Guess what also attaches to the ATLA? The levator ani muscles, which are the deep pelvic floor muscles. Because they are both attached to the ATLA, by design, the OI and the pelvic floor muscles have to work as a team.
The OI's job is to rotate the hip laterally and prevent the hip from falling the opposite way (into medial hip rotation). The OI has to control the hip during standing, and even more during walking, and even more during running. If the OI isn't doing its job on the team because of weakness (often accompanied by spasm), the pelvic floor muscles have to contract more than normal to compensate. This overworks the pelvic floor and contributes to chronic pelvic floor muscle tension. So, good OI function is critical to good pelvic floor function (and vice versa).
The other reason that the OI is more interesting than the other deep six is because of its angle of pull. Most of the deep six muscles lie and pull along a straight line. But the OI bends 90 degrees as it turns the corner from the inside to the outside of the pelvis. This gives it improved efficiency compared to the other muscles. The OI also has a greater number of small motor units (a motor unit is a nerve plus the muscle fibers it innervates). This means it is capable of greater precision with small, finely controlled movements.
Another amazing thing about the OI is that the fascia of the OI is continuous with the fascia surrounding the bladder. If the OIs are habitually held in an overstretched position (hip medial rotation, a.k.a. sitting with your knees close together and your feet apart), that creates increased, abnormal tension in the bladder fascia. That tension can cause or contribute to urinary urgency or frequency.
Speaking of fascia, there's a canal that runs through the fascia of the OI, called Alcock's canal, and the pudendal nerve runs through it. So, tension in the OI can cause or contribute to compression of the pudendal nerve. That can present as a wide variety of symptoms, because the pudendal nerve sends branches to most of the pelvic floor.
This is a really geeky detail, but the OI is very interesting to palpate. Pelvises come in different shapes, so that means the OI isn't always where you expect to find it. Sometimes it runs parallel to the front of the body, and sometimes almost parallel to the sides of the body, but usually about halfway in between at a diagonal angle. Also, the top part of the OI can be palpated above the "hammock" of the pelvic floor muscles (through the walls of the vagina or rectum), but the bottom part can be palpated externally on the medial side of the "sits bones" (ischial tuberosities).
The OI doesn't just laterally rotate the hip – it also abducts the hip, especially when the hip is flexed. Hip abduction causes your thigh bone to move out to the side, away from your other thigh bone. As with hip lateral rotation, control of hip abduction is critical to having good balance and control when walking and running. The "Hip Ice Cream Cone" exercise below combines both of these movements. Hip abduction and lateral rotation almost always occur together, and improving one improves the other.
Those are the main reasons that I'm obsessed with the obturator internus, and I hope I've piqued your interest in this fascinating muscle. 99% of my pelvic PT patients have weakness and spasm of the OIs, and as we work to improve their strength, relaxation, and motor control, whatever pelvic-related symptoms that they have tend to improve. The first class I ever took on pelvic PT was a class called "Rotator Cuff of the Pelvis," taught by Janet Hulme, and she connected the dots between hip rotation and pelvic floor function in a clear, unforgettable way. That connection runs through the obturator internus!
For more information on the OI and how it relates to your health and healing, ask your PT at CTS!
