Tutorial: Foot and Ankle Angles

Joint Angles are conceptually challenging to present concisely and accurately.

A joint angle is the relative orientation of one segment relative to another segment, and mathematically is often represented as a 3x3 rotation matrix.

The most common way to present the joint angle, however, is to parse this 3x3 rotation matrix into 3 components using an Euler sequence. The conceptual challenge is that there are 16 different Euler sequences by which this 3x3 rotation matrix can be parsed, and each sequence produces 3 different component values for a given 3x3 rotation matrix.

It seems intuitively clear that there must be a standard choice of the correct sequence for a given anatomical joint, but unfortunately, this is not the case.

Preparing for the Tutorial

1) Ensure that version 3.0 or later of Visual3D has been downloaded and installed.
2) Download the cmo file with the standing calibration file and model template from the website: Tutorial1.cmo
3) Launch the Visual3D program from the Start menu.
The program will open to the main workspace.

Purpose

1. Discuss the minimal marker placement for a single segment foot

Foot Segment - Marker Placement

2. Discuss the right-hand rule and its application to defining joint angles

Ankle Angle Explained

3. Create a simple foot definition which can be used for kinetic calculations

Kinetic Right Foot Segment

3. Use three different methods to define the foot for kinematic calculations:

Method 1 - Heel to Toe

Method 2 - Normalized to proximal segment

Method 3 - Projected landmarks

Discussion

There are many ways to define the foot. With a simple single segment foot, two feet are often used.

1) The first foot is used for kinetic calculations, with the proximal joint is define at the ankle.
2) The second foot (often referred to as Virtual Foot) is used for kinematic calculations. The segment coordinate system of the kinematic foot is defined in such a way that the joint angle has a more clinically relevant meaning.

There is no set definition of neutral ankle angle, but neutral is approximately when the foot is flat on the floor and the shank segment is vertical. Since the angle joint and toe target are not parallel to the floor, an initial offset is introduced in the ankle angle. The segment coordinate system of the kinematic foot is defined to remove this initial offset and create a more clinically relevant ankle joint angle.

This tutorial will explain 3 ways to define a kinematic only foot. Keep in mind, there is no default Visual3D foot definition, and the correct definition is dependent on your laboratory's definition of neutral.

Note that although segments are defined using different proximal and distal landmarks, all segments are tracked using the same targets (RFT1, RFT2, RFT3). Also, since these feet are kinematic only, the radius is irrelevant and can be set to 0.1.

Foot Segment - Marker Placement

This tutorial presents a one segment foot model: CA(FCC),SMH(FM2),VMH(FM5),VMB(FMT)

The minimal useful marker set is as follows: CA(FCC),SMH(FM2),VMH(FM5)

The placement of the calcaneous marker is then very important. The height of the calcaneous marker relative to the height of the toe marker defines dorsi-plantar flexion in the standing posture. Medial lateral placement of the calcaneous marker is important because the sagittal plane of the foot is defined by the calcaneous marker, the toe marker, and the virtual ankle center.

CA(FCC) ^{:p. 160} = Posterior Surface of Calcaneus

ST(FST)^{:p. 160} = Sustentaculum Tali of Calcaneus

SMH(FM2)^{:p. 160} = Head of 2nd Metatarsus

VMH(FM5)^{:p. 160} = Head of 5th Metatarsus

VMB(FMT)^{:p. 160} = Tuberosity of 5th Metatarsal

PM(PM6)^{:p. 160} = Proximal Medial Phalanx

FMB = Base of First Metatarsal

SMB = Base of Second Metatarsal

• As previously mentioned, Visual3D does not have a default marker set or segment definition. It is therefore important to keep in mind that the marker set and segment definitions described in this tutorial as solely provided as an example. There are a number of alternatives for marker placement and segment definition.

Create a Kinetic Right Foot Segment

This is a simple representation of the foot that is adequate for many of the Kinematic and Kinetic calculations in Visual3D. It is not, however, adequate for the calculation of the ankle joint angle. If using the sample data provided in the beginning of this tutorial, the Right Foot is defined in this way.

