Here is a nice tutorial on useing the Maya spring IK with an automatic lower leg flex control.
"Dog Hind Leg Tutorial
By, Tyler Thornock
This tutorial will help you setup a dog/animal's hind leg with a single control to make it easier for the animator to move. This basically artificially sets up a spring ik. So if you have Maya 7, you may want to use it, though this setup does give you more control in the end. This setup may be confusing for some, but the stuff you learn can be applied to many other areas.Well, the first part of this tutorial is "borrowed" from the Inspired 3D Advanced Rigging and Deformations book, however the book does not setup the rotation of the second part of the leg to be driven automatically which is a pain because the animator would have to rotate it appropriately every time they move the leg. With this setup, the animator can control the rotation or leave it automatic, unlike using a spring ik."
And here is another site with a spine rig tutorial, a reader of the book, taking ideas from it and expanding on them and then sharing it with the world. I just wanted to share the links to their sites and say thank you for giving back to the community.
Maya Stretchy Spine
The objective of this tutorial is to create a Spine Chain that will stretch and Give you warning if stretch too far or Squash too much."http://www.darksuit.com/tutorials/StretchSpine.html