THE AIRBUS A300 (AMERICAN FLT 699) WAS BEING PUSHED BACK BY A RAMP CREW IN PREPARATION FOR A SCHEDULED INTL PASSENGER FLT. WITNESSES STATED THAT THEY OBSERVED THE RAMP GUIDE (WHO WAS USING THE GROUND-TO-COCKPIT MICROPHONE) WALK BEHIND THE NOSE GEAR AT LEAST ONCE BEFORE THE ACDNT OCCURRED. SUBSEQUENTLY, AS HE WAS WALKING BEHIND THE NOSE GEAR AGAIN, HE STUMBLED & THE NOSE GEAR TIRES ROLLED OVER HIS UPPER BODY, WHICH RESULTED IN CRUSHING THORACIC INJURIES. RECORDS SHOWED THAT THE RAMP GUIDE HAD RCVD SAFETY TRAINING & WAS QUALIFIED FOR THE DUTIES THAT HE WAS PERFORMING.