Normalized Joint Angle - Method 1 illustrated how to align a Virtual Foot Segment precisely with the Segment Coordinate System for the Shank. The example on this page shows how to create a virtual foot that is aligned with the lab floor. This example uses Tutorial2.cmo from the Visual3D Tutorial Files page. Download this file and follow the steps below. Tutorial2.cmo

Open the Tutorial2.cmo file, click on the Model tab. The model has been completely constructed.

The first task is to project the markers that were used to define the foot onto the same plane (for convenience we will use the floor of the laboratory). To do so, we must create some laboratory landmarks.

Click on the Model tab and click on the Landmark Tab. Follow these steps to create these landmarks: Lab_Origin, Lab_X, and Lab_Y.

1. Create Lab_Origin landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: Lab_Origin
   Define Orientation Using:
   Existing Segment: Lab
3. Offset Using the Following ML/AP/AXIAL Offset: X= 0 , Y= 0 , Z= 0
4. DO NOT Check: Offset by Percent (1.0 = 100%)
5. DO NOT Check: Calibration Only Landmark
   

2. Create Lab_X landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: Lab_X
   Define Orientation Using:
   Existing Segment: Lab
3. Offset Using the Following ML/AP/AXIAL Offset: X= 0.1 , Y= 0 , Z= 0
4. DO NOT Check: Offset by Percent (1.0 = 100%)
5. DO NOT Check: Calibration Only Landmark
   

3. Create Lab_Y landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: Lab_Y
   Define Orientation Using:
   Existing Segment: Lab
3. Offset Using the Following ML/AP/AXIAL Offset: X= 0 , Y= 0.1 , Z= 0
4. DO NOT Check: Offset by Percent (1.0 = 100%)
5. DO NOT Check: Calibration Only Landmark
   

In this step, we will create 3 landmarks (RLA_Floor, RMA_Floor, and RFT1_Floor) that are the projection of the 3 markers used to define the foot (RLA, RMA, and RFT1) onto the floor. We will use the newly created Lab landmarks in Step 1 to project the foot markers on the floor.

1. Create RLA_Floor landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: RLA_Floor
   Define Orientation Using:
   Starting Point: Lab_Origin
   Ending Point: Lab_X
   Lateral Object: Lab_Y
   Projection From: RLA
3. Check: Calibration Only Landmark
   

2. Create RMA_Floor landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: RMA_Floor
   Define Orientation Using:
   Starting Point: Lab_Origin
   Ending Point: Lab_X
   Lateral Object: Lab_Y
   Projection From: RMA
3. Check: Calibration Only Landmark
   RMA Floor Landmark Normalized Joint Angle Method 2.png

3. Create RFT1_Floor landmark:

1. Click Landmarks button
2. Click Add New Landmark button
   Landmark Name: RFT1_Floor
   Define Orientation Using:
   Starting Point: Lab_Origin
   Ending Point: Lab_X
   Lateral Object: Lab_Y
   Projection From: RFT1
3. Check: Calibration Only Landmark
   RFT1 Floor Landmark Normalized Joint Angle Method 2.png

The following method will not set the Ankle Joint Angle to Zero degrees in the standing trial, but it will create a segment coordinate system that is flat to the floor, which is convenient for describing the angle of the foot segment relative to the laboratory, and also produces an ankle joint angle that is close to zero in the standing posture (if the shank is vertical, the angle will be zero).

The following definition will create the Segment Coordinate System for the Virtual Foot Segment aligned parallel to the floor. This can be accomplished by using the projected landmarks just created in Step 2.

Create Right Virtual Foot Segment:

1. In the Segments tab, type Right Virtual Foot 1 in the Segment Name box.
2. Check Kinematic Only
3. Click on the Create Segment button.
4. In the Right Virtual Foot tab, enter these values:
   
   Define Proximal Joint and Radius
   Lateral: RLA_Floor Joint: None Medial: RMA_Floor
   Radius: numbers grayed out
   
   Define Distal Joint and Radius
   Lateral: RFT1_Floor Joint: None Medial: None
   Radius: 0.06
   
   Extra Target to Define Orientation
   Location: None None
   Select Tracking Targets:
   Check RFT1, RFT2, RFT3
   
5. Click on Build Model. Since it is a Kinematic Only segment, no new image will be appear.
6. Click on Close Tab before proceeding.
   Right Virtual Foot Segment Normalized Joint Angle Method 2.png

Note that the segment coordinate system is parallel to the floor with the z-axis in the plane of the floor rather than vertical (e.g. aligned more-or-less with the shank coordinate system). To resolve this, we need to rotate the coordinate system.

Rotate Segment Coordinate System:

1. In the Right Virtual Foot segment, click on the Segment Properties tab.
2. Click on Modify Segment Coordinate System
   Select A/P Axis= +Z
   Select Distal to Proximal= -Y
3. Click Ok

The coordinate system is now aligned with z-axis vertical.

Let's plot both ankle joint angles (shank with respect to foot and shank with respect to virtual foot) to see the difference.

1. Create the angles

Create Right Ankle Angle - Right Foot with respect to Right Shank:

1. In the Model menu, select Compute Model Based Data.
2. In the dialog enter the following:
   Data Name: Right Ankle Angle
   Model based Item Properties: Joint_Angle
   Segment: Right Foot
   Reference Segment: Right Shank
3. Click on Create
4. Click on Close before proceeding.
   Right Ankle Angle Normalized Joint Angle Method 1.png

Create the Normalized Right joint angle - Right Virtual Foot with respect to Right Shank:

1. In the Model menu, select Compute Model Based Data.
2. In the dialog enter the following:
   Data Name: Right Ankle Angle Virtual Foot 1
   Model based Item Properties: Joint_Angle
   Segment: Right Virtual Foot 1
   Reference Segment: Right Shank
3. Click on Create
4. Click on Close before proceeding.
   Right Ankle Angle Virtual Foot Normalized Joint Angle Method 2.png

2. Graph the Angles for Comparison

Now let's graph the two angles and compare their signals.

Graph the Right Ankle Angle and Right Ankle Angle Virtual Foot

1. Graph the Right Ankle Angle
   1. In the Signal and Events tab, make the Walking Trial 1.c3d active
   2. Select the Link_Model_Based to expand the folder
   3. Right-click on the Right Ankle Angle to bring up the submenu
   4. From the submenu, select Graph X,Y, and Z to plot all three components of the right ankle angle.
   5. Another submenu will appear, select New Graph to plot all 3 components in the pane to the right.
2. Add the Right Ankle Angle Virtual Foot 1 to the Graph
   1. In the Signal and Events tab, select the Link_Model_Based folder.
   2. Right-click on the Right Ankle Angle Virtual Foot 1 to bring up the submenu
   3. From the submenu, select Graph X,Y, and Z to plot all three components of the right ankle angle virtual foot.
   4. Another submenu will appear, select Add to Existing to add all 3 components to the existing graph.

The top graph represents ankle dorsi/plantarflexion angle. Two signals are shown. The top most signal is right ankle angle and the bottom signal is the normalized angle angle (wrt to virtual foot).

For another normalization method, go to Normalized Joint Angle - Method 1. In addition, there is a great tutorial Tutorial: Foot and Ankle Angles that discusses normalization.