Arthur's current quest is single-minded enough that he's caught unawares when John bucks under him. He's not thrown off, but he's knocked off-balance, and his left hand automatically goes to catch him-- escaping John's attempt to grab him, only to slam elbow-first into a floor that's much closer than anticipated.

"Shit!" he says loudly, as his arm seizes up painfully for the second time in quick succession; and now he rolls off of John.