3D Kinematics is the description of motion in 3D space without regard to the forces that cause the motion. The variables most often used in movement analysis of linked rigid structures are joint/segment angles, angular velocities and angular accelerations.
Joint angles are Special and get a separate section in the documentation. The other kinematic variables get little attention in the clinical literature.
At the heart of 3D Kinematics in Visual3D is the definition of a Segment in terms of its local Segment Coordinate System because there are few kinematic measures in Biomechanics that aren't related to Segments. Therefore, to understand the computation of kinematic measure, such as a Joint Angle in Visual3D, you should understand how segments are constructed and represented in Visual3D. See Segment Construction in the online documentation
Visual3D Models are actually just collections of Segments. This generalization means that users are not restricted to pre-defined models, or to understanding the potentially elaborate interactions between segments. Visual3D takes care of the interactions automatically, which allows the user to focus on defining individual segments, and since all segments are defined using the same terminology and process, this means that learning to build Visual3D models requires the user to only learn how to create a segment
There are many nuances to building segments, but the principles are straightforward. Users are encouraged to contact C-Motion support when a subtlety (e.g. when is it useful to change the segment coordinate system definition of a segment; for example changing the anterior direction) is presented to them (usually by receiving a model from another user).
One of the powerful features of Visual3D is that a segment coordinate system (which is really a local coordinate system) can be defined without there being a physical segment (e.g. a virtual segment). Kinematic computations in Visual3D work equally well with virtual segments and segments that are not associated with the subject (e.g. an obstacle in the environment).
The kinematics are calculated when the user requests a signal. The request can be made by opening a report template or by creating the signal. At the time the model is built, the 6 degrees of freedom describing the motion of each segment is calculated. This is sufficient information to animate the model but not to calculated the kinematics or kinetics. The number of possible kinematic variables is extremely high, and Visual3D only calculates the ones that are requested. Link Model calculations are only performed after they have been defined by the user (opening a report will define the variables included in the report).
Note: if the model is changed, the RECALC button should be pressed to ensure that all calculations are updated.
SEG_CGPOSITION: the Segment Center of Gravity Position can be resolved into any segment coordinate system.
SEG_PROXIMAL_JOINT: the Segment Proximal End Position can be resolved into any segment coordinate system.
SEG_DISTAL_JOINT: the Segment Distal End Position can be resolved into any segment coordinate system.
SEG_VELOCITY: The velocity of the center of mass of a segment.
JOINT_ANGLE : Relative orientation of one segment and another (not necessarily contiguous) segment
Segment Angle : The angle of a segment relative to the laboratory is simply a Joint Angle with the Laboratory as the reference segment.
JOINT_VELOCITY: Relative angular velocity of one segment relative to another segment.
JOINT_ACCELERATION: Relative acceleration of one segment relative to another segment. This is the first derivative of the Joint Angular Velocity