‘Once you’ve been taught a good posture, you feel gratitude and spontaneously do gassho. You feel good and want to sit in zazen, but if your posture is bad, your mood is as well.’ (Commentary on The Song Of Awakening)
I might add, and vice versa. Many times, when my mood was bad, my posture in zazen was not upright…
Shundo David Haye 
Some people are blessed with supple, firm bodies from youth on. Their disposition is sweet as apple dumplings. However thier spiritual proclivities are dim. I believe however that one part of the emotional, intellectual, spiritual, physical parts of a human being all effect each other.
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