the thoracic vertebrae do not have much give in them because they are tied together by the ribs. extension is typically limited to the excursion of the ribcage during inspiration but a bit more is possible. gravity can be brought into play by serious back bends of say 60 degrees or more. this because i got back from aikido class & she said: straighten up, you're all bent over with your chest collapsed. yes dear, thankyou. it was true. i was tired. i had dropped my ball. i had reverted.
2 people quit the class recently. they did not want to go through the multi-week process of finding out that that they could not indeed walk and chew gum at the same time, even just walk right. each new phyical thing demonstrated to be performed suboptimally, each finger, each joint angle, the senses not trained in any useful way.
so i felt like i found a balance point at t12-L1 that i could use to get a relaxed extended ribcage so i could carry my torso "proud" and stay relaxed. the habit is to not even try & just let that balance point collapse into flexion & i end up all bent over creeping along with my walker.
that reminded me of the other balance point, a certain feeling in the middle of the neck. the control of the head is at C1, if the back of the head is extended & the chin is tucked there is a point at which the "bow" of the cervical spine is neutrally cocked.
of course all of this specificity falls apart a soon as i depart from very slow.