Yoga Anatomy

Alternately, having the knees lower than the hips (by elevating the seat) prevents the
pelvis from tipping backward and makes it easier to maintain the lumbar curve of the spine.
The challenge in this arrangement of the legs is that it can tip someone too far forward
on his sitz bones. The curves of the spine, particularly the lumbar curve, can be greatly
exaggerated by this anterior tilt, and then the muscles of the back have to remain active
to prevent falling forward.
In either case, tipping too far forward or too far backward necessitates using the muscles
continuously to prevent falling into gravity.
The goal should be to find the position of the legs that allows the weight to fall most
clearly from the spine through the pelvis into the sitz bones and the support of the floor,
regardless of how high or low the knees are relative to the pelvis. In this way, a minimum
amount of muscular effort is needed to align the bones for support. For some people this
involves raising the seat a great deal or even sitting on a chair for ease in the spine until
more mobility can be cultivated in the pelvis and legs. In a well-supported seated asana,
the intrinsic equilibrium of the pelvis, spine, and breathing mechanism supports the body,
and the energy that has been liberated from postural effort can be focused on deeper
processes, such as breathing or meditation.