Model Based Items

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All Kinematic and Kinetic calculations in Visual3D are described as either KINETIC_KINEMATIC Items or Link_Model_Based Items

Kinematic calculations are based on segment coordinate systems with no reference to inertial properties.

Kinetic (Inverse Dynamic) calculations are based on assuming linked rigid segments, "connected" by joints with 1 to 6 degree of freedom joints

The Kinematics and Kinetics are computed as KINETIC_KINEMATIC data based on the model, the movement trial, and the assigned forces. These data are resolved into the Laboratory Coordinate system, and therefore often have little or no anatomical meaning.

Model Based Items, on the other hand, allow the user to select a resolution coordinate system. An appropriate choice of resolution coordinate system imbues anatomical meaning to the signal.

The dialog for creating the variables can be accessed by selecting the following option under the Model Menu item or by the Pipeline Command Compute_Model_Based_Data.

Tutorials

Tutorials describing model based computations.

Tutorial Model Based Computations

Using PROCESSED Signals

Under the settings menu there are three options:

Use Processed Analogs for Ground Reaction Force Calculations
Use Processed Forces for Segment to Force Assignments
Use Processed Targets for Model/Segment/LinkModelBased items

In most situations, you want "Use Processed Analogs" and "Use Processed Targets" set to TRUE and Use "Processed Forces" set to FALSE.

If "Use Processed Analogs" and "Use Processed Targets" are checked (set to TRUE), Visual3D computes Kinetics and Kinematics using the data in the PROCESSED folder of TARGET, and ANALOG.

If "Use Processed Analogs" and "Use Processed Targets" are checked (set to TRUE), if the PROCESSED folder does not exist, the ORIGINAL folder is used. If you have filtered the FORCE/COFP/FREEMOMENT folders (which is not recommended - explained here) the PROCESSED will always be used.

If "Use Processed Forces" is checked (set to TRUE), Visual3D computes the frames for the force assignments using the FORCE PROCESSED folder. There is more information about this here.

Link_Model_Based signals are calculated using the Compute_Model_Based_Data command. These signals are typically stored in the Recalc pipeline, which means that they get recomputed whenever a Recalc occurs. These signals are stored in the LINK_MODEL_BASED ORIGINAL folder regardless of whether ORIGINAL or PROCESSED targets were used to create the signals.

At what point should signals be filtered

This is a difficult question because there isn't a single correct answer.

The answer depends on the pose estimation method, the marker placements, the movement being recorded.

An excellent discussion on this topic can be found here

Deleting Model Based Signals

Model based signals can be deleted from the data tree. Right mouse click on the signal (or folder) and select delete.

This is not sufficient, however, because the Recalc Pipeline contains the definition of the signal, which means that when the Workspace is recalculated, the model based signal will re-appear. In order to delete the signal completely, edit the Recalc Pipeline and delete the command that defines the signal.

Miscellaneous Defaults

Model Based Units

Visual3D computes model based items using MKS units; eg. based on meters, kilograms and seconds. An exception to this rule are joint angles, which are represented in degrees.

Model Based Items Default Normalization

Default scaling factors.

Default Segment Names

Visual3D contains a number of default segment names that are provided for the convenience of the user. These default segments contain geometry, the inertial properties, and the wire frame model. Users that create their own segment names need to add this information in the segment properties tab in Model Builder Mode

Default Joint Names

The calculation of Joint Moment, Joint Power, and Joint Force refer to the proximal end of the segment selected. For convenience we have mapped segment names to joint names automatically because we assumed that users would find it more intuitive.

Link_Model_Based Items

Pose_Estimation

Signals related to the estimation of the pose (position and orientation) of segments.

SEGMENT_RESIDUAL

Visual3D computes the 6 Degree of Freedom pose of a segment using a Least Squares fit of the tracking marker locations in the standing trial to the tracking marker locations at each frame of the movement trial. The Goodness of Fit is described by the residual.

SEG_CGPOSITION

SEG_PROXIMAL_JOINT

SEG_DISTAL_JOINT

Ground Reaction Forces

COP_Path

Creating a LINK_MODEL_BASED signal called the COP_PATH is a more general way to compute the center of pressure. The command checks all contacts between the specified segment and any force platform. The signal is then transformed (or resolved) into the specified local coordinate system). All occurences (eg. all force platforms) are included to determine if the segment has been assigned to the force signal.

GRF_Data

Creating a LINK_MODEL_BASED signal called GRF_DATA is a more general way to reference an external Force. The user identifies the segment to which external forces have been assigned.

This allows the user to define the segment rather than the Force Platform, thus eliminating the need to make notes as to which segment is in contact with which platform.

Kinematics

JOINT_ANGLE

The joint angle defined using a Cardan sequence (including the Joint Coordinate System.

HELICAL_ANGLE

The joint angle defined as a helical angle.

JOINT_ROTATION

The joint angle represented as a 3x3 rotation matrix.

JOINT_ACCELERATION

The joint angular acceleration is a vector that describes the relative angular acceleration of one segment relative to another segment.

JOINT_VELOCITY

The joint angular velocity is a vector that describes the relative angular velocity of one segment relative to another segment.

TARGET_PATH

Computing the movement of the tracking markers relative to the segment coordinate system.

SEG_VELOCITY

The velocity of the center of mass of a segment relative to the velocity of the center of mass of the reference segment. Computationally, this value is obtained by taking the segment center of mass velocity and subtracting the reference segment center of mass velocity, with the resultant vector being transformed into the resolution segment coordinate system.

MODEL_COG

Model Center of Gravity

MODEL_COG_VELOCITY

Model Center of Gravity Velocity

MUSCLE_LENGTH

Path Length of a Muscle

SEG_PROGRESSION_ANGLE

SEG_PROGRESSION_ANGLE is the projected angle of the segment (e.g. projection of the long axis of the segment onto the ground) with the path of the center of mass of the segment over a window of 51 frames centered at the current frame.