In my early 50's and healthy, not overweight.

At 39, I had my first knee issue.  Just moved into a new house and the basement stairs have a 90 degree. First tread is 45 then a straight fleight down.
Anyway, I noticed that taking that twist caused a quick twinge on the inner front of my knee.  Just a second long, but I would get achiness an hour later behind the knee. This went on and off. I would go 6 months pain free and then back of the knee achiness for a few days or weeks. This now cycles even without the twinge in the front of the knee.

Just a mild general achy feeling.  I strength train and that doesn't seem to aggrivate it. It seems to just happen randomly. Also seems to be more prevalent in the fall. And in arfcom tradition, I haven't seen a doctor about it.

Are there any specific exercises that strengthen the knee. I already do squats and leg presses and calf raises.
Thanks

*Not an expert, but this is what I use. YMMV

That looks interesting.  Never seen one before.