Landmarks provide a powerful method of representing significant locations within space or locations defined by the model.

There are several landmark types or ways to create landmarks. Landmarks can be created:

1. as a point relative to one marker/landmark
2. as a point on a Line
3. as a point in a Plane
4. as a point Projected on a Line
5. as a point Projected on a Plane
6. as offsets (using regression equations or relative to a segment)
7. relative to three points
8. digitized with a pointer
9. as a functional point

The simplest landmark is a landmark which is defined in a segment coordinate system. There are many variations of this but a landmark can be defined in lab space, in a segment coordinate system, or by a 4x4Rotation Signal. The definition's offsets define where the landmark is created relative to the coordinate system.

This landmark is defined in Lab space, or the laboratory coordinate system.

As an example definition of a landmark relative to specific axes in the laboratory coordinate system:

1. Click Landmarks button
2. Click Add New Landmark button
3. Create Landmark: Relative_Axes
   1. Landmark Name: Relative_Axes
   2. Define Orientation Using:
   3. Existing Segment: Lab
   4. Offset Using the Following ML/AP/AXIAL Offsets:
      1. X: LAJC::X
      2. Y: LAJC::Y
      3. Z: 0.0
   5. Do NOT Check: Offset by Percent (1.0 = 100%)

Landmarks can also be created as an offset from the specified marker in a segment coordinate system or in the LAB coordinate system. In the dialog below the vRLK landmark is created from RLK as an offset in the x direction in the LAB coordinate system.

Other Examples include:

• Virtual Lab Tutorial: The LAB_O, LAB_X, and LAB_Y landmarks in the virtual lab tutorial are defined in LAB space, where the offsets refer to the distance the landmarks are created in meters.
• The following two examples describe how to define landmarks that represent stair corners. The position of targets placed on the steps are calculated from dynamic trials is used to define the landmarks.
  • Using Motion Trial Example 1
  • Using Motion Trial Example2

Landmarks can be created relative to a segment, i.e., in the segment coordinate system.

As an example definition relative to a segment:

1. Click Landmarks button
2. Click Add New Landmark
3. Create Landmark: Relative_Segment
   1. Landmark Name: Relative_Segment
   2. Define Orientation Using:
   3. Existing Segment: Left Shank
   4. Offset Using the Following ML/AP/AXIAL Offsets:
      1. X: 0.0
      2. Y: 0.0
      3. Z: -1
   5. Check: Offset by Percent (1.0 = 100%)

Other examples include:

• Create a landmark relative to the thigh segment using ML, AP, and AXIAL offsets as percents.
• Creating a landmark at the end of a segment that is tracked with the segment.
• Hip Joint Centers are often defined using regression equations which locate the hip joint centers in the pelvis coordinate system:
  • Bell and Brand Regression Equations
  • Hip Landmark from Custom Regression Equation
• A Knee Alignment Device (KAD) defines the joint centers by defining landmarks in the KAD segment coordinate system
• Create a Landmark Mimicking contralateral segment. This example uses several different types of landmarks, but the end result is a landmark created in the foot coordinate system.

A sample Landmark definition using a 4×4 rotation matrix:

1. Create RTH_Origin: This will create a Landmark at the position of the Rotation signal RTH4x4
2. Click Landmarks button
3. Click Add New Landmark button
4. Create Landmark: RTH_Origin
   1. Landmark Name: RTH_Origin
   2. Define Orientation Using:
   3. Existing Coordinate System: RTH4X4
   4. Offset Using the Following ML/AP/AXIAL Offsets:
      1. X: 0.0
      2. Y: 0.0
      3. Z: 0.0
   5. Check: Offset by Percent (1.0 = 100%)

Point projection onto a line can be used at the shoulder to project a lateral target on to a vertical line passing through the acromium or it can be used at the knee to project lateral or medial targets on a functional axis. To define a landmark as a point projected on a line (two other points define a line in space):

1. Select the Targets and/or Landmarks radio button;
2. Select 2 targets and/or landmarks for the Reference/Axial Starting Point and Axial Ending Point;
3. Select a point to project using the Projected From combo box.

In the image below, the RAnkleCenter is the point that is created by projecting the RHL heel marker onto the line created from the lateral and medial ankle markers RLA and RMA.

Another example can be found on the project marker onto a line example page.

By projecting multiple landmarks onto the same plane you can build segment coordinates systems in a specific plane. To define a landmark as a point projected onto a plane defined by 3 points:

1. Select the Targets and/or Landmarks radio button;
2. Select three targets and/or landmarks for the Reference/Axial Starting Point, Axial Ending Point and Lateral object to define a plane; and
3. Select a point to project using the Projected From combo box.

In the image below, the RPPinPlane is the point that is created by projecting the RPP marker on the illiac crest onto the plane created from the right and left ASIS's (RASI,LASI) and the right PSIS RPS marker.

