To identify subjects and to process their motion data, Vicon Nexus requires information, such as a description of the generic relationship between segments and joints and the Vicon markers for a certain type of subject, such as a human being. This information, which identifies the markers that Nexus is to track and how these markers are connected to the underlying segments, is stored in the following file types:

VST – labeling skeleton template (.vst file). Describes the generic relationship between physical markers attached to a subject and the skeletal structure to which the markers are attached for a certain type of subject, for example, a human being.
VSK – labeling skeleton (.vsk file). Describes the relationship between physical markers and the skeletal structure to which the markers are attached for a specific subject of the type described in the associated VST. For example, if the VST represents a human being, the VSK contains the marker arrangement and skeletal structure calibrated for an individual person.

The workflow for creating and using these files can be summarized as follows:

Creating a VST is a one-time task. You create a template once for a specific marker set or a general type of labeling skeleton, such as a human being. One generic labeling skeleton template (VST) can be used on many different sizes of subjects (from someone very short to someone very tall), all wearing the marker set described in the VST.
You then calibrate the VST to a specific subject at the beginning of each capture session (but not before every trial).Subject calibration updates the generic relationships from the VST with subject-specific information, such as the length of the segments or the distance between specific markers for the subject, which is saved to a labeling skeleton (VSK) for the subject.

You only need to recalibrate if the subject changes, for example, if a different subject wears the same marker set, or if the markers are moved.

Important
The labeling skeleton template (VST) is used only for labeling, so the aim is to define a labeling skeleton template that labels movement trials reliably, not one that is anatomically correct.

For more information, see:

When is a custom labeling skeleton template needed?

A labeling skeleton template defines a marker set and enables Nexus to perform automatic labeling. In many situations, the standard templates for Plug-in Gait that are supplied with your Nexus installation will produce the required labeling. However, in some circumstances, you may need to create your own custom template to provide additional information to Nexus. Examples of situations when you may need to do this include:

When you want to include segments that are not present in Plug-in Gait, such as hand models
When you want to include additional marker placements, such as marker clusters
When you want to track objects, such as a golf club
When you want to track non-human subjects, such as a horse

After you have created a labeling skeleton template, you can use it to automatically process markers and obtain labeled trajectories for any subjects that use the same marker set.

Important
A labeling skeleton template (VST) defines a marker set and enables Nexus to perform automatic labeling. It is not a biomechanical model that will output valid joint angles or other kinematic/kinetic variables. To compute valid kinematics or kinetics, you must either use a pre-defined biomechanical model (such as Vicon Plug-in Gait, which involves running specific pipelines in Nexus) or create your own model using Vicon BodyBuilder, MATLAB, Python, or Vicon ProCalc.

Labeling skeleton template contents

The labeling skeleton template (VST) contains the following information:

The names of the markers in the marker set (e.g. LASI, RASI)
The skeletal structure of the subject you are tracking (e.g. the Femur connected to the Pelvis, the Tibia connected to the Femur)
The relationship between the marker set and the skeletal structure (e.g. the LASI marker is attached to the Pelvis segment)
The type of joints that connect the segments to other segments (e.g. ball joint, free joint)
Other properties, such as segment, marker and joint names and colors, and how the markers are displayed in the 3D Perspective view.

Requirements for creating a custom labeling skeleton template

For the best results, plan your initial trial, in which you will capture the marker set for the custom labeling skeleton template, to meet the following requirements:

Avoid occlusion (hidden markers). For example:
- Do not include more than one subject in the volume.
- Ensure that the subject can complete the required movement without assistance, or the use of crutches or other mobility aids, or that, if these are necessary, they do not hide the markers from the cameras.
Ensure that the base pose (the subject's body position that is used for initial labeling) is easily reproduced by the subject.

In addition, ensure that the labeling skeleton template (VST) is built using the recommendations given in this guide, specifically that it:

Is built using the Labeling Template Builder in Nexus 2, as described in this guide.
Has at least three markers per segment

Note
If you have created your own labeling skeleton template outside of Nexus, these instructions may still be useful, but you may need to make some adjustments to suit your particular template.

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