9:35 AM PT -- Brown just commented on the gym gaffe ... taking a shot at his former rival, Vontaze Burfict!
"This sh*t hit harder than Vontaze Burfict......... #MrBigChest #315Easy"
Antonio Brown's got lightning-fast reflexes ... but even that wasn't enough to keep him from getting hit in the head by an errant piece of gym equipment.
The out-of-work NFL star appeared to be working out at his 84/7 training facility in Florida -- pushing big weight on the bench press -- when another piece of gym equipment got loose and crashed down on his head.
It's unclear why the workout machine got loose -- but don't worry, AB didn't appear to be injured.
Just a reminder ... be careful in the gym!!
As for his career, Brown has been training like a maniac and recently told TMZ Sports he plans on meeting with team reps at the NFL Combine in Indianapolis when that event kicks off this weekend.