Exercises to prevent knee pain

In curi blog comments someone posted a few videos about how to stop knee pain. I watched the first video and I have some comments.

The hip exercise shown at the end was interesting. I've never seen it before.

I do something that seems similar. I think it activates the same muscles.

What I do is stand on one leg, initially just letting my free leg be above the floor a bit. Then I rotate my free leg in such a way where I extend it fully in one direction and then fully in the other direction. So like at one point my free leg is behind me while my torso is somewhat near parallel to the floor. At the other end of it, my free leg is in front of me, pointing as far as I can in the direction of the standing leg, while my torso is straight up. (At all times my arms are extended in opposite directions to help keep balance.)

I do this exercise partly to help strengthen my hips but also to strengthen my ankles and to improve my balance overall.

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EDIT: I should have explained how I move from one end of the movement to the other. I swing my leg such that it's as close to parallel to the floor as I can get it. (If that's too hard for you re balance, then lower the leg such that you are balanced.)