I used to think that when clients had a limited internal rotation of the femur it was likely from tightness in the deep six muscles including the Piriformis.
I noticed with some clients, however, that when I do an internal rotation of their femur they will feel tension or pain in the front of the hip, near the ASIS. This can be from tightness in the Sartorius or Iliacus muscles, as both are external rotators. Releasing tension in these muscles can help relieve chronic pain in this area and improve the internal rotation of the femur.
In this video, I show ways of doing a gentle pin and stretch of the Sartorius and Iliacus, both with the client prone and supine.
Be careful when doing an internal rotation of the hip with your client prone if they have medial knee problems such as a tear of the medial meniscus or the medial collateral ligament. You may do this technique prone while supporting the knee, or do the technique in a supine position.