1. Create the Right Foot Segment: In the Segments tab, select Right Foot in the Segment Name box. Click on the Create Segment button. In the SRight Foot tab, enter these values:    Define Proximal Joint and Radius    Lateral: RLA     Joint: None     Medial: RMA         Define Distal Joint and Radius    Lateral: RFT1     Joint: None     Medial: None         Radius: 0.06    Select Tracking Targets:      RFT1, RFT2, RFT3 Click on Build Model. A 3D image of a foot will appear distal to the shank. Click on Close Tab before proceeding.

Virtual Foot Method 1 - Heel to Toe

This method will use the heel and toe targets to define the proximal and distal ends of the foot.

Creating the Ankle and Toe Joint Centers

In this tutorial we consider the Ankle Joint Center to be the distal end of the shank segment:

1. Create Right Ankle Joint Center (RAJC): Click Landmarks button Click Add New Landmark button Create Landmark: RAJC    Landmark Name: RAJC    Define Orientation Using:     Existing Segment: Right Shank Offset Using the Following AXIAL Offset: -1 Check: Offset by Percent (1.0 = 100%) DO NOT Check: Calibration Only Landmark

2. Create Right Toe Joint Center (RTOE) at the same height as the heel marker: Click Landmarks button Click Add New Landmark button Create Landmark: RTOE    Landmark Name: RTOE    Define Orientation Using:     Click Existing Segment: LAB Offset Using the Following ML/AP/AXIAL Offsets:    X: RFT2::X    Y: RFT2::Y    Z: RHL::Z DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

Once the Ankle Joint Centers have been created, the Foot segments can now be defined.

Defining the Virtual Foot Segments

Note: This definition assumes that the heel, toe, and ankle center define the sagittal plane of the foot. Care must be taken to place the heel marker carefully. Any medial/lateral misplacement of the heel marker will result in an inappropriate inversion/eversion of the segment.

3. 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: None     Joint: RHL     Medial: None         Radius: 0.01    Define Distal Joint and Radius    Lateral: None     Joint: RTOE     Medial: None         Radius: 0.01    Extra Target to Define Orientation    Location: Anterior     RAJC    Select Tracking Targets:      Check Use Calibration Targets for Tracking Click on Build Model. A 3D image of a foot will appear distal to the shank. Click on Close Tab before proceeding.

Rotating the Virtual Foot Segments

4. Rotate the Segment Coordinate System of the Right Virtual Foot Segment: The segment coordinate system should be rotated so the coordinate system follows the convention below: The x-Axis (red) represents flexion/extension of the ankle The y-Axis (green) represents inversion/eversion The z-Axis (blue) represents abduction/adducation Please see the Rotate Segment Coordinate System section of this tutorial for instructions on rotating the segment coordinate system.

Virtual Foot Method 2 - Normalize to Proximal Segment

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.

Note: this definition assumes that the posture in the standing trial is to be considered an ankle angle of zero degrees regardless of the actual posture.

6. 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         Define Distal Joint and Radius    Lateral: RLA     Joint: None     Medial: RMA         Select Tracking Targets:      RFT1, RFT2, RFT3 Click on Build Model. A 3D image of a foot will appear over the shank segment. NOTE: The image of the foot can be removed by deleting the associated .v3g file from the Segment Properties tab. Click on Close Tab before proceeding.

NOTE Since the two segment coordinate systems are perfectly aligned the segments have identical orientation in the standing trial and hence have a joint angle of zero degrees.

(The segment coordinate system in this section will not need to be rotated to follow the convention in the rest of the tutorial)

Virtual Foot Method 3 - Projected landmarks

Note: This definition assumes that the foot segment is parallel to the floor regardless of the actual posture in the standing trial.

Create 3 Laboratory Landmarks

Create 3 landmarks (Lab_Origin, Lab_X, Lab_Y) to represent the lab.

