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President Bush marked Earth Day with a lung-busting mountain bike ride high above Napa County wine country, dodging ruts that sent several of his riding partners crashing into the mud.

The president spent Saturday morning with a small pack of riders in a foggy redwood forest about 90 minutes north of San Francisco. He relished the swampy conditions on parts of the trail in this remote state-owned tract, leading his partners repeatedly through huge puddles and streams running high after weeks of heavy rain.

"I still ride the mountain bike primarily to help settle the soul and to burn off the excess energy one gets when you're living life to its fullest," Bush told an Associated Press reporter who accompanied him on the ride.

"We're able to enjoy the beauty without really disrupting the pristine nature of the place," the mud-splattered president said after the 65-minute ride. "It's a classic way for mankind to enjoy God's gift. Plus, we get some exercise."

The ride started at an elevation of about 1,700 feet above sea level and dropped steeply for several miles, prompting a bit of anxiety in the president. He is not accustomed to the sustained drops and climbs of California's mountains, having ridden mostly in the Washington, D.C., area and on his ranch near Crawford, Texas.

Bush settled into a steady rhythm on the way back up — an ascent he said pushed him to near his physical limits. His heart rate monitor-wristwatch reported he achieved a maximum heart rate of 176 beats per minute — just off his maximum, 183.

When a fellow rider spoke up from behind him, Bush said between grunts, "Can't you at least act like you're breathing hard?"

Asked at mid-climb whether he still lifts weights, Bush replied good-naturedly but pointedly, "Don't talk to me." A moment later, he answered in the affirmative.

"I don't spend a lot of time chitchatting," Bush said after the ride. "But I get great pleasure in riding with a group of people, and afterward we shoot the breeze and have fun and laugh and go about our way."

The president set the pace throughout the ride, with four Air Force men, a White House legislative aide and Secret Service agents tightly bunched behind him. Rep. Dan Lungren, R-Calif., began the ride with the president, but he quickly sensed he was not up for the rugged terrain and fast pace. He peeled off and pedaled solo.

"I'm used to riding in the flats," Lungren said later.

A long convoy of SUVs and off-road vehicles rumbled behind the group, carrying medics and security agents with machine guns.

One of the bicycle-borne agents had mapped out the route, and he was the only person who knew where to turn when the road forked. Bush, however, made plain he was in charge. "Drop back," the commander in chief ordered the agent, with a thumb over his shoulder.

The president explained later that he cherishes both the solitude and the social aspect of mountain biking.

"Generally when I ride it is the one time when I feel alone, even though I know people are behind me," he said. "I ask people a lot of times not to be in my line of vision because all I can see straight ahead is, you know, space."

Often, he said, when he is riding with his usual group near Washington he plugs headphones into his ears and cranks up his iPod, "and it's like I'm alone."