There are a few tricks to help solve these problems. Take a character picking up an object and moving it, when the hand touches the object, constraints are used to connect the object to the hand. The constraint’s influence is switched from 0 to 1 when the hand contacts the object. When the object is released, the constraints are switched from 1 to 0. There are a few gotchas that you need to know to make this work. Key frame the loc/rot of the object on the frame before the hand contacts it. Key frame the constraints influence at 0 on this frame as well, on the very next frame, key frame the influence to 1. On the frame right before the object is released, key frame the influence at 1, and the objects loc/rot. Then on the very next frame, key frame the objects visual loc/rot first, then turn the influence to 0 and key frame it. Here’s why you have to do it this way: while the object is constrained to the hand and moved by the hand, the object doesn’t actually move in the 3d world, it appears to move because it is constrained to do so. Inserting a key frame on the frame before the object is released sets keys for the object’s loc/rot at the same position as the object before it is picked up. Then on the frame where the object is released, you insert a visual loc/rot key frame. The visual key frame sets the objects loc/rot to what is seen on the screen. Where as setting just a loc/rot key frames sets it for where the object really is, with out the constraint’s effect.
What constraints are used to do this? I prefer the child of constraint, others I’ve seen parent an empty to the hand bone at the location of where the object should be, and use copy location & copy rotation constraints.
For feet that should stick to the floor, or hands that should stick to a surface, you have to be using the rig in IK, or inverse kinematics. With IK, when you key frame a foot control it will stay at that position until moved, no matter how much the hips are moved, and it figures out how the bones between the 2 should be posed.
An explanation of the differences in FK (forward kinematics) and IK:
See attached file for an example of moving an object.
25_ChildOf.blend (370 KB)