Before building a model, it is important to understand how a Segment Coordinate System (SCS) is created in Visual3D.

To create a SCS:
1) The user must specify targets to identify the distal/proximal end of a segment, using these targets V3D will:

A) Define the anatomical (or frontal) plane & identify the joint centers that lie on the anatomical plane

B) Align the inferior/superior axis of the SCS between joint centers (the inferior/superior axis will lie along the anatomical plane)

2) Visual3D must create unit vectors to define the SCS

The targets chosen to define the distal/proximal ends of the segment will affect the location and orientation of the SCS.

This information can also be downloaded as a power point from here.
More information regarding segment coordinate systems can be found on the C-Motion YouTube channel.
The YouTube Tutorial on Segment Definition creates several coordinate systems for the shank segment using various targets to define the proximal and distal ends of the segment.

In this tutorial the following target labels are used:

Please note that this is not a recommended marker set, and this wiki page does not discuss tracking targets. This page only describes the various options within Visual3D to define a segment using anatomical landmarks.

Although Visual3D is marker set independent, a conservative full body marker set and various modifications are described here.

There are many possible options to define a segment. Visual3D will use the specified targets to define the anatomical plane and the joint centers. Depending on the specified targets, Visual3D will use different methods to define the segment joint centers.

The location of the segment joint centers is important because Visual3D defines the inferior/superior axis of the SCS along the vector between the joint centers. The orientation of the anatomical plane is important because the anterior/posterior axis will be perpendicular to the plane defined by the targets.

If the joint center is specified by the user, Visual3D will use this location. If the joint center is not specified, Visual3D will need to locate it.

Four options to define a SCS are:

= RadiusA vs RadiusB =

RadiusA - When the joint center landmark is the only target used to define the proximal or distal end, it is necessary to enter the segment radius, but this value will not be used to define the joint center (because it is already defined).
RadiusB - When a lateral or medial target is the only target used to define the proximal or distal end, it is necessary to enter the segment radius, and this value will be used to define the joint center.

= Targets vs Landmarks =

In this document, referencing “targets” indicates motion capture markers and “landmarks” indicate virtual markers.

In this document, there is not a distinction between the definition of a landmark, so the term “landmark” could refer to Functional Landmarks, Digitized Landmarks, relative landmarks, etc.

In this example, three border targets are used. So the frontal plane is simply the plane defined by the three targets.

In this example, four border targets are used. A plane is defined by three points, so when four targets are used, the frontal plane is fit between the four targets using a least square fit. The least squares fit is applied such that the sum of squares distance between the targets and the frontal plane is minimized.

In this example, three border targets are used. So the frontal plane is simply the plane defined by the three targets.

In this example, three border targets are used. So the frontal plane is simply the plane defined by the three targets.

The Inferior/Superior is aligned with the vector which is drawn from the distal joint center to the proximal joint center.

By default, the inferior/superior axis is the Z-Axis (drawn in blue), but different labs may use a different conventions.
For the four methods listed above, the inferior/superior axis of the segment coordinate system will be aligned with the vector “m” drawn in the images below:

1) Method 1: SegmentDefinition_ProxJC_DistLatMed_ppt_M.jpg{{/images/thumb/b/b9/SegmentDefinition_ProxJC_DistLatMed_ppt_M.jpg/250px-SegmentDefinition_ProxJC_DistLatMed_ppt_M.jpg Method_22) Method 2: SegmentDefinition_ProxLatMed_DistLatMed_ppt_M.jpg]] Method_44) Method 4: SegmentDefinition_ProxLat_DistLat_ppt_M.jpg{{/images/thumb/d/d8/SegmentDefinition_ProxLat_DistLat_ppt_M.jpg/250px-SegmentDefinition_ProxLat_DistLat_ppt_M.jpg?250×231]]