EE AND COWORKER WERE REPLACING CATWALK BOARDS. EE WAS SITTING ON CONVEYOR #6 BELT. THIS CONVEYOR NOT LOCKED OUT, TAGGED OUT. CONVEYOR #6 WAS INADVERTENTLY ENERGIZED. AND EE WAS PULLED UP TO A BEWLT SUPPORT. THIS ACTION PULLED EE AROUND & THE SUPPORT HIT HIM IN THE BACK. STRAINING A PRE-EXISTING BACK CONDITION.