EE WAS PULLING GREASE HOSE (FROM A SERVICE TRUCK) ACROSS FROZEN GROUND CONDITIONS. THE HOSE HUNG UP ON A PIECE OF FROZEN GROUND WHICH CAUSED UNEXPECTED FORCE AND TWISTING MOTION TO THE EMPLOYE E WHEN THE LENGTH OF HOSE SUDDENLY TIGHTENED. HECONTINUED WORKING UNTIL 2-26-01. AT THAT TIME HE WAS UNABLE TO WORK DUE TO BACK PAIN.