One method for setting the Ankle Joint Angle to Zero degrees in the standing trial is to define the Segment Coordinate System for the Virtual Foot Segment to be aligned precisely with the Segment Coordinate System for the Shank. This can be accomplished by using the same proximal and distal targets.
This example uses Tutorial2.cmo from the Visual3D Tutorial Files page. Download this file and follow the steps below.
Open the Tutorial2.cmo file, click on the Model tab. The model has been completely constructed.
Step 1: Construct Right Shank Segment
Create Right Shank Segment:
 In the Segments tab, from the Segment Name pulldown select Right Shank.
 Click on the Create Segment button.
 In the Right Shank tab, enter these values:
Define Proximal Joint and Radius
Lateral: RLK Joint: None Medial: RMK
Radius: numbers grayed out
Define Distal Joint and Radius
Lateral: RLA Joint: None Medial: RMA
Radius: numbers grayed out
Extra Target to Define Orientation
Location: None None
Select Tracking Targets:
Check RSK1, RSK2, RSK3, RSK4


 Click on Build Model. A 3D image of the shank will appear
 Click on Close Tab before proceeding.


Create Right Virtual Foot Segment:
 In the Segments tab, type Right Virtual Foot in the Segment Name box.
 Check Kinematic Only
 Click on the Create Segment button.
 In the Right Virtual Foot tab, enter these values:
Define Proximal Joint and Radius
Lateral: RLK Joint: None Medial: RMK
Radius: numbers grayed out
Define Distal Joint and Radius
Lateral: RLA Joint: None Medial: RMA
Radius: numbers grayed out
Extra Target to Define Orientation
Location: None None
Select Tracking Targets:
Check RFT1, RFT2, RFT3


 Click on Build Model. Since it is a Kinematic Only segment, no new image will be appear. Note that the coordinate system for the right virtual foot segment is the same as the right shank segment.
 Click on Close Tab before proceeding.


Note: that having the two segment coordinate systems perfectly aligned means that the segments have identical orientation in the standing trial and hence have a joint angle of zero degrees.
Step 3: Plot Angles for Comparison
Let's plot both ankle joint angles (shank with respect to foot and shank with respect to virtual foot) to see the difference.
Create the angles
First, let's create the angles:
Create Right Ankle Angle  Right Foot with respect to Right Shank:
 In the Model menu, select Compute Model Based Data.
 In the dialog enter the following:
Data Name
Right Ankle Angle
Model based Item Properties
Joint_Angle
Segment
Right Foot
Reference Segment
Right Shank


 Click on Create
 Click on Close before proceeding.


Create the Normalized Right joint angle  Right Virtual Foot with respect to Right Shank:
 In the Model menu, select Compute Model Based Data.
 In the dialog enter the following:
Data Name
Right Ankle Angle Virtual Foot
Model based Item Properties
Joint_Angle
Segment
Right Virtual Foot
Reference Segment
Right Shank


 Click on Create
 Click on Close before proceeding.

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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
 Graph the Right Ankle Angle
 In the Signal and Events tab, make the Walking Trial 1.c3d active
 Select the Link_Model_Based to expand the folder
 Rightclick on the Right Ankle Angle to bring up the submenu
 From the submenu, select Graph X,Y, and Z to plot all three components of the right ankle angle.
 Another submenu will appear, select New Graph to plot all 3 components in the pane to the right.
 Add the Right Ankle Angle Virtual Foot to the Graph
 In the Signal and Events tab, select the Link_Model_Based folder.
 Rightclick on the Right Ankle Angle Virtual Foot to bring up the submenu
 From the submenu, select Graph X,Y, and Z to plot all three components of the right ankle angle virtual foot.
 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).
Other Normalization Methods
For another normalization method, go to Normalized Joint Angle  Method 2. In addition, there is a great tutorial Tutorial: Foot and Ankle Angles that discusses normalization.