1. Create Lab_Origin: Click Landmarks button Click Add New Landmark button Create Landmark: Lab_Origin    Landmark Name: Lab_Origin    Define Orientation Using:     Existing Segment: LAB Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.0    Y: 0.0    Z: 0.0 DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

2. Create Lab_X: Click Landmarks button Click Add New Landmark button Create Landmark: Lab_X    Landmark Name: Lab_X    Define Orientation Using:     Existing Segment: LAB Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.1    Y: 0.0    Z: 0.0 DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

3. Create Lab_Y: Click Landmarks button Click Add New Landmark button Create Landmark: Lab_Y    Landmark Name: Lab_Y    Define Orientation Using:     Existing Segment: LAB Offset Using the Following ML/AP/AXIAL Offsets:    X: 0.0    Y: 0.1    Z: 0.0 DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

Create 3 projected landmarks

Create 3 landmarks (RLA_Floor, RMA_Floor, RFT1_Floor) that are the projection of the 3 markers used to define the foot onto the floor.

1. Create RLA_Floor: Click Landmarks button Click Add New Landmark button Create Landmark: RLA_Floor    Landmark Name: RLA_Floor    Define Orientation Using:    Starting Point: Lab_Origin    Ending Point: Lab_X    Lateral Object: Lab_Y    Project From: RLA Offset Using the Following AXIAL Offset: X.X DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

2. Create RMA_Floor: Click Landmarks button Click Add New Landmark button Create Landmark: RMA_Floor    Landmark Name: RMA_Floor    Define Orientation Using:    Starting Point: Lab_Origin    Ending Point: Lab_X    Lateral Object: Lab_Y    Project From: RMA Offset Using the Following AXIAL Offset: X.X DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

3. Create RFT1_Floor: Click Landmarks button Click Add New Landmark button Create Landmark: RFT1_Floor    Landmark Name: RFT1_Floor    Define Orientation Using:    Starting Point: Lab_Origin    Ending Point: Lab_X    Lateral Object: Lab_Y    Project From: RFT1 Offset Using the Following AXIAL Offset: X.X DO NOT Check: Offset by Percent (1.0 = 100%) Check: Calibration Only Landmark

Create the Virtual Foot Segment

The following method creates a foot coordinate system that is aligned with the laboratory coordinate system (as established by the 3 landmarks created above). Since the landmarks are projected onto the floor, the Segment Coordinate System for the Virtual Foot will be parallel to the floor.

This orientation is convenient for describing the angle of the foot segment relative to a surface. In this example, the ankle joint angle will be close to zero in the standing posture (if the shank is vertical, the angle will be zero).

1. Create the 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: RLA_Floor     Joint: None     Medial: RMA_Floor         Define Distal Joint and Radius    Lateral: RFT1_Floor     Joint: None     Medial: None         Radius: 0.01    Select Tracking Targets:      RFT1, RFT2, RFT3 Click on Build Model. A 3D image of a foot will appear distal to the shank. Click on Close Tab before proceeding.

Rotate the Virtual Foot Segments

2. Rotate the Segment Coordinate System of the Right Virtual Foot Segment: The segment coordinate system should be rotated so the coordinate system follows the convention below: The x-axis (red) represents flexion/extension of the ankle The y-axis (green) represents inversion/eversion The z-axis (blue) represents abduction/adducation Please see the Rotate Segment Coordinate System section of this tutorial for instructions on rotating the segment coordinate system.

Rotate Segment Coordinate System

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

Rotate the foot coordinate system: 1. In the Segment Properties tab, select Right Virtual Foot in the Segment Name box. 2. Click the Modify Segment Coordinate System button.

3. In the Segment Orientation dialog box, enter these values:    A/P Axis +Z    Distal to Proximal Axis -Y 4. Click on Apply 5. Click on Build Model

Ankle Joint Angle

A final example of this tutorial can be seen in this file.

Once this tutorial complete, a total of four foot segments have been defined:
Right Foot was defined using Kinetic Foot Segment
RFT_2 was defined using Method 1 - Heel to Toe
RFT_3 was defined using Method 2 - Normalized to proximal segment
RFT_4 was defined using Method 3 - Projected Landmarks

The Ankle Joint Angle can be defined using the Virtual Foot relative the Shank segment. The joint angle created by each foot definition can be plotted and compared to see which is the most ideal definition for your specific task.

Ankle Angle Explained

Understanding the Ankle Angle Signal

This section will explain a little more about the definition of the ankle and the right hand rule.
The graphs in this section are saved in a cmo file which can be downloaded here.

