Tune In to Turn In
Another internal rotator of the hip is the tensor fasciae latae. Located on the outer front hip, this muscle also works with the iliopsoas and the sartorius to flex the hip. So if the tensor fasciae latae is weak in relation to the iliopsoas (an external rotator), your hip will externally rotate whenever flexed—in other words, every time you take a step.
Finally, the hip is rotated internally by the two innermost hamstrings: the semitendinosis and semimembranosis. These muscles also extend the hip—the action you create when you lift your hips off the floor to come into Setu Bandha Sarvangasana (Bridge Pose). The massive gluteus maximus and the biceps femoris also extend the hip, so if these two muscles overpower your two inner hamstrings, you'll externally rotate your hips whenever you extend them.
As a yogi, you want to avoid this undesirable combination of actions. When you extend your hip, as in backbends or when you lift your leg in Virabhadrasana III (Warrior Pose III), external rotation can create compression in your lower back and sacroiliac joints.
To eliminate or at least reduce this compression, you need to strengthen your internal rotators sufficiently to counter the powerful and, typically dominant, gluteus maximus.
Let It Roll
Start on your back, legs straight out on the floor and heels together. Let your legs relax completely; if you're like most people, your legs will roll out in this position. Now try to roll them back in so the inner edges of your feet come together. If you are a strong external rotator, it may feel a bit tiring to hold your feet parallel, or you may not even be able to do it.
To make this action easier and to build endurance in the internal rotators, slide to a wall and press into it with the soles of your feet. Again relax your legs and give in again to your stronger, tighter external rotators. Your feet will turn out and you'll feel the outer edges of your feet press into the wall while the base of your big toes and your inner heels will feel lighter.