Eka Pada Sirsasana (Foot-behind-the-Head Pose)
An Ounce of Preparation
Before you even think about doing Eka Pada Sirsasana, you should maintain a well-rounded practice for many months. This is true even for practitioners who begin yoga with enough flexibility to perform Eka Pada Sirsasana or who may achieve it fairly quickly. Flexibility is necessary, of course, but strength, stability, and integrating your whole body within the pose are just as important.
In fact, I often tell my students that it is more difficult to be flexible than to be stiff. An expression that says, "Oh, sure," usually crosses the faces of the stiff ones. All they know is that when they stretch, they are really uncomfortable, and they don't move anywhere nearly as much as their more flexible classmates, who seem to slide into many poses with such ease. Those more flexible (and seemingly more fortunate) students have the difficult task, however, of trying to find balance in their poses without constantly overworking the areas that move so readily. Super-flexibility, without the balance of strength, can result in instability in the joints—which, in time, can lead to pain and injury. I have found over the years that loose, very flexible students seem to have physical problems more often and of a more serious nature than stiffer students. So maintaining a balanced practice for a prolonged period is valuable not only for building up to Eka Pada Sirsasana; it also allows you to practice the pose in a safe way.
Despite all my caveats about the need to balance flexibility with strength, you clearly do need flexibility in your legs and hips to do Eka Pada Sirsasana. It is usually best, therefore, to practice this pose as the culmination of a series of forward bends and hip openers. To avoid overstretching your spine and straining your lower back in any forward bend, it is important to lengthen your hamstrings and fold forward from your hip joints rather than bending at the waist. Eka Pada Sirsasana may not appear to be much of a forward bend since you do not lower your torso forward toward your legs. But all the principles of forward bends apply; you are simply varying the forward-bending process by bringing your leg up toward (and beyond) your torso instead of bending your torso down.
You can develop the flexibility required to work productively on Eka Pada
Sirsasana by practicing all of the forward-bending poses. The variations of Supta Padangusthasana (Reclining Hand-to-Big-Toe Pose) are particularly helpful, especially the one B.K.S. Iyengar presents as the second variation in Light on Yoga (Schocken Books, 1995). And proficiency in Kurmasana is a prerequisite.
Even if you've realized that strength is as important as flexibility for a balanced body, you may be surprised to learn that strength is necessary for Eka Pada Sirsasana. The pressure that the leg exerts is powerful, and needs to be balanced by the strength of the neck and back muscles. Sirsasana (Headstand), Sarvangasana (Shoulderstand), and their variations are especially helpful in strengthening your neck and back. Akarna Dhanurasana (Archer's Pose) is also a particularly good preparation for Eka Pada Sirsasana, not only because it increases mobility in the hips and legs, but also because it helps build strength in the spinal muscles.