Right Hand Rule

For this segment coordinate system (z-up, y-anterior) rotation about the x-axis represents flexion/extension. Visual3D always computes all signals based on the Right Hand Rule. For example, if you point your thumb in the direction of the x-axis of the hip (shown in Red in the animation viewer) pointing laterally to the right. Knee extension will be zero when the thigh segment coordinate system and the shank segment coordinate system are aligned. Your curled fingers indicates the positive direction of rotation. Thus, in this case knee extension will be seen as a positive angle, and knee flexion as negative. Note that the x-axis for both the left and right thigh segment coordinate systems points laterally to the right. Using the same schema inversion of the ankle is a positive rotation about the y-axis for the right leg and a negative rotation about the y-axis for the left leg. Inward internal rotation of the ankle is a positive rotation about the z-axis for the right leg and negative rotation about the z-axis for the left leg. This reflection of the data anatomically from right to left is a result of applying the right hand rule rigidly within Visual3D. When presenting data it is quite common for Visual3D users to negate the y and z terms for the left knee angle so that the frontal and transverse plane rotation use the same sign convention for both right and left joint angles.

Define the Ankle Angle (Following the Right Hand Rule for both sides)

Define the Left and Right Ankle Angles in Compute Model Based:

1. Go to the Model drop down menu

2. Select Compute Model Based Data

3. Define the ankle angle using the following convention

NOTE: RFT_2 and LFT_2 are virtual feet which were created using Method 3 in this tutorial.

Graph the Ankle Angle (without consistent sign convention)

Graph the X Y and Z components of the Ankle Angle:
The signals can be graphed as an interactive graph under the Signals and Events tab, or as a 2D graph under the Report Tab.

Left: + Dorsiflexion Right: + Dorsiflexion

Left: + Eversion Right: + Inversion

Left: + Abduction Right: + Adduction

As per the above description, because the Right Hand Rule is being applied strictly followed for both right and left joint angles, the sign conventions are opposite between right and left side for the frontal and transverse plane. Though not a problem in itself, in biomechanics it is often recommended to use the same sign convention for both sides so that interpreting joint angles is easier.

Define the Ankle Angle (Sign convention using the Right Hand Rule from the Right side only)

Define the Left and Right Ankle Angles in Compute Model Based:

Negate the Y and Z components of the Left Ankle Angle

Negating a component simply multiplies the signal by -1, to change its sign convention.

Graph the Ankle Angle (with consistent sign convention)

Graph the X Y and Z components of the Ankle Angle:

Left: + Dorsiflexion Right: + Dorsiflexion

Left: + Inversion Right: + Inversion

Left: + Adduction Right: + Adduction

The resulting rotations about the y-axis/frontal plane (abd/adduction) are positive in the same direction.
The resulting rotations about the z-axis/transverse plane (internal/external rotation) are positive in the same direction.

Foot Progression Angle

The foot progression angle is a measure often used in clinical settings, to assess toe-in/toe-out during gait. While the exact way of computing the Foot Progression Angle can vary, this tutorial describes two of the most common ways.

Please note that neither of the proposed methods in this tutorial is the method used by Vicon in Nexus and referred as Foot Progression Angle.

Using a Virtual Lab

When using a model that includes a Pelvis segment, it is recommended to create a Virtual Laboratory that will change with the direction of walking. This allows to compute consistent Pelvic angle, without being affected by the direction of walking.

The same also applies to the Foot Progression Angle. To compute the progression angle relative to the lab, it is important that the reference coordinate system used is consistent with the direction of walking.

Create a Virtual Lab

First, create a Virtual Laboratory that changes with the direction of walking. Once completed, you can proceed to the next step to compute the foot progression angle.

Define the Foot Progression Angle

Compute the Foot Progression Angle using Compute Model Based Data:

Negate the Y and Z components for the Left Foot Progression Angle

Graph the Foot Progression Angles

Considering that the Foot Progression Angle is a measure of Toe-In/Toe-Out, only the "Z" component of the L/RFT_Prog_Angle should be used and graphed:

Using the General Pelvis Direction

Another method to compute Foot Progression Angle is to use the general direction of the pelvis as the main direction of progression.

