THE EMPLOYEE WAS HELPING INSTALL A STAINLESS PLATE UNDER THE WASHER COAL SCREENS IN THE PREP. PLANT. WITH NOT MUCH ROOM TO WORK IN THIS AREA, THE EMPLOYEE WAS SITTING DOWN AND PUSHING THE PLATE UP. HE DIDN'T FEEL ANY DISCOMFORT UNTIL HE FINISHED WELDING THE PLATE. HE WENT TO THE DOCTOR ON 7/20/04; LOWER BACK STRAIN.