B777 CREW STARTED THE ENGS WITH THE PARKING BRAKES NOT SET. THE ACFT ROLLED FORWARD. A FLT ATTENDANT WAS INJURED WHEN SHE WAS KNOCKED UNCONSCIOUS AS THE BRAKES WERE SET ABRUPTLY. THE CAPT DID NOT REQUEST MEDICAL ATTN FOR THE FLT ATTENDANT UNTIL AT LEAST 5 HRS AFTER THE INCIDENT HAPPENED. SHE WAS TAKEN TO THE HOSPITAL.

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PERFORMED RECEIVING AND BEFORE START CHKLISTS IN ENTIRETY. STARTED L ENG; THEN DURING R ENG START ACFT BEGAN TO ROLL FORWARD. FO CALLED 'WE'RE MOVING.' BOTH HE AND I SIMULTANEOUSLY APPLIED BRAKES AND ACFT CAME TO AN ABRUPT STOP. CHKED WITH PURSER IF EVERYONE WAS OK. HE INFORMED ME THAT WE HAD 1 FLT ATTENDANT INJURED. WAITED ON THE RAMP FOR APPROX 5 MINS TO MAKE SURE ALL WAS WELL. THE PURSER ADVISED ME THAT FLT ATTENDANT WAS KNOCKED UNCONSCIOUS FOR APPROX 30-40 SECONDS. HE SAID THAT EVERYONE; INCLUDING INJURED FLT ATTENDANT; WAS READY TO CONTINUE ON TO DEST. I MONITORED CONDITION THROUGHOUT FLT AND SHE TOLD ME HER STOMACH FELT UPSET AND HER PELVIC AREA HURT BADLY. I USED SATCOM AND REQUESTED A DOCTOR MEET US AT GATE. UPON ARR; DOCTOR RECOMMENDED FLT ATTENDANT GO TO HOSPITAL FOR FURTHER EVALUATION. ONE OTHER FLT ATTENDANT HAD A MINOR BRUISE. BEFORE STARTING ENGS WE WERE CONTINUALLY INTERRUPTED BY LOAD PLANNER; MIL PERSONNEL; FLT ATTENDANTS; AND COUNTLESS MESSAGES FROM DISPATCH. DISPATCH SENT 3 OR 4 DUPLICATE MESSAGES EACH REQUIRING IMMEDIATE REPLY. WE RECEIVED NO FEWER THAN 10 WT AND BALS WHICH CONSTANTLY DUMPED OUR TKOF AND PERFORMANCE DATA. OUR FINAL ATC CLRNC WAS SIGNIFICANTLY DIFFERENT THAN WHAT WE HAD LOADED. THE GND SUPVR GAVE THE CLRNC TO START ENGS AND I GAVE THE BRAKES SET SIGNAL. IT IS MY BELIEF THAT THE CONSTANT DISTRS WERE A DIRECT CONTRIBUTING FACTOR IN THIS INCIDENT.