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Since the ribs and thoracic spine are attached it makes sense that breath will effect the movement of the thoracic spine.
Every inhalation is accompanied by a gentle forward bending of the thoracic spine, while on every exhalation the opposite happens, and the spine moves through extension… So it makes absolute sense to move into extension postures as we breathe out, if our intention is to access the thoracic spine. Anything else would be going against the intelligence of the body. Taken from ‘Intelligent Yoga’, Peter Blackaby.
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