Hip to Be Square
Throughout the body, two antagonists (muscles or muscle groups having opposite actions) often work together like this, co-contracting to help stabilize a joint. Other examples of this phenomenon include the wrist flexors and extensors in Handstand, and the abdominals (which flex the spine, curling it forward) and the erector spinae (the back muscles that extend the spine, arching it back) when you are standing upright.
The stabilizing function of the hip adductors and abductors is vitally important in many standing poses. When you are practicing a one-legged standing asana like Vrksasana (Tree Pose), it can make an enormous difference in your balance and your endurance in the pose. If you are standing on your left leg in Vrksasana, press your right foot firmly into your upper left thigh and simultaneously press your inner left thigh into your foot, just as if it were the block you used in Tadasana. Notice how strong these actions make your foundation.
Similarly, you can improve your stability in Utthita Hasta Padangusthasana (Extended Hand-to-Big-Toe Pose) by eliciting a co-contraction of the adductors and abductors. As you stand on your left leg and lift your right leg forward and up—holding your right foot with your hand or a strap, or resting it on a chair or ledge—bring your awareness to your left leg. Recall the feel of the block touching your inner thighs in Tadasana, and press your left thigh into the imaginary block just enough to bring a sense of strength and groundedness to the standing leg.
In both these poses, be careful not to press too hard with the left leg; if you do, the right side of your pelvis will probably rise. Remember, one of the goals in both Vrksasana and Utthita Hasta Padangusthasana is to have a level pelvis.
Balancing Strength and Stretch