Angular Acceleration Analysis 
The Angular Acceleration Analysis allows you to calculate the acceleration of angles between 2 vectors or a vector and a plane. You can graph up to 4 angular velocities at a time. The vectors are defined as vector 1 being the vector between Marker 1 and Marker 2 and vector 2 being the vector between Marker 3 and Marker 4 in the Angular Acceleration Analysis Setup dialog box. In many cases it is practical to use the same marker for Marker 2 and 3. For instance, when calculating the knee angle, you may want to have one marker on the hip, one on the knee and one on the ankle.
When you are doing 3 dimensional angular analysis you can create a 2D angle (a.k.a. "shadow angle") by projecting the 3D angle onto a specified plane.
You can graph the angular data relative to the starting position by selecting the Normalize option. This option will calculate an average of the first n frames and then subtract this angle from each consecutive frame creating a “change of angular acceleration” graph.
Click on any item in the picture below for more information.
