Bradley Martyn -- the famous fitness influencer known for starting ZOO Culture -- is being sued by a man who says Martyn's security got rough with him ... slamming his head into a pole for no reason.
Charles Mancusco says he attended an event at the popular Encino gym back in January for the gym's 2-year-anniversary ... an event he says was open to the public.
During the event, Mancusco says he thinks Martyn and others at ZOO Culture created a frenzy by distributing free promotional materials .