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Virtual segments provide a convenient way to include multiple definitions for a segment within a single model, but these multiple definitions do not easily allow for external constraints (like an Inverse Kinematic chain) to be applied consistently across all segment definitions. For example, if a Virtual Segment is defined with relation to a segment and then an Inverse Kinematics constraint is applied to the original segment, the changes in the original segment's pose estimation do not propagate to the Virtual Segment.

Shadow segments resolve this issue by allowing that segment's pose to be defined as a transformation of the original segment's pose estimation. Users can use the Expression function Pose_4x4 when defining their own shadow segments.

This tutorial will demonstrate how to create custom shadow segments and how they can be used.

The shadow segment must be defined based on a pre-existing segment therefore, before continuing ensure you have a model with a already defined segment that you wish to shadow. More information on segments and how they can be defined can be found here.

Once a segment has been created, the shadow segment can be created the same way as any other segment in the Segments tab of the model builder. Type in your desired shadow segment name (i.e. Shank_Shadow) and change the segment type to shadow. Select Kinematic only and press create. When using a shadow segment, it should not be used in the kinetic chain as they are primarily reference segments rather than physical body segments. As they don't have any mass or inertial properties they should not be included in the kinetic chain.

Once created, you will be automatically taken to the new shadow segment tab. First select the segment you wish to shadow (The pelvis will be used for this example). Now, you must define the 4×4 rotation which is the heart of defining the shadow segment. More information on this command can be found Here. The following shadow pelvis would adjust the X coordinate rotation by 10 degrees. The first three zeros are for translational changes and the next three are rotational. The next conditions are just further defining the new segment with 0.1 chosen for the radius as it is a common default value.

You should now be able to see your shadow segment in your model. In this example the new segment is rotated 10 degrees about the segment X-axis from the original.

Shadow segments are often used in comparison to the anatomical results. Due to shadow segments not being part of the kinetic chain only kinematic analysis can be done on these segments.

Here are some samples of analysis that can be done on a shadow segment:

Joint Angles

Joint Velocity