Landmarks are like virtual markers, whose positions are computed automatically by Visual3D based on the positions of the actual markers, segments, or equations, according to rules which you define. Landmarks are created by either specifying a location in a segment coordinate system or by transforming markers or other landmarks into a significant location. These landmarks are like virtual markers, whose positions are computed automatically by Visual3D based on the positions of the actual markers, segments, or equations, according to rules which you define.

The landmark mechanism is the key to Visual3D’s highly flexible modeling capabilities. A single example suffices to suggest the usefulness of markers. Frequently, target markers are mounted on short posts which protrude away from the skin surface, and consequently, defining a segment using medial and lateral markers as described above will tend to over-estimate the segment’s radii. You can eliminate this problem by creating landmarks which are like “shadow versions” of the actual markers, moved inward by the length of the marker posts to better approximate the skin surface, and then define your segments based on these landmarks.

Landmarks can be 'calibration only' or can be calculated for the motion files.

1. Calibration Only: These landmarks are used only for segment calibration, but are not needed (or produced) for any associated motion file(s).
2. Generated for Assigned Motion Files: These landmarks are generated for both the calibration file and the associated motion files. These landmarks may be used for tracking a segment.

The landmark dialog can be broken into 3 sections:

1. Define Orientation
2. Offset
3. Calibration Only

The following examples use the markers related to the left shank illustrated below.

1. LMK: Left medial knee
2. LLK: Left lateral knee
3. LSK: Left shank
4. LMA: Left medial ankle
5. LLA: Left lateral ankle
6. LTUB: Left tibial tuberosity (shown projecting a shadow onto the plane)
7. LKJC: Left knee joint centre (landmark)

Landmarks can be defined relative to a target, along a line, on a plane, or as a projection onto a line or a plane. Landmarks can be defined in lab space or in a segment coordinate system.

Any type of landmark can be defined using offsets in three directions, a user has the option to define the offsets manually (X/Y/Z or ML/AP/AXIAL) or they can use the “Offset to Existing Calibration Target or Landmark” option.

The Offset to Existing Calibration Target or Landmark option allows users to get the location of a target (or landmark) relative to a plane or segment coordinate system. The offsets are populated automatically. This option can be used any time all three offsets can be specified, but is described as a special type for Type 4 since this is a crucial example.

Offsets should be specified in meters unless “Offset by Percent” is checked. It is possible to specify offsets as Expressions

When a landmark is set to “Calibration Only” it means that the landmark will only exist in the calibration file. When this option is not checked, it means that the landmark will be created in the associated motion files. If a landmark is created in the associated motion files, it can be used to track a segment or for other calculations.

Although there are many different ways to define a landmark, all of these methods result in a “virtual marker” whose position is calculated by Visual3D throughout motion trials. These landmark types include:

See the Landmark Types page for a detailed explanation of each definition type.

Landmarks can be created relative to a plane or technical coordinate system.

Sample Landmark Definition (click to expand)

1. Create LAND: Click Landmarks button Click Add New Landmark button Create Landmark: LAND

Landmark Name: LAND    Define Orientation Using:    Starting Point: P1    Ending Point: P2    Lateral Object: P3

Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.01    Y: 0.02    Z: 0.01 Do NOT Check: Offset by Percent (1.0 = 100%)

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

Other Examples:

• Digitized Landmarks are stored and tracked relative to tracking targets
• Functional Landmarks (Joints) are stored and tracked relative to tracking targets
• Muscle insertion points are stored and tracked relative to tracking targets

Create a landmark relative to three targets. The offset of the landmark is defined using the location of a target relative to the three targets during the static trial.

Landmark Definition (click to expand)

1. Create Relative_Tracking: Click Landmarks button Click Add New Landmark button Create Landmark: Relative_Tracking

Landmark Name: Relative_Tracking    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Lateral Object: LSK

Offset to Existing Calibration Target or Landmark: LKJC Do NOT Check: Offset by Percent (1.0 = 100%)

Example 1 (click to expand)

To define a landmark as a point on a plane defined by 3 points * Select the Targets and/or Landmarks radio button * Select three targets and/or landmarks for the Reference/Axial Starting Point, Axial Ending Point and lateral object. The landmark/target reference offset is also selected. The RKnee calibration only landmark is the point that is being tracked by 3 target markers on the thigh RTH1, RTH2, and RTH3.

Other Examples:

• Create a Landmark Using two standing trials
  • Example 1
  • Example 2
• Example using a reference location
• Example defining a segment endpoint relative to 3 tracking targets

A target or landmark may be projected onto a line defined by two points.

Landmark Definition (click to expand)

1. Create Project_Line: Click Landmarks button Click Add New Landmark button Create Landmark: Project_Line

Landmark Name: Project_Line    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Project From: LTUB

Do NOT Check: Offset by Percent (1.0 = 100%)

Project the heel target onto the line defined by the mediolateral ankle targets (click to expand)

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. 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.

Other Examples:

• Project onto a line

A point can be projected onto a plane defined by three points.

Landmark Definition (click to expand)

1. Create Project_Plane: Click Landmarks button Click Add New Landmark button Create Landmark: Project_Plane

Landmark Name: Project_Plane    Define Orientation Using:    Starting Point: LLK    Ending Point: LMK    Lateral object: LMK    Project From: LTUB

Do NOT Check: Offset by Percent (1.0 = 100%)

Example: Project the iliac crest target onto the plane defined by the R/LASIS and RPSIS (click to expand)

By projecting multiple landmarks onto the same plane you can build segment coordinates systems in a specific plane. 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.

Other Examples:

• Project the hip joint onto a plane passing through the GT's
• Example 2: Project onto a principal plane of the lab

Digitized landmarks are defined using a digitized pointer and again result in a “virtual marker” being computed by Visual3D relative to the positions of other, existing markers. The goal of digitized landmarks is to allow users to track anatomical landmarks without having to place markers on that location. See instructions for using a digitizing pointer to learn more.

Functional landmarks indicate the locations of functional joints. Calculating functional joint positions requires movement of one segment relative to another segment. The chosen algorithm then searches for a point (or for a one degree of freedom joint, an axis) that is stationary relative to the 2 segments (or 2 sets of markers).

Visual3D's algorithms for computing functional joint positions are adapted from:

Schwartz MH, Rozumalski A (2005) A new method for estimating joint parameters from motion data. Journal of Biomechanics, 38, 107-116] and from 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, in press] See the in-depth explanation of functional joints to learn more.