isner is pretty easy to implement in Maya. Quick outline:
- Set up a basic spline IK (remember maya hates it if you do spline ik and don’t use X as your primary joint orient!)
- extend the top of the spline a little so you don’t get kinks if the spine is hyperextended
- add 3 knots to each end of the control spline. You need 3 to make sure that the tangents are completely stable at the ends of the chain.
- skin the CVs of the spline to control objects for the hip and shoulders. You can control the tension in the spline by scaling the control objects.
- set up advanced twist on on the spline IK so the start and end rotations follow the control objects
Some extra info here http://www.giantbite.com/Steve/BP/img30.html and on the following slide
The main drawback to the isner setup is that (like any IK) you can hyperextend the control; not much you can do about that with standard maya.