The Gronk Brothers: MEATHEADS UNITE ... For Running Man Challenge

The Running Man challenge has been Gronk'd -- with all 5 Gronkowski brothers coming together in a gym (where else?) ... to bust out the dance moves.

FYI -- the Running Man Challenge has taken over social media ... with stars like Kyrie Irving, Justise Winslow and scores of college hoops players dancing along to "My Boo" by Ghost Town DJ's.

Now, the Gronks have entered the mix.

The best part ... when one of the bros slips and falls during a box jump. Nice recovery though.