Leg presses are an excellent choice to add some overall strength to your legs.My knees are in great shape and I attribute it personally to doing alot of free squats without weight. I have observed dramatic improvement in my ability to squat and overall body strength since I began a free squate, push up and sit up exercise program over ten years ago.
*edit: When I asked my sister which kind of excercises I should do she mostly reccomented isometric (non weighted) exercises. Weights can be good, but also very bad especially when joints are concerned, so look into isometric alternatives, and dont underestimate them because no weight is involved.
Thanks all for your advice. This time I have no access to any exercise machine. So I think I should try Rider's Stance.
I just started seated excercise. I am sure it will work.