More About Bones

Joints will tend to move in a straight line between two keyframed positions. Because bones always keep the same length, keyframing one joint can cause other joints to move unexpectedly. As shown below, the movement of the leftmost joint affects the rightmost joint. Understanding this effect will help you control your animations.

Nailing the rightmost joint will lock it into place. Now, the leftmost joint travels in an arc.

Alternatively you can reduce the motion in the joint by reducing it's Flex setting.