EE WAS SITTING INT HE OPERATORS CAB OF A CAT 777A OFF HIGHWAY TRUCK WAITING TO GET LOADED. WHEN THE LOADER OPERATOR WAS LOADING THE TRUCK. A ROCK DROPPED ON THE SIDE OF THE BED OF THE TRUCK WH ICH JARRED THE OPERATORS CAB WHERE THE EE WAS SITTING. HE SUSTAINED A STRAIN THE UPPER BACK AREA.