Rigging: Digital Orthopedia - Blender Foundations: The Essential Guide to Learning Blender 2.6 - page 254

set Chain Length to 2, as it appears in the
figure. Chain Length tells the constraint
how many bones to include in the effect.
In this case, we only want the lower and
upper leg bones included. If we had left
this setting at 0, the IK constraint could
possibly have affected the entire armature,
which, while it can lead to some wild ani-
mation, will not usually give you a con-
trollable, natural result.

Constraints can be added to a bone through
the Add Constraint control on the panel,
and rearranged for different results. Like
modifiers, each has its own unique effect on
the bone and its own battery of settings.

Constraints can also be added to bones by
using the Shift-Ctrl-C hotkey in the 3D
view, which provides a pop-up menu of the
large variety of constraint types. Adding an
IK constraint is so common that it gets its
own hotkey: Shift-I . Notice in Figure 9.13
how the Target and Bone fields are already
filled in. When adding a constraint directly from the panel controls, you have to populate these fields by
hand. However, the fact that we first selected bone B when creating the constraint with the hotkey told
Blender to use that as the target bone.

Figure 9.13 The Bone and Bone Constraint panels.

You'll find that you can now manipulate the entire leg by grabbing the bone that extends backward from
the heel (D) and moving it around. Begin by doing this in a side view. Translating the heel gives some
fairly nice knee-bending motion. Rotating the heel bone gives a good result too. If you move the heel
bone too far away from the leg, the foot detaches. While there are some nice rigging solutions to prevent
this from happening, there's an even simpler solution: Don't move the heel too far when you animate.
This highlights one of the “personal preference” aspects of rigging and animation. Some animators prefer
to have everything automated. Others prefer a rig that is simpler and assumes that you're going to pay
attention when you're animating. My preference is for the second, but yours may differ.

Switch to a front view and translate the heel bone again. The leg moves from side to side, but the motion
is clearly wrong. The knee shouldn't twist like that. A real knee only rotates along a single axis. We'll
look at how to lock that down in a moment.

Return the heel bone to its rest position (Alt-G to clear translation and Alt-R to clear rotation) and switch
to a top view. With the heel still selected, rotate it. The foot structure rotates as you would expect, but
the leg doesn't follow. We'll fix that first.