Have you ever think how many degrees of freedom the human arm has (excluding palm and fingers)? I would like to share a short excerpt from a book by Saeed Benjamin Niku, Introduction to Robotics -Analysis, Control, Applications. The shoulder, the elbow, and the wrist are three joint clusters in the human arm. The shoulder has 3 degrees of freedom. The upper arm can swing up and down in the coronal plane. It can also swing back and forth in the transverse plane. And rotation with respective to the axis along humerus can be done also. The following is the illustration of human anatomy planes from Wikipedia.