Inverse Foot IN XSI

The Inverse Foot technique is quite simple. Instead of trying to give away the recipe, let's try to discuss the needs and get to the final result through the discussion.
The Inverse Foot is intented to control a "regular" foot or, as called on the tutorial, the "Forward Foot". The term "Forward Foot" is just meant to be opposed to "Inverse Foot", as I am not an English speaker and don't quite know what it could really mean :-)
A "Forward Foot" would be governed by the leg, parented to its effector or to the last bone, as shown below .

To make things easier, I'll try to paint the bones so we can understand what's going on. As you can see, the "Forward Foot" is painted yellow and the leg is painted in the default green.

In this case, as we move the leg, the foot would get off the floor with it and if we wanted to keep its front section on the floor, we would have to do it by adjust its effector through Global Translation or something, not a good idea anyhow.

The principle of the Inverse Foot is having another kinematic chain, that looks pretty much like the yellow one, but it's constructed on the opposite direction, and this new chain is going to control the "Forward Foot" and the leg by a serie of simple constraints. If your Forward Foot is already in postion, it's a good idea to construct the Inverse Foot with the Snap option ON (snap to centers), as shown below.

I just WON'T rename all the bones with cute little names and give you a list of the constraints so you can follow the recipe. Let's think of it just for a moment. If you build the red chain (the Inverse Foot chain) with snap to centers on, it means that the center of a certain bone in the red matchs another certain bone center on the yellow chain. You'll have to constraint each bone of the yellow chain to another bone on the red chain by "constraint position". The center of the bone is usually where it begins, hence the "fat section" of the bone. So, pick a bone on the yellow chain, press "constraint position" and pick the red bone with the closest "fat section". That's the closest center. This "rule" can also be applied to roots and effectors, link it to the closest center of the opposite chain. ALSO: constraint the effector of the leg to the effector of the Inverse Foot so it can control the leg going up and down.
As the constraint have been done, Inverse Foot is almost ready to hit the road. A few more adjusts to make it easier to select, since it's going to be the driver of the Forward Foot and leg. Select the bones, open up the chain bone dialog box by hitting ENTER. Change the settings for Shadow Display so you can easily select your Inverse Foot Bones and not the Forward Foot bones (which are good only for skinning your character).
Another good thing to do: since the Forward Foot is connect by "position" it won't follow the Inverse Foot rotations in Y. You could have done the constraint by POSE with constraint compensation ON, i think. Otherwise, create a Null to act as a chain upvector for the first bone of the Forward Foot Parent it to the root of the Inverse Foot chain so it can turn and follow ou driver properly (see explorer in the next image). If the bone twists after this procedure, just hit ENTER and go to Kinematic Joint/ Resolution plane and set its value as 180 to twist back.

Ok, kids, it's ready to roll. The animation will be done by rotating the Inverse Foot bones. The Inverse Foot MUST stay on another hierarchy. So, you can have the skeleton hierarchy, the inverse feet hierarchy and both attached to the same model.

HOW TO ANIMATE IT: The inverse foot must be animated by rotating the bones, that's been told. Don't try to use the effector, impredictable movements will occur and you'll not have the accurate control you can achieve with rotation. IK is good for lots of things, not for feet. To move the legs (to take a step, for instance), you move the root of the inverse foot hierarchy and everybody will happily follow.

ood luck!