Somewhere underneath you right now are two protrusions known as the sitting bones. If you cup your buttocks in the palms of your hands (NSFW?), you’ll feel these prominences which form the southernmost points of the pelvis. Try slumping back and you’ll notice that the sitting bones rotate forward. Now pull yourself into an overly upright position so that your chest is arched. Can you feel the sitting bones start to roll back?
In a comfortable sitting position, your sitting bones are somewhere between those extremes, resting like two feet on the chair. If your spine is in a healthy alignment, the sitting bones provide a easy conduit for weight to be transferred into the sitting surface so you won’t have to hold yourself up with stomach and back tension.
I’m writing today to champion the benefits of a firm sitting surface. It’s much easier to stack your spine on a firm surface because the sitting bones have “traction” and are less likely to roll. Sitting on a couch or squishy cushion is like hugging someone in a bubble coat – you can never quite find their body beneath the padding.
I recommend buckwheat cushions for this purpose, although the Zen kneeling bench and other firm-style cushions are fine as well. Find a balance between allowing blood flow to go into your legs (i.e. don’t sit for an hour on a stone slab) while giving your sitting bones a firm, easy target to rest on. On a soft cushion, I picture the sitting bones like sled dogs in a blinding snowstorm trying to find their way home. They don’t know which way is up or down and where to deposit their weight.