The Presidential candidate hit up the Muhammad Ali Center in Louisville, Kentucky this week ... and decided to take a swing at one of the speed bags on display ... fun, right?!
Bernie threw a hard right and connected ... but the 77-year-old didn't realize the bag was going to swing back at his head -- and dude got ROCKED!!!
Sanders went flying backwards and both hands up in the air like Tyson Fury on the receiving end of a Deontay Wilder shot ... it didn't look good for the Senator from Vermont!