The purpose here is to create a landmark defining the location of a segment endpoint relative to 3 tracking markers (or landmarks).
In this example, we will compute midpoint between the medial and lateral knee markers, LEFT.KNEE.LATERAL and LEFT.KNEE.MEDIAL and track it with three tracking markers on the thigh.
First compute the location of the joint in the standing trial as the midpoint between a medial and lateral knee markers. Switch to the Landmarks TAB in Model Builder Mode and Select Add New Landmark. Enter in the following:
Landmark Name= Left Knee Static
Starting Point= LEFT.KNEE.LATERAL
Ending Point= LEFT.KNEE.MEDIAL
Offset Using the Following ML/AP/AXIAL Offsets= 0.5
Offset by Percent Checked
Calibration Only Landmark Checked There should now be a landmark, Left Knee Static at the midpoint between the two markers LEFT.KNEE.LATERAL and LEFT.KNEE.MEDIAL. These two markers are normally removed after the standing trial because these markers typically have substantial movement artifacts during the movement trials.
Since the medial and lateral markers are removed for the motion trial, the midpoint between the two markers LEFT.KNEE.LATERAL and LEFT.KNEE.MEDIAL.will not be created unless we define a landmark that is tracked by the thigh markers. To create a Landmark at the Knee Joint that is defined relative to 3 tracking markers (LEFT.THIGH1, LEFT.THIGH2, LEFT.THIGH3) rather than the two static markers, enter in the following:
Landmark Name= Left Knee Joint
Starting Point= LEFT.THIGH1
Ending Point= LEFT.THIGH2
Lateral Object= LEFT.THIGH3
Offset to Existing Calibration Target or Landmark= Left Knee Static
Offset by Percent Not Checked
Calibration Only Landmark Not Checked This Landmark will now be tracked by the 3 landmarks attached to the thigh.