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Forearms
Much like the shoulders, the forearms consist of two bones. Most skeletons use only one forearm bone, and this is fine for most applications. In real life, the forearm has two bones. The radius is the bone that ends along the thumb side; the ulna is the bone that terminates on the pinky side of the hand. Creating a separate radius and ulna bone for the skeleton will allow for much more realistic motion.
The one catch in this situation arises when you want to use IK, which requires a single bone from the elbow to the wrist. The simplest way to set this up is to keep this "forearm" bone simply for IK—it does not deform the mesh. For this task, create two additional bones, the radius and ulna (see Figure 22).
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Figure 22 To simulate a radius and ulna, add two additional bones.
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Start with a standard arm with a two-joint chain, illustrated in Figure 23. The box at the end of the chain represents the palm of the hand.
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Create two single-joint IK chains. Name them radius and ulna, as in Figure 24.
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Link both the radius and the ulna to the humerus at the elbow. Then link the IK handles of the two bones to the palm of the hand, as shown in Figure 25.
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Position these so that the radius runs from the elbow to the thumb, and the ulna runs from the elbow to the pinky, as shown in Figure 26.
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Figure 23 Start with a standard arm.
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Figure 24 Create two bones.
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Figure 25 Link the bones to the wrist and the elbow.
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Figure 26 Position the bones.
That's it. Rotating the palm will cause the radius and the ulna to behave in a realistic manner, as shown in Figures 27 and 28.
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Figure 27 Rotating the palm causes a natural forearm motion.
Figure 28 The skin should deform properly.
This article just covers just the basics of skeletal setups. Other parts of the body, such as hands and faces, should also be governed by the same principles: Keep the setup simple and easy to use, with enough control that the animator can do the job easily and effectively.