See the example project the hip joint onto a plane passing through the greater trochanters for more detail.

Landmarks are often offsets from existing target markers. The origin of the offset is the starting point (reference point). The offset is specified as an absolute distance in meters, or as a percentage. The percentage is relative to a segment or markers.

Regression equations can be used as an offset in the landmark definition.

This is notably used when defining hip joint centres:

1. Hip Landmark: Landmarks defining the location of the hip joint center are not particularly notable except that two of the default Pelvis segment types (Coda and Helen Hayes) create hip joint landmarks automatically when the segment is created.
2. Bell and Brand Hip: Specifying the Hip Joint Landmark using Bell and Brand. This page will also illustrate how to create a generic pelvis technical coordinate system and hip landmarks.
3. Hip Landmark from Custom Regression Equation: Specifying the Hip Joint Landmark using a custom regression equation

Landmarks can be created relative to a segment, which is to say within in the segment coordinate system. To define a landmark in a segment coordinate system in the Landmarks Mode:

1. Select the Existing Segment radio button;
2. Select a segment in the existing segment combo box below the button. Segment landmarks are, by default, offset from the segment origin. If you would like to offset the landmark from a different location you must select a Reference/Axial Starting Point marker/landmark location;
3. The landmark offsets can be specified in any coordinate system that has been defined in the model. When defining a landmark relative to a segment, the 3 components of the offset are defined anatomically as ML, AP, and AXIAL boxes.

As a special example, if you would like to offset the landmark in lab space simply select the LAB segment in the existing segment combo box. When the landmark is relative to the LAB the offsets are the X, Y and Z directions of the lab.

Landmarks can be created relative to three locations, which can be handy when removing markers in the motion trials.

A right handed orthogonal coordinate system is defined by the three points (P1, P2, P3) with the origin of the coordinate system at the point P1. The frontal plane of the coordinate system is defined by the three points and the positive AP direction is into the screen.

The objective for using the pointer is to identify anatomical landmarks without having to place motion capture markers at the location.

Visual3D's functional joint calculations have been adapted from two papers:

1. Schwartz MH, Rozumalski A (2005) A new method for estimating joint parameters from motion data. Journal of Biomechanics, 38, 107-116; and
2. Jensen E, Lugade V, Crenshaw J, Miller E, Kaufman K (2016) A principal component analysis approach to correcting the knee flexion axis during gait. Journal of Biomechanics, 49(9), 1698-1704

If you want to create a landmark using a pipeline command and you already know how to create the landmark interactively in Model Builder Mode, you can save the model template to disk. Open the model template and search for the Landmark definition. You can then cut-and-paste the definition into a pipeline. The only difference will be that you must specify the Model that is to be used; e.g. the model template already defines which model is to be used to create the landmark.

Muscles] are defined by linear segments connecting a series of landmarks. These landmarks represent insertion points and are tracked relative to the trial's markers. Any of the rules for creating landmarks in Visual3D described above can be used to describe these landmarks.

It was not possible to place anatomical landmarks on the medial and lateral ankle of the subject because the subject was missing one lower limb. It was possible to place markers on the foot prosthesis consistent with the existing foot. Assuming symmetry in the two foot segments the user wanted to create landmarks representing the location of the medial and lateral ankle of the missing shank relative to the foot coordinate system consistent with the location of the medial and lateral ankle in the existing segment.

The Knee Alignment Device (KAD) is manufactured by Motion Lab Systems. The KAD is used to assist in defining the frontal plane of the thigh segment. This page describes the KAD and shows how it is implemented to create the frontal plane and landmarks in Visual3D.

Landmarks can be created from two static standing trials.

We collected 2 sets of studies for the same person, and we collected one static trial of the lower extremity for each set of the study. We forgot to add the GT markers for the first static trial. Based on what we have right now (a complete 2nd static trial and the 1st static trial with GT markers missing), is it possible for us to create a virtual GT marker for the first static trial? The answer is yes, landmarks can be created to calculate missing tracking markers.

Consider the following right thigh segment constructed using static markers RPPT, RAPT, RPDT, RADT. These tracking markers are saved to a model template, but for the next subject the static trial is missing the RAPT marker. A landmark can be created to replace the missing tracking marker.

Landmarks can be created from the motion trials.

This example creates landmarks from Targets in a movement trial but not in the standing trial. In this example, a movement trial contains 4 markers that have been placed on the corners of an object. The standing trial does not contain these markers, but needs the corner locations for defining an object (segment).

A structure (i.e. stairs) can be defined by placing markers at vertices, then connecting these vertices. It isn't always practical, however, to place these markers during the standing trial, so it would be useful to have a separate motion trial that contained only these markers. In this example, we demonstrate how to get the marker locations from a movement trial and create landmarks. In the movement trial for this example, markers are placed on the steps of a set of stairs.