I created separate bones for animating twist of the arm muscles and applied a copy rotation constraint based on the wrist bone. (Y axis only, local space to local space, mix=replace):
I had to disable their rotation inheritance from the main forearm bone because with it, the twist bones would over-rotate as shown below:
When inherit rotation is disabled for the twist bones, they rotate correctly, but no longer follow the main forearm bone, which is used for IK to allow control of the hand and the elbow:
How do I parent the twist bones (green) back to the forearm bone (yellow) without adding its own rotation to the rotation coming from "copy rotation" constraint for the twist bones? Please help.