EE WAS INSPECTING EQUIPMENT ON GROUND FLOOR OF THE MILL BUILDING WHEN HE STEPPED ON A SMALL STEEL GRINDING BALL THAT WAS COVERED BY MATERIAL AND NOT VISIBLE, CAUSING HIM TO TWIST AND STRAIN HI S BACK. DID NOT REQUEST TO SEE DOCTOR UNTIL 3-20 WAS RETURNED TO FULL DUTY AT THAT TIME, HOWEVER ON VISIT OF 3-27 DOCTOR PRESCRIBED PHYSICAL THERAPY TO COMMENCE WEEK OF 3-31-03.