All kinematic and kinetic calculations in Visual3D result in either KINETIC_KINEMATIC items or Link_Model_Based items. Kinematic calculations are based on segment coordinate systems with no reference to inertial properties while kinetic calculations (e.g., inverse dynamics) are based on assuming rigid segments, “connected” by joints with 1 to 6 degrees of freedom. The results of these kinematic and kinetic calculations are KINETIC_KINEMATIC signals and are resolved into the laboratory coordinate system, often with little or no anatomical meaning.

To resolve this issue, Link_Model_Based items allow the user to specify 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.

Visual3D allows users to specify a large range of Link_Model_Based items that we have grouped into different categories: those related to pose estimation, ground reaction forces, kinematic quantities, and kinetic quantities. We also note those related items that are not Link_Model_Based items themselves.

These Link_Model_Based items are signals related to the estimation of the pose (position and orientation) of segments.

Some calculations are based on model-based items but are not themselves LINK_MODEL_BASED items.

In order to get the most value out of these LINK_MODEL_BASED items it is important to understand how Visual3D works with these items, the assumptions that are made, and the options available to users.

Visual3D computes model-based items using MKS units, e.g., meters, kilograms and seconds. The exception to this rule are joint angles, which are represented in degrees.

Visual3D uses certain default scaling factors to normalize model-based items.

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. Similarly, 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 default joint names because we assumed that users would find it more intuitive.

Under the settings menu there are three options:

1. Use Processed Analogs for Ground Reaction Force Calculations
2. Use Processed Forces for Segment to Force Assignments
3. 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 the TARGET and ANALOG data types. If the PROCESSED folder does not exist for either of these data types then the respective ORIGINAL folder is used in its place. If you have filtered the FORCE/COFP/FREEMOMENT folders (which is not recommended) then the PROCESSED folder will always be used.

If “Use Processed Forces” is checked (set to TRUE), Visual3D computes the frames for the force assignments using the FORCE data type's 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 data type's ORIGINAL folder regardless of whether ORIGINAL or PROCESSED targets were used to create the signals.

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 on the ISB's Biomch-l forums. In all cases we encourage users to be very deliberate about their choices to filter signals.

Model-based signals can be deleted from the data tree by right-clicking on the signal in question (or foler in question) with the mouse and selecting “Delete”.

This is does delete the model-based item. Since the Recalc Pipeline contains the definition of the signal, the model-based signal will reappear every time the workspace is recalculated. If you want to delete the model-based item completely, edit the Recalc Pipeline and delete the command that defines the signal.

Follow the Model Based Computations tutorial to learn more about defining and using model-based items.