A MAINTENANCE MECHANIC AND A SUPERVISOR WERE DESCENDING A CONCRETE STAIRWAY WHEN THE MM LOST HIS FOOTING. HE FELL ONTO HIS BUTTOCKS WITH HIS LEFT LEG UNDER HIM. HE WAS HOLDING ONTO THE HANDRAI L SO HE WAS ABLE TO KEEP FROM FALLING DOWN THE STAIRS. THE MM EXPERIENCED SORENESS IN HIS LEFT KNEE AFTER THE FALL. THE MM LOST ONE DAY OF WORKAND IS ON RESTRICTED DUTY.