If you spent your childhood taking ballet lessons or were ever teased for walking like a duck, there's a good chance you have an imbalance between two important muscle groups: those that internally rotate the hips and those that externally rotate them.
The splayed-feet look is the result of external hip rotators that are stronger and tighter than internal rotators. If you have such an imbalance, you may have to work harder to create some basic alignments in your yoga poses. You may not find it easy or comfortable, for example, to stand with your feet parallel in Tadasana (Mountain Pose) or to keep them parallel when you're upside down in Headstand or Shoulderstand.
In many backbends, habitual turnout can lead to compression in your lower back. And if your external rotators are really short and tight, they can literally be a pain in the butt; by putting pressure on your sciatic nerve, they can create numbness or pain, which can then radiate down your leg.
Fortunately, understanding the interplay between your internal and external hip rotators can help you avoid or overcome such problems. Even if you don't have an imbalance between these two muscle groups, learning about their actions can help you achieve more stability and better form in your asanas. Strengthening your internal rotators appropriately will also give you more stability in all kinds of sporting activities and in your daily life.
Tweak in the Knees
Stand as you normally would and look carefully at your knees. If your kneecaps point straight ahead, most likely your hips are fairly evenly balanced between internal and external rotation. If your kneecaps turn in, your hips are internally rotated. If your kneecaps point away from each other, your hips are externally rotated.
The muscles responsible for external rotation are a large and potentially strong group. They include the biceps femoris (the hamstring on the outer back thigh) and the more posterior muscles of the buttocks (the powerful gluteus maximus near the surface and six much smaller deep external rotators underneath). At the front of the body, the iliopsoas and the sartorius externally rotate the hip as well as flex it (pull it toward the chest).
To improve the flexibility of these muscles, you need to regularly practice a wide range of poses, especially ones that involve stretching your hamstrings. When doing hamstring poses, make sure you don't let your leg turn out, since that movement will let the biceps femoris escape the full stretch. To lengthen the gluteus maximus and the deep external rotators, use Parivrtta Trikonasana (Revolved Triangle Pose) and the seated twist Ardha Matsyendrasana (Half Lord of the Fishes Pose). And don't forget to practice Virabhadrasana I (Warrior Pose I) and plenty of lunges to stretch the iliopsoas.