EMPLOYEE STATED HE WAS MOUNTING THE FRONT LADDER OF THE 769 CAT WATER TRUCK WHEN HIS RIGHT KNEE LOCKED UP AND HE FELL APPROXIMATELY 3 FEET TO THE GROUND. HE STATED HE HAD TWO PREVIOUS SURGERIE S ON THIS KNEE WHICH COULD BE PART OF THE PROBLEM. HE HAS NOT RETURNED TO WORK AT THIS TIME.