How to move the part of the rigidbody by script?

I just learn unity for one week and I want to move the lower arm. How can I achieve this? really need help!!

First I import a free model from Mixamo, then I use the Humanoid function to automatic create bone and muscle. But I can't find the solution to move the lower arm. I think the problem is the local coordinate, but how can I know the local coordinate's name, or just use "transform.up"?

The link below is my final goal. If someone know how to do it or just some advices. https://www.youtube.com/watch?v=vAFCrMgJrmI![alt text][1]

[1]: /storage/temp/90705-17547494-934307163371674-167195685-o.png