A vector is created using the position of the Pelvis' CG at the first and last frame of each trial, a second vector created using the long axis of the foot (i.e. running anterior-posterior). The angle between these two vector, projected onto the lab floor is then computed, for both the left and the right foot.

Pipeline Script

This pipeline script is markerset dependent. The user will need to modify the marker names accordingly in order for it to work correctly. The changes should be made in the Evaluate_Expression, following the Compute right-side unit vector and Compute left-side unit vector comments. The markers are those of the "toe" and "heel", representing the center line of the foot.

Event_Explicit
/EVENT_NAME=START
/FRAME=1
! /TIME=
;

Event_Explicit
/EVENT_NAME=END
/FRAME=EOF
! /TIME=
;

!
! Find Pelvis at start
!
Metric_Signal_Value_At_Event
/RESULT_METRIC_NAME=Pelvis_At_START
! /RESULT_METRIC_FOLDER=PROCESSED
/SIGNAL_TYPES=KINETIC_KINEMATIC
/SIGNAL_NAMES=CGPOS
/SIGNAL_FOLDER=RPV
/EVENT_NAME=START
/GENERATE_MEAN_AND_STDDEV=FALSE
! /APPEND_TO_EXISTING_VALUES=FALSE
! /GENERATE_VECTOR_LENGTH_METRIC=FALSE
! /RETAIN_NO_DATA_VALUES=FALSE
;

!
! Find Pelvis at start
!
Metric_Signal_Value_At_Event
/RESULT_METRIC_NAME=Pelvis_At_END
! /RESULT_METRIC_FOLDER=PROCESSED
/SIGNAL_TYPES=KINETIC_KINEMATIC
/SIGNAL_NAMES=CGPOS
/SIGNAL_FOLDER=RPV
/EVENT_NAME=END
/GENERATE_MEAN_AND_STDDEV=FALSE
! /APPEND_TO_EXISTING_VALUES=FALSE
! /GENERATE_VECTOR_LENGTH_METRIC=FALSE
! /RETAIN_NO_DATA_VALUES=FALSE
;

!
! Convert metric signal by grabbing an arbitrary signal multiplied by zero and add metric
!
Evaluate_Expression
/EXPRESSION=0.0*KINETIC_KINEMATIC::RPV::CGPOS+METRIC::PROCESSED::Pelvis_At_END
/RESULT_NAME=Signal_Pelvis_At_END
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

!
! Convert metric signal by grabbing an arbitrary signal multiplied by zero and add metric
!
Evaluate_Expression
/EXPRESSION=0.0*KINETIC_KINEMATIC::RPV::CGPOS+METRIC::PROCESSED::Pelvis_At_START
/RESULT_NAME=Signal_Pelvis_At_START
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Evaluate_Expression
/EXPRESSION=vector((DERIVED::PROCESSED::Signal_Pelvis_At_End::X-DERIVED::PROCESSED::Signal_Pelvis_At_START::X),
(DERIVED::PROCESSED::Signal_Pelvis_At_End::Y- DERIVED::PROCESSED::Signal_Pelvis_At_START::Y),
(0*(DERIVED::PROCESSED::Signal_Pelvis_At_End::Z-DERIVED::PROCESSED::Signal_Pelvis_At_START::Z)))
/RESULT_NAME=DirectionOfProgression
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Signal_Magnitude
/SIGNAL_TYPES=DERIVED
/SIGNAL_NAMES=DirectionOfProgression
/SIGNAL_FOLDER=PROCESSED
/RESULT_NAMES=DirectionOfProgression_Magnitude
/RESULT_TYPES=DERIVED
/RESULT_FOLDER=PROCESSED
! /RESULT_SUFFIX=
;

Evaluate_Expression
/EXPRESSION=DERIVED::PROCESSED::DirectionOfProgression/DERIVED::PROCESSED::DirectionOfProgression_Magnitude
/RESULT_NAME=DirectionOfProgression_Unit
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

