The body has to be rendered fit by a course of purification and training in posture to practise meditation. It is not ordinarily ready to do so without such previous preparation, geniuses excepted. The posture training is of two kinds. First, the spine and head must be straightened by a slight contraction of the anus, a pull of the navel region backwards and upwards, a drawing-up of the neck and head. There are psychic and energy currents from the solar plexus passing up the spine during meditation to the brain. Not only is their free movement hindered by a bent body or a sunken chest, but they are unable to attain their proper strength. The second kind of posture training is to find the fixed position in which one can sit steadily for a long time without getting uncomfortable. This is necessary because if the body is moving about, or working, or shaking, the mind cannot attain the proper depth of thought or subtlety of attention or absorption needed for meditation, nor can the collection and concentration of the vital forces needed for the same purpose occur.
-- Notebooks Category 5: The Body > Chapter 5: Exercise > # 1