Positioning the Chest
There are differing views on the chest. Some experts suggest that you should lift the chest up. (Kapleau 362) Others say to anchor the chest down and in. (Porter 59, Gokhale 82) Many teachers give no particular instruction with regard to the chest at all. (Dogen, Aitken, Gunaratana, Sogen)
I have experimented with lifting the chest, anchoring the chest, and ignoring the chest. I have generally found that I am most comfortable just ignoring the chest. Either consciously lifting or depressing the chest simply creates unnecessary muscle tension in the middle of my back. If the pelvis, the hands, and the head are appropriately positioned, the chest remains in a neutral position and needs no special attention.
Many people, when they think about good posture, tend to lift the chest. In most cases, they are creating unnecessary strain and an unsustainable situation. This is basically what people do when they are trying to stand up as straight as possible or adopt a military posture with formal rigidity. Such an effort does not produce a healthy posture for meditation, which should be natural and relaxed.