!
! Compute right-side unit vector
!
Evaluate_Expression
/EXPRESSION=vector((TARGET::PROCESSED::RTOE::X-TARGET::PROCESSED::RFCC::X),
(TARGET::PROCESSED::RTOE::Y-TARGET::PROCESSED::RFCC::Y), 
(0.0*(TARGET::PROCESSED::RTOE::Z-TARGET::PROCESSED::RFCC::Z)))
/RESULT_NAME=RFoot_Line
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Signal_Magnitude
/SIGNAL_TYPES=DERIVED
/SIGNAL_NAMES=RFoot_Line
/SIGNAL_FOLDER=PROCESSED
/RESULT_NAMES=RFoot_Line_Magnitude
/RESULT_TYPES=DERIVED
/RESULT_FOLDER=PROCESSED
! /RESULT_SUFFIX=
;

Evaluate_Expression
/EXPRESSION=DERIVED::PROCESSED::RFoot_Line/DERIVED::PROCESSED::RFoot_Line_Magnitude
/RESULT_NAME=RFoot_Line_Unit
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

!
! Compute left-side unit vector
!
Evaluate_Expression
/EXPRESSION=vector((TARGET::PROCESSED::LTOE::X-TARGET::PROCESSED::LFCC::X),
(TARGET::PROCESSED::LTOE::Y-TARGET::PROCESSED::LFCC::Y),
(0.0*(TARGET::PROCESSED::LTOE::Z-TARGET::PROCESSED::LFCC::Z)))
/RESULT_NAME=LFoot_Line
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Signal_Magnitude
/SIGNAL_TYPES=DERIVED
/SIGNAL_NAMES=LFoot_Line
/SIGNAL_FOLDER=PROCESSED
/RESULT_NAMES=LFoot_Line_Magnitude
/RESULT_TYPES=DERIVED
/RESULT_FOLDER=PROCESSED
! /RESULT_SUFFIX=
; 

Evaluate_Expression
/EXPRESSION=DERIVED::PROCESSED::LFoot_Line/DERIVED::PROCESSED::LFoot_Line_Magnitude
/RESULT_NAME=LFoot_Line_Unit
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
;

!
! Steps left is to find unit vector of two landmarks on the right virtual foot Y
!
Evaluate_Expression
/EXPRESSION=cross(DERIVED::PROCESSED::RFoot_Line_Unit, DERIVED::PROCESSED::DirectionOfProgression_Unit)
/RESULT_NAME=RFoot_Line_Cross_Product
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Evaluate_Expression
/EXPRESSION=asin(DERIVED::PROCESSED::RFoot_Line_Cross_Product::Z)
/RESULT_NAME=RFoot_Angle
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Multiply_Signals_By_Constant
/SIGNAL_TYPES=DERIVED
/SIGNAL_NAMES=RFoot_Angle
/SIGNAL_FOLDER=PROCESSED
! /RESULT_NAMES=
! /RESULT_TYPES=
! /RESULT_FOLDER=PROCESSED
/RESULT_SUFFIX=_Degrees
! /SIGNAL_COMPONENTS=
/CONSTANT=57.3
;

!
! Steps left is to find unit vector of two landmarks on the left virtual foot Y
!
Evaluate_Expression
/EXPRESSION=cross(DERIVED::PROCESSED::LFoot_Line_Unit, DERIVED::PROCESSED::DirectionOfProgression_Unit)
/RESULT_NAME=LFoot_Line_Cross_Product
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Evaluate_Expression
/EXPRESSION=-1*asin(DERIVED::PROCESSED::LFoot_Line_Cross_Product::Z)
/RESULT_NAME=LFoot_Angle
! /RESULT_TYPE=DERIVED
! /RESULT_FOLDER=PROCESSED
; 

Multiply_Signals_By_Constant
/SIGNAL_TYPES=DERIVED
/SIGNAL_NAMES=LFoot_Angle
/SIGNAL_FOLDER=PROCESSED
! /RESULT_NAMES=
! /RESULT_TYPES=
! /RESULT_FOLDER=PROCESSED
/RESULT_SUFFIX=_Degrees
! /SIGNAL_COMPONENTS=
/CONSTANT=57.3
;

Graph the Foot Progression Angles

This method created a derived signal, make sure to select DERIVED in the Signal Type dropdown list, in the Y Axis Properties in the Add / Modify Graph window.