excuse me ...
Lee Salzman wrote:
Especially the fixation of the upright, rigid spine, ...
What do you mean?
It's a torque-minimization problem. Imagine an individual bone. Push or pull on the side of it, it is a lever, a brittle one, with shear, torque, etc. Push or pull instead on the ends, it instead bears tension or compression. Now if you are standing upright, like a statue, and push on something with your arm, your body is now one giant fused lever, exerting a big torque on you.
But if you laid out all the individual bones of the skeleton, and played a "one of these things is not like the other" game, the spine kind of stands out - well, not just the spine, but the spine is sure weird. When you are standing or walking, your spine is not actually straight, it has an S curve. Your spine is partly a shock absorber, a spring, it can twist and flex. So okay, when we sink the tail-bone, raise the skull as if on a string, etc. etc. this is straightening out the S-curve, making the spine into a better bridge from up to down, getting rid of those torques acting in the vertical.
But then again, the forces we need to express or bear are not always going straight up, or straight down, and we're not always just carrying a force from end to end, sometimes we're absorbing, sometimes we're generating. You want to push something in front of you, but the ground is below you, so if the spine is rigid, no matter how you orient it, it's just a lever or a really poor bridge with big stresses sliding off at the end points, with the force absorbing or generating capabilities of it having been stamped out by fixating it. So what do you do about it?