Armstrong had already covered 97 miles of the stage and was at the front of the pack when he fell, Maertens said, adding that Armstrong knew immediately what had happened, although he had never before broken his collarbone in his 17 years as a cycling professional.
When he fell, Armstrong's first reaction to the crash was reportedly to utter the single word "sh**."
From the time he returned to racing in 1998 after his battle with testicular cancer to his retirement after the 2005 Tour de France, Armstrong was not sidelined for any significant time periods from crash-related injuries in training or competition, although he did take an occasional spill.
"Crashes are a part of what we do in training and in racing," he told Sports Illustrated in 2000. "It seems like every year, I have two big crashes. I did last year, I did this year. Like I said, it is just part of our job."
While training for the 2006 New York City Marathon Armstrong experienced some of the most taxing injuries of his athletic career.
"I'm icing my shins right now," Armstrong told USA Today in November 2006. "I've been riding the bike lately because of these nagging little injuries."
That shin splint problem was diagnosed as a stress fracture 10 days after the marathon, according to Armstrong's Web site.
Zooming down roads at speeds averaging more than 25 miles per hour, professional cyclists risk serious injury even from a simple fall, according to Dr. Sherwin Ho, an associate professor of surgery and the director of the sports medicine fellowship at the University of Chicago.
"Any fall directly onto the shoulder basically collapses your shoulder in against your body," said Sherwin.
Under normal circumstances, two body parts work in tandem to keep the shoulder in place -- the clavicle bone that keeps the shoulder joint at the right distance from the body and a ligament called the AC (acromioclavicular) joint that connects the shoulder to the clavicle.
In a fall, "something has to give, and it's either one of these two," said Sherwin. "It seems like the higher the impact, the faster they break, the more often well see the collarbone break." Sherwin said he sees more AC tears from people falling at lower speeds.
Sherwin said it is sometimes possible to tell whether someone fractured a collarbone or tore an AC joint just by looking at the person. For instance, a clavicle bone jutting out under the skin by the shoulder often indicates an AC tear.
"If you see it, that's actually the end of the clavicle sticking out," said Sherwin. On the other hand, a fractured clavicle often looks like general bruising and swelling on the center of the bone.
In either case, Sherwin said a fractured clavicle or torn AC ligament are relatively easy to treat. "It's just a lot of pain, and it's hard to use your arm," said Sherwin. "In most cases you don't need to fix it, you just need to protect it and leave it alone."
However, it is likely that Armstrong will need several weeks -- Sherwin often recommends six at minimum -- before he can go back on the road.
But Jacinto Vidarte, the press director for the Vuelta of Castilla, said he has seen athletes return from similar injuries much sooner -- around four weeks, or even earlier.