Try warming your knee joints up 1st.
Feet Together gently go into a half squat and rotate your knees in a circular direction. 10 to the left & 10 to the right. That should warm up your ligaments.
It MAY also be that you are forcing the stretch beyond your limits. How Heavy are you? Do you have thin or thick set legs?
Try sitting on the floor with your legs wide appart (Watching TV) and keeping your knees flat on the floor and bending at the waist (keeping a straight back).. Try to touch your forearms on the floor in fron whilst still watching the TV. That may be a way to gently stretch your ligaments whilst watching decent Martial Arts films like "Fighter in the Wind", "Arahan" and "Fearless"
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