EE WAS GRADING HAULROAD UNDER VERY MUDDY CONDITIONS. AS HE WAS GRADING, THE BLADE OF THE GRADER STRUCK A LARGE ROCK. THE SEAT BELT RESTRAINED HIM IN THE SEAT BUT HIS UPPER BODY WAS THROWN FORW ARD IN A WHIPLASH ACTION RESULTING IN A SPRAIN TO HIS LOWER BACK. THIS WAS HIS LAST SHIFT OF THE WORK WEEK AND HE ASKED TO LEAVE EARLY DUE TO DISCOMFORT IN HIS BACK. ON HIS 1ST SCHEDULED DAY