Why do we emphasise posture so much?
We take up the posture of Buddha and we are Buddha. If this sounds absurd, it is only because we identify ourselves with our thoughts.
Body and Mind are not separate. They are two aspects of an ungraspable wholeness.
We need to sit in a stable balanced posture, preferably cross legged, our weight dropping down through our sit bones, our head balanced but not held, our spine uncompressing.
If we are physically balanced in this way, we can gradually find a state which is alert but peaceful, awake but not effortful. Habitually in our lives we alternate between exertion and collapse. Zazen enables us to find a state other than this; in our practice, in our lives.