You create segments from the markers you reconstructed previously (see Capture and reconstruct the marker set) using the Labeling Template Builder in the Subject Preparation tab of the Tools pane. Typical segments you would create for a live subject include Pelvis, Head, Femur, etc.
You create the segments for your template by grouping markers that move in roughly the same way. For example, if you have three markers on the shank, one at the knee joint, one half-way down the shank and one at the ankle joint, these three markers can be considered to be attached to the same segment (known as the TIBIA segment).
You can also consider markers that are placed close to a joint to be part of both the proximal and distal segment. For example, a marker placed on the anterior superior iliac spine (ASIS) can be considered to be part of both the pelvis and the thigh segments, even though it is actually attached to the pelvis segment.
Ideally, a segment should be defined by three or more markers: one to define the origin, one to define the primary axis of the segment and one to define the secondary axis. If the segment is the child segment of another segment, the secondary axis marker can be omitted. In addition, any number of additional markers that do not specifically define the segment's axes can be added.
To create segments:
- Go to the Subject Preparation tab of the Tools pane .
- In the Labeling Template Builder section, click in the Create Segments field. Enter a name for the first segment, for example Pelvis, and then click Create.
Segment names can contain spaces, and if they are preceded by L or R; or Left or Right (followed by a space), they are automatically colored appropriately if you are using the default color scheme.
The mouse pointer changes to a cross-hair with the label Select the Origin Marker.
In the 3D Perspective view, click on the marker that defines the origin of the segment you are creating. For the Pelvis, this could be the LASI or the RASI marker.
Choose the origin marker for distal segments to be on a more proximal segment. However, remember that the purpose of the labeling skeleton template is to define a labeling skeleton that is suitable for labeling. The segments are related to marker positions, not to joint centers as in a biomechanical model.
Nexus highlights the selected marker. The mouse pointer label changes to Select the Primary Axis Marker.
- Click on the marker that defines the primary or long axis of the segment you are creating. For the Pelvis, this could be the unused LASI or RASI marker.
The marker is highlighted and the mouse cursor label changes to Select the Secondary Axis Marker
- Click on the marker that defines the secondary axis. The marker is highlighted and the mouse cursor label changes to Select an Additional Marker.
This marker will fully define the segment by requiring the primary axis and the secondary axis marker to be in the same plane. For the Pelvis, this could be the SACR marker.
- Click on the chosen marker.
In the same way, select any other markers that you want to associate with the same segment.
If you have problems selecting a marker, it may be because the selection operation selects something else, like the bounding box of a segment. To turn off the visualization of the segment, clear the Subjects option in the Options dialog box.
When you have finished creating the segment, click again on the Create button.
In the 3D Perspective view, the selected markers are now displayed in gold. The segment's axes are displayed as X=red, Y=green and Z=blue. By convention, the Z axis is the primary axis and the secondary axis marker defines the YZ plane.
If, at any point in the steps above, you make a mistake (for example, you click on the wrong marker), you must finish creating the segment first and then use the Undo operation to remove the segment. You can do this by either using the CTRL+Z shortcut or clicking the Undo button on the Nexus toolbar. You can then recreate the segment.
Repeat the above steps to define all segments.
If possible, make the origin marker of each new (proximal) segment on the distal segment at a 'joint'.
All markers must be part of at least one segment.
When creating child segments, remember that the markers you select will need to be linked using an appropriate joint type. It is a good idea to look ahead to the next procedure (see ) to familiarize yourself with the different joint types now, so that you select the appropriate markers for segments in this step. For example, a free joint is typically used to link two segments that do not share any markers, and a ball joint is used to link two segments that share one marker. So when defining child segments, for example, a thigh segment that you intend to link to the pelvis segment using a ball joint, you must use one of the pelvis segment's markers as the origin marker of the thigh segment. In the standard gait marker set, this would mean that you would use the LASI marker as the origin, and the LKNE and LTHI markers as your other two markers for the left thigh segment. Do not worry that the LASI marker is not actually positioned on the thigh segment. Remember that the aim is to define a labeling skeleton template that helps the labeler produce the required labeling, not one that is anatomically correct.
Tips for creating segments
When you select the markers to define a segment, keep the following basics in mind:
- One marker defines the location of one point, but not a whole segment
- Two markers define a line, with length and direction.
- Three non co-linear markers define a complete segment as well as a plane
- Four markers provide extra information which enables occluded markers to be re-created based on visible markers
- One or more of the points used to determine the location and attitude (orientation) of a segment may be derived from a neighboring segment. In fact, if the positions and attitudes of the neighboring segments are fully determined, and the degrees of freedom of the joints are defined, it may be possible to determine the location and attitude of a segment without any markers being attached to it.
You can choose any marker within the segment to be the origin of that segment, and any set of mutually perpendicular directions as the axes. However, there are a few guidelines which simplify these choices.
- The majority of models in biomechanics involve linked segments which form a chain, such as Pelvis-Femur-Tibia-Foot, and such models describe the movements of each segment relative to the more proximal segment.
- The auto-labeler will perform better if the skeletal topology defined in the template file roughly matches the real anatomical structure. Therefore, Vicon strongly recommends that you define the labeling skeleton template using segments that are as close to the real anatomy as possible.
The following are additional tips and recommendations to help you in defining segments:
- Start by defining the root segment (the most proximal) and work your way towards the leaf segments (the most distal ones). Often, the root segment will be the pelvis or the thorax segment when defining a human labeling skeleton template, but it could be any segment.
- When defining child segments, remember that they can share markers with the parent segment. For example, the anterior superior iliac spine (ASIS) marker can be used to define the pelvis segment, but can also be the origin marker of the femur segment. Similarly, a marker placed on knee can be used as the primary axis marker for the femur segment, but also the origin marker of the shank segment.
- Where possible, align a segment's primary axis with the biomechanical long axis. Where there is no obvious long axis (as in the pelvis or trunk segment), the long axis can be defined using any two markers.
- If two markers are assigned to the same segment and the distance between the two markers will change significantly during the trial, the labeler will not perform as well. Markers placed on the top of the shoulders, for example, can be part of the trunk segment if the intended movement to be captured does not involve a lot of shoulder movement. Conversely, if the shoulders are likely to move significantly relative to the trunk, these markers should define the primary axes of clavicle segments, the origin of which can be a marker placed at the clavicular notch.
- If you are creating a segment with restricted movement, for example, finger markers, it may make sense for your segment to consist of only two markers.
Remember that the purpose of the labeling skeleton template is to define a skeleton that is suitable for labeling. The segments are related to marker positions, not to joint centers as in a biomechanical model.