Stand and Deliver
To refine and strengthen this action, try practicing Warrior II with your back to a wall. Stand close enough so your right buttock touches the wall but your feet do not. Turn your right foot out so the foot is parallel to the wall, and turn your left foot in slightly more than perpendicular to the wall. Now come down into the pose, bending your right knee and keeping your left leg straight and strong. Place a yoga block or firmly rolled towel between your right knee and the wall, and press your right knee into this prop. It is the deep hip rotators that are putting pressure on the block—and at the same time aligning your knee perfectly with your foot.
As you press the right knee toward the wall, make sure that you don't let the left thigh buckle away from it. In fact, if you'd like to increase the challenge, place a second block or rolled sticky mat between the wall and the center of your left femur; hold it in place by firmly pressing your left leg back while continuing to press the right knee into its prop. You will feel a strong action in the back of both hips as the piriformis and its friends externally rotate both legs. A similar action should occur in Parsvakonasana (Extended Side Angle Pose) to the right: Keep the right knee pressing into the right arm while maintaining a straight line through the left leg.
The knee is also at risk of being twisted in some straight-leg standing poses, and a strong contraction of the deep hip rotators is required to align the femur, knee, and foot. While in Trikonasana (Triangle Pose) to the right, turn your head and look at your right knee. Chances are, it's slightly internally rotated.
Now feel the firmness in your right buttock as you externally rotate the thigh to align the knee with the foot. (Notice that this action also creates a beautiful arch in your foot.) Even more challenging, maintain this alignment as you bend your knee and prepare to transition to Ardha Chandrasana (Half Moon Pose). In this transition, the rotators must act strongly to prevent the knee from angling in toward the big toe, and they must continue to contract as the knee straightens fully in the pose.
Let's look at one more straight-leg standing pose, Tadasana (Mountain Pose). Many people tend to stand with their feet pointing out slightly. Sometimes this may be caused by short and tight rotators that externally rotate the whole leg. If that is the case, the knees will also be pointing out, and you need to spend more time stretching the backs of your hips.
However, it's also fairly common for the feet to point out while the knees point in, an alignment that can contribute to pronation of the feet (collapsed arches), knee problems, and low back pain. Weak rotators can be the culprit behind this pathological alignment. If the external rotators aren't strong enough, the internal rotators (which include the gluteus medius and tensor fascia lata on the outer hip as well as the adductors) will pull the femur into internal rotation. In an attempt to align the femur, the outer hamstring, which is also an external rotator, takes over for the deep hip rotators. Unfortunately, the outer hamstring inserts into the lower leg, and so, instead of aligning the shin and thigh, it turns the lower leg out even more deeply, exacerbating the misalignment.