However, over a period of 6 weeks, I've gone from a natural spread of about 120° to a full 180°. I'm not ready to try it on ice yet, but I can do it completely without pain in my knees, hips, or back. So, how'd I do it?
First, instead of going to the 180° position, I just went slightly beyond my natural spread, increasing it by small amounts day to day.
Secondly, I repeated the exercise several times a day and I used figure skater posture (shoulders back, head up, leaning back).
Thirdly, I only held the position for seconds each time I did it. I never held it more than twenty seconds until I was without pain in that position.
I had some knee pain at the beginning of each extension, but it was small. Since I didn't hold the position for very long, and I didn't go from my normal position to a full 180° right away, I didn't damage the knees. I just did it every time I got in the elevator, or while I was waiting for the water to boil at home, or when I was alone in my office, or on any other occasion where I had a few private seconds.
I think I need to get to a 190° spread before I do it on ice, and I need to make the transitions smoother, but those are details. Lookin' good for elderly lady on ice in spreadeagle!
So, that's what I've been doing, in addition to the half hour of Physical Therapy exercises that I will have to do for the rest of my life. If you watch this video, this is my life, but in my hips (but my doctors are very responsive).