Whenever you hear a yoga teacher say, "As you bend your knee, point your kneecap directly toward your middle toe," she's reminding you to stabilize your thighbone and knee in healthy alignment. But that's often easier said than done. Even if your alignment is fine when you're standing with straight legs, you may collapse your front knee inward when you come into Virabhadrasana II, or Warrior II.
To correct this misalignment, you need to focus on two actions in Warrior II. The first is stretching your hip adductors. This large muscle group, which fills your inner thighs and pulls your knees toward each other, includes the pectineus, adductor brevis, adductor longus, adductor magnus, and gracilis.
To get a good, long, passive stretch for these muscles, practice this pose lying on your back: Lie perpendicular to a wall, with your feet on the wall and your knees and hips each bent to 90 degrees, as though you were sitting on a chair that had tipped over backward. Then open your knees to the sides and move your feet farther apart, so your shins remain perpendicular to the wall and parallel to the floor. Stay in this position for four or five breaths and allow your inner thighs to relax and stretch.