FLC OF A B757 JUST ABOUT TO REACH LEVELOFF ALT ENCOUNTERED A MOUNTAIN WAVE ACTION WHICH POSSIBLY KICKED OFF THE AUTOPLT. THEY EXCEEDED ASSIGNED ALT AND TRIED TO LEVEL OFF BUT DSND BELOW THE ASSIGNED PRIOR TO RETURNING TO CRUISE ALT. THE ACTION WAS STRONG ENOUGH TO CAUSE FLT ATTENDANT INJURIES.

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LEVELING OFF AT FL350; WE EXPERIENCED MODERATE MOUNTAIN WAVE TYPE ACTION. THE MACH ROLLED FROM .800 TO .851 AND THE ACFT STARTED A 2500 FPM CLB. THE AUTOPLT KICKED OFF AND THE FO; WHO WAS PF; SMOOTHLY RECOVERED THE AIRSPD AND ALT. THE ALT WENT FROM APPROX FL348.5 TO APPROX 352.5. THE FRONT 2/3 OF THE AIRPLANE EXPERIENCED A LITTLE LIGHTNESS IN THEIR SEATS. BUT OF THE 4 FLT ATTENDANTS IN THE REAR OF THE PLANE; 3 OF THEM HIT THE CEILING. ONE FLT ATTENDANT COMPLAINED OF A HARD BLOW TO HIS HEAD. WE IMMOBILIZED HIM AND A DOCTOR ON BOARD CHKED HIM OVER AND TAPED HIS HEAD TO THE SEAT HE WAS SITTING IN. WE CONSIDERED A DIVERT TO PITTSBURGH BUT THE FLT ATTENDANT SAID THEY WERE OK TO CONTINUE TO NEWARK. WE COORDINATED THROUGH AIRLINE DISPATCHER. THEY PUT OUT A PIREP FOR THE INCIDENT AREA AND COORDINATED PARAMEDICS TO MEET THE PLANE IN NEWARK. UPON ARR INTO NEW YORK APCH CTL; I REQUESTED PRIORITY FOR LNDG. THE PARAMEDICS MET THE AIRPLANE; ATTENDED TO THE FLT ATTENDANT; AND TOOK HIM TO A LCL HOSPITAL. HE WAS RELEASED LATER THAT EVENING. THE OTHER 3 FLT ATTENDANTS; WHO WERE IN THE REAR OF THE PLANE; LATER COMPLAINED OF NECK AND SHOULDER ACHES.