Wearing Belt of Stelgaer reduces the duration of Fighter's Knockdown ability, even when above 50% endurance, and seemingly by more than 66%. Here are some screenshots:
http://i.imgur.com/AL8DGni.jpg - without the belt on
http://i.imgur.com/a9xqJLi.jpg - while wearing a belt.
My character has 8 Int. No bards were harmed without consent during the test.