Well, in order to rig a leg you have to do this: create three hierarchies. The first is used to transform the leg in IK, the second in FK, and the last is used to blend between those two. Some sort of UI controls have to be created to control the switching and matching mechanism and other expressions has to be used in order to implement squash & stretch… that’s the most common technique using default rigging tools.
Now there is other alternative, it’s an IKFK solver, and it’s job is to automate all that process by just having only one hierarchy that can be transformed in both modes (IK and FK) depending on the needs of the animator.
It would be nice if you can give it a look. It is a beta version and it could have some bugs, i haven’t saw any by now, maybe i have to look deeper .
Download it from here.