EMPLOYEE WAS ROOF BOLTING ON UNIT 3. HE WALKED BACK TO THE REAR OF THE ROOF BOLTER TO MAKE UP A ROOF BOLT. ON HIS WAY TO THE REAR OF THE ROOF BOLTER HE STEPPED ON A PIECE OF ROCK CAUSING HIM T O TWIST HIS RIGHT KNEE. EMPLOYEE WORKED UNTIL 2-28-01 AND STARTED MISSING 3-1-01.