There are others here who know anatomy much better than me so they can correct me if I'm wrong, but here's my understanding of spine flexibility: Cervical spine: limited flexibility, with most movement coming at the axis & atlas Thoracic spine: very little flexibility, because of the ribcage Lumbar spine: the most flexible part Sacral spine / sacrum: no movement Also, keep in mind that from the side, the thoracic spine will be almost all the way to the back. In your second piece posted, it's too much towards the middle.

That's a good question. That part of the spine does have limited movement but not enough to lose it's curve because the Thoracic part of the spine is attached to the ribs and other bones in the shoulder area. For the most part the thoracic will stay the same shape as the back of the rib cage. But have some fun and test it out on yourself or maybe someone can be your model. When I try to straighten that area out I have to pull my shoulders back and stick out my chest.

And a the rest is here. I drew some of the poses more times. I forgot about the sacrum in the first studies.