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Two basic physical models seem appropriate for the human body
1) Torsion tube
2) Double (or multiple) pendulum
The torsion tube can apply to the torso but also to the limbs and to the limbs and torso considered in a continuum.
The double pendulum can be considered as the legs from hips to ground plus the torso from hips to head. Each limb may also be separately a double pendulum, or all of them together form a chain of dependent pendula.
At first glance, these models would seem very different, a torsion tube quite static, the double pendulum quite dynamic, but the structure and dynamic of these models are, in fact, closely related.
The effect of the torque creates shear on the radial and longitudinal axis of the tube. The diagonal figure shows the resulting linear stresses of the shear -- tension in one diagonal and compression on the other. If you extend these diagonal lines around the surface of the tube, (and torsional shear is always greatest at the surface) then you get two interlaced spirals around the body of the tube.
One spiral is in compression and the other spiral is in tension. The two lines of stress are oriented 90 degrees from one another, and they are both 45 degrees off the longitudinal axis of the tube.
"Wait!" you say, "What about the double pendulum?" Well, since you askedů