EE WAS OPERATING A HAULER. AS HE WAS BEING LOADED WITH SHOT ROCK, A ROCK SLIPPED OFF OF THE LOADER BUCKET, FELL AND STRUCK THE SIDE RAIL ON THE HAULER BED JARRING THE EE. THIS RESULTED IN RE I NJURING THE BACK STRAIN FROM A PREVIOUS INJURY. THIS BECAME REPORTABLE DUE TO FIRST LAST WORK DAY ON 2-20-02.