Think your day was bad? Mark Zuckerberg is working with a broken arm

Mark Zuckerberg
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You won't see a cast on his arm, but Facebook founder and CEO Mark Zuckerberg apparently has a broken arm. Not that it's stopping him from, say, running marathons whenever he wants.

"People don't believe me because I don't have a splint or a sling," Zuckerberg said Tuesday. "Apparently when you're trying to heal an elbow or a shoulder, if it's not too badly separated, the mobility actually helps it heal, more than just having it in a sling that makes it a little stiff."

The confession was part of his first-ever question-and-answer session on Facebook Live, as the company steps up its emphasis on live video broadcasting. Zuckerberg was joined by comedian Jerry Seinfeld, who dug into Zuckerberg's everyday routine using his hallmark focus on ordinary minutiae.

Zuckerberg publicly shares his yearly resolutions, which this year included a goal to run a 365 miles this year. But Seinfeld wasn't impressed — at least, until he found out about the broken arm.

"That is not a challenge," Seinfeld said. "A mile a day."

Zuckerberg responded: "I have a way of making things more intense than they should be ... It's gone too far. I've how gotten to the point where I can basically wake up in the morning and bang out a half marathon or a marathon if I want."

Since the goal was supposed to be "super easy," Zuckerberg explained that he decided to train for a triathlon instead, only to fracture his radius bone last week in a tumble "ten seconds" into a ride on his new bike. "Your last one was ten — now let's go for a 15-second bike challenge," Seinfeld quipped.

Basil Besh, a Bay Area doctor and spokesman for theAmerican Academy of Orthopaedic Surgeons, said that while he doesn't personally know the details of Zuckerberg's condition, it wouldn't be unusual to walk around without immobilization if he had a condition called a "non-displaced fracture of the radial head."

"The radius is one of the two long bones of the forearm," Besh said. "It has a smaller end at the elbow and a larger end at the wrist. Some fractures of the radial head, particularly when they are relatively minor will be treated for a few days with immobilization with either a sling or a split for comfort, at which time, early range of motion will be begun."

While not in a sling, Zuckerberg said he'll be "easing" in to his next challenge. Indeed, too much movement can cause bones to be displaced, according to the American Academy of Orthopaedic Surgeons.