IK (short for 'Inverse Kinematics'). When you take a skeleton and you create a 'reaching' animation by moving the shoulder, then the elbow, then the forearm, then you are doing 'forward kinematics'. If you were to grab the hand and 'pull' the whole skeleton to reach an object, then you are doing 'inverse kinematics'.
An IK solver takes a point in space that you want a given bone to reach and then adjusts the bones so that you get what you asked for. IK is most easily done during authoring, but it is most *interesting* when done at runtime.
Blender had IK the last time I checked. I don't remember how well it was supported, though. I have not touched that product in many years. That was an authoring-time solution, though, and would not adapt to the environment like what you are wanting. Do a search on 'inverse kinematics algorithms' and you'll probably turn up something like what you want.
-- Jeff
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