EMPLOYEE STATED HE WAS ASSISTING A FELLOW EMPLOYEE TO REMOVE THE FEED CHUTE DOOR OF THE PELLET MILL. HE STATED THE DOOR WAS HEAVIER THAN HE EXPECTED AND HAD TO LOWER IT QUICKLY. AS HE DID HE F ELT A TWINGE IN HIS LOWER BACK. COMPLETED SHIFT DIDN'T REPORT INCIDENT TILL NEXT REGULAR SCHEDULED SHIFT.