The helical axis is a useful tool to analyze the relative motion of a rigid object to another, such as the joint motion. One can effectively find the instantaneous axis of rotation of a joint and the relative translation of the bones involved in the joint.
P is rotated around n to P' .
As vector r u rotates through angle f around n , P moves to P' . Vectors Q , P , and P ' are the position vectors of points Q , P , and P' , respectively. Remember here that both P and P ' are described in a common reference frame, the XYZ system shown in Figure 5 . Now, let's get back to the original problem illustrated in Figure 3 . Now, let's take a look at the methods to compute the helical axis-related parameters.