Stretching for the knee

Hi everyone,

My knees are pretty good (some moderate osteoarthritis & moderate pains) and I am 51 (6 ' 1", 175 lbs) and have had a heavy competitive sports wear & tear (tennis & soccer). But I am still quite active (no jogging or intense running though). No surgeries and from what a few MDs have said pretty stable knees and good strength in quads.

I have been doing some yoga recently. One of the exercises is to "sit on your ankles" (where the front of the knee to the front of the ankle is flat on the floor, back strait, and you move your buttocks down toward your ankles to "sit").

When I do this I don't go even close to actually sitting on my ankles just to be careful. As I go into the movement, I feel some tightness or pain on the outside of my knees (mostly on lower, outside part of knees) but after I finish the stretch and I shake my legs out (some more moderate pain there too) I feel normal and pain free.

Would appreciate getting some feedback as to weather I should gradually try to do this stretch as I seem to be extremely inflexible with this movement? Or avoid as there is some discomfort when I do it?

Any advice would be greatly appreciated.