INJURED EMPLOYEE WAS RIDING IN R35 TRUCK FROM QUARRY TO PLANT FOR LUNCH. TRUCK OPERATOR PARKED TRUCK, SET PARKING BRAKE. INJURED EXITED CAB AND PROCEEDED ON CATWALK TO LADDER OF TRUCK. WHEN OP ERATOR TOOK FOOT OFF BRAKE, TRUCK LURCHED FORWARD, CAUSING INJURED TO LOSE BALANCE AND FALL APPROX. 7' TO GROUND. INJURED STATED HE SHOULD HAVE WAITED FOR TRUCK TO BE SAFELY STOPPED.