To achieve a realistic forearm, we are going to have to look at the anatomy of the arm. In figure one, you can see that there are two bones that make up the forearm, the radius and the ulna. The Radius goes from the elbow down to the wrist and connects by the thumb. The ulna goes from the elbow down to the wrist by the fifth finger.
What we are going to do is place some joints in to work as the ulna and radius. Open the forearm_start.mb file.
Download these Mel scripts and place them in the "My Documents\maya\5.0\scripts" directory. These scripts will then be loaded when you start up maya and are necessary for these tutorials. If you have already done this in a different tutorial disregard these steps.

















Start by creating a joint for the ulna which starts at the elbow and goes to the pinky side of the hand. The bump on the top side of your wrist is the ulna. Name the joint ulna (figure 2).
Now create a joint for the radius which runs from a little below the elbow to the thumb side of the wrist (figure 3). Name the joint radius. Place the joints so that they are over the polygon bones (check the other views) and then run the jsOrientJointUI script (see orient_joints.html ). You want to orient the new joints. Careful not to select the wrist joint the script won't orient the wrist correctly because there are four joints parented to it.
Create an ik chain for the radius which goes from the elbow to the wrist, parent the ik chain under the wrist. Do the same thing for the ulna. (figure 4)
Notice when you rotate the wrist that the ulna moves at the elbow effectively breaking it.
To fix this create a locator and place it over the ik handle (hold 'v' for point snap and middle mouse drag it to the ik handle) for the ulna.
Point constrain the ik handle to the locator. You want to ik handle to follow the locator so select the locator and then the ik handle and select constrain->point. (figure 5)

When using point constrains, selection order matters. A good rule to remember is second follows first.

Parent the locator under the wrist and parent the ulna ik under the lower arm. (figure 6)
Hide both the ik and the locator.
Parent the radius joint and the ulna joint under lower arm.
The wrist looks alright when it rotates around its y-axis however there should be little to no motion on the x and z axis.
Duplicate the wrist joint and remove all of its children. Rename it forearm_wrist.
Parent locator1 and the radius ik handle under forearm_wrist (figure 7).
Open the connection editor and connect the rotateY of the wrist to the rotateY of the forearm_wrist (figure 8).
Hide the forearm_wrist. Rotate the wrist.
The elbow has no rotation ability in the x and y axis and only limited in the z. Set these limits on the lower arm joint under the rotate tab(figure 9).