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."
Unless Armstrong completely snapped his clavicle in half and has a gap between the pieces, Sherwin predicted he may not need surgery. 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.
"If you start too early you run the risk of re-injuring it," said Sherwin. "You need that arm to control the bike or you'll end up crashing again. So it's key that he heal before he starts riding."
Most cyclists can continue training on a stationary bike while their shoulders heal.
Unlike many other contact sports, Sherwin said cyclists face only the occasional acute injury.
"The incidence of injury is not as high as a contact sport," he said. "However, when you do get a traumatic injury because they're moving at a high rate of speed, and because they're falling from a high position on the bike, and because they're falling on a concrete, then the injuries tend to be more severe."
ABC News' Miguel Marquez contributed to this report.