Lance Armstrong, branded a drug cheat and banned from cycling by the US Anti-Doping Agency, was back on a bike in Colorado on Saturday and loving every minute of it.

"Had a blast racing the #poweroffour this morning," Armstrong tweeted after finishing second in the Power of Four mountain bike race, a mostly local affair featuring tough climbs and descents on four peaks in the Aspen-Snowmass ski resort area.

"Got whooped up on by a kid young enough to be my son! Keegan Swirbul - remember that name!" added Armstrong, a father of five who finished second behind 16-year-old Swirbul in the race.

Armstrong, now retired from elite level cycling, was making his first public appearance since the USADA said on Friday that he would be banned for life from cycling's top pro events.

His seven Tour de France titles will also be expunged from his career record because of "numerous anti-doping rule violations, including his involvement in trafficking and administering doping products to others."

USADA said Armstrong will forfeit all titles, medals and prizes earned from August 1, 1998, which means that in addition to the seven Tour titles he earned from 1999-2005 he also stands to lose the Olympic bronze medal he won in Sydney in 2000.

The International Cycling Union (UCI) and Tour de France organizers have yet to comment officially, but USADA made it clear it believes they must honor its findings under the World Anti-Doping Code.

"Because Mr Armstrong could have had a hearing before neutral arbitrators to contest USADA's evidence and sanction and he voluntarily chose not to do so, USADA's sanction is final," the agency's statement said.

Armstrong, whose cycling exploits in the wake of his recovery from life-threatening cancer made him a global inspiration, has vehemently denied the doping accusations that have confronted him throughout his career.

Many fans leapt to his defense on Friday, not necessarily to protest his innocence but to laud his efforts in fundraising for cancer awareness and his support of those touched by the disease.

The Lance Armstrong Foundation launched in 1997 has garnered almost $500 million -- and donations through its Livestrong website were up on Friday, foundation chief executive Doug Ulman said.

Sponsors also expressed support, with firms such as apparel-maker Nike and brewer Anheuser-Busch saying they would continue their relationships with Armstrong and his foundation.

Armstrong himself looked relaxed as he set off with the rest of the weekend warriors for Saturday's race in the Colorado mountains. He said on Twitter he planned to follow up with a marathon on Sunday.

Known as a fierce fighter on the bike and off, he surprised many on Thursday night when he said he would not seek to clear himself of the official charges levied by USADA through independent arbitration.

Such a case would have allowed him to hear the evidence USADA says it has gathered and contest it in a public forum in a process that could have eventually gone to the Swiss-based Court of Arbitration for Sport.

Instead, Armstrong repeated his view that the arbitration process was loaded in favor of USADA, an agency on a 'witch hunt' against him.

USADA chief executive Travis Tygart characterized the case as a fight against a 'win-at-all-cost culture', and the agency said it had evidence from more 10 witnesses prepared to testify about their first-hand experience with Armstrong, or knowledge of those involved with him, in a doping conspiracy in his former US Postal Service cycling team.

Armstrong, who retired from top-flight cycling last year but had planned to compete in high-level triathlons from which he is also now banned, said he passed hundreds of drug tests during his career and followed the rules in place at the time of his Tour de France victories.