I read a Frederic Delavier’s book « Strength Training Anatomy » and I learned good stuff.
Sitting on the floor with one leg stretched out and the other leg bent with your foot on the floor and moving from the outer side of your leg stretched. Press on the outer side of your knee bent with your opposite elbow. This exercise mainly stretches the muscles of the gluteus maximus and the lateral rotator group (piriformis, gemellus superior, obturator internus, gemellus inferior, quadratus femoris and obturator externus).
Variant
It’s possible to press on your knee with your 2 hands.
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-Steph