his new pastime to an Associated Press reporter Monday, roaming the dirt roads and far-flung pastures of his 1,600-acre ranch. About halfway through, he sailed over the handlebars during a dangerous descent, but dusted himself off, picked up his $3,100 bicycle and kept riding.

Bush, who was wearing a helmet and a mouth guard, escaped injury other than a small cut on his knee. But he conceded he was a little shaken up, riding tentatively as he descended the rest of the downhill.

Crashing is a routine part of mountain biking, a sport in which riders roll over loose dirt, rocks and other obstacles. Nevertheless, the president said, it's easier on his body than jogging, which was grinding his knees.

"This is like running except I don't feel bad afterward," he said Monday after burning about 1,200 calories over an 18-mile ride that lasted an hour and 20 minutes.

"You can cover a lot more, and you can go very fast on a bike," Bush said. Most important, he gets his heart rate up. "At my age, you're more concerned about the cardiovascular" benefits of a workout, the 58-year-old president said.

Bush has only been riding the knobby-tired mountain bikes since February, yet he takes on dangerous sections that would give veterans pause. He keeps a cramp-inducing pace on long uphill sections, pouring it on to reach 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.

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

He had it specially fitted by a Washington bicycle retailer.

"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 by you, nor can you swim your favorite piece of property."

But mountain biking is inherently dangerous, as the president has learned firsthand.

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 minutes before Monday's crash, he warned 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 said.

He hit the brakes and steadily advanced downhill when his front tire lost its grip on the loose rocks. His foot got stuck in a strap that keeps it on the pedal.

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

He lay motionless for a few moments. The reporter hoisted the bike off him just as medics arrived.

There were trees and a drop-off nearby and a road littered with rocks, but Bush was largely unscathed.

A reflector snapped off the bike and was left as a warning marker for next time. Bush straightened the handlebars, threw a leg over the bike and kept rolling.

"We've got thrills, spills — you name it," he said.

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

But out on the trail, it's officially a politics-free zone. When the reporter points out that Democratic challenger John Kerry has a $8,000 road bicycle, Bush replied: "Who?"