Modeling

Motion

Motion

External forces may be used to activate or manipulate the model so that the joints and/or muscles record the necessary patterns that allow BodySIM to run driver servos in a forward dynamics analysis.

Much of the motion comes from digitized sources such as motion capture equipment in the form of active/passive marker tracking systems.

Various motion parameters may be adjusted. See Parameters to tune the model for simulation. Choosing Model Parameters offers more information on data sources and on how to select the parameters mentioned in this section.

Sections:

Anatomy of a Motion Agent
Motion Agent Sets
Changing the Location of the Data Markers on the Body Before Creating Agents
Importing Motion Capture Data
Creating a Base Set of Motion Agents on the Model
Augment a Motion Agent Set
Creating an Individual Motion Agent
Creating a Tracker Agent
Using a New Set of Motion Data
Deleting Motion Agents

Anatomy of a Motion Agent

Motion capture (MOCAP) systems track the trajectories of markers attached at various locations on the body. The marker trajectories are then used to train the human model. During the training, the response of the body (joint rotations, muscle shortening/lengthening patterns ) is recorded and later used for a forward dynamics simulation (see Bicycle Rider example).

Marker trajectory data is used to drive elements called "motion agents," which are massless parts fixed to the body segments using spring elements (see Figure 1). Through this attachment, motion agents are motion influencers not motion governors. This accommodates the geometric differences between the body model and the actual human subject, as well as motion agent location discrepancies.

Figure 1: Motion Agent configuration

Motion agents are displayed on the model as small spheres. A yellow sphere is the represents the location of the massless part governed by the trajectory data. The red sphere (inside the yellow sphere in Figure 1) is a point rigidly attached to the segment of the human model that anchors the motion agent. The two spheres are joined via bushing spring force. A bushing spring force is a 6 component spring (3 forces and 3 torques).

In addition to using MOCAP data to drive the body motion, motion agents may also be driven manually to guide the model into a motion prescribed by the user, such as flexing and extending a detailed cervical spine (see Cervical Spine example) or stabilizing a knee (see Total Knee Replacement Tutorial).

Motion Agent Sets

To quickly create the motion agent on the model, LifeMOD/BodySIM™ supports several standard video-tracking marker placement protocols. The supported motion marker protocols include:

In these protocols, marker locations exist at standard locations on the body and are scaled with the body during the segment-creation phase of modeling.

Changing the Location of the Data Markers on the Body Before Creating Agents

There may be examples where the motion capture markers on the test subject and the location of the motion agents on the human model are quite different. For example, the length of the wand may have several inch disparity in the Plug-in marker data set. (See appendix for more information on data sets). If these differences from the motion protocol are known, offsets may be introduced into the model before creating the motion agent sets.

To identify the locations of the marker attachments to the human segments, first display the proper motion protocol markers on the model using the LifeMOD display toolbox. Then select the individual marker and edit the location or move it using the A/view move tools on the A/view toolbar.

Figure 2: Displaying the marker set locations for each of the three protocols

Figure 3: To move motion agent attachments to the body, move the underlying markers with Adams/View move tools.

Importing Motion Capture Data

Motion -> Import Motion Capture Data
Many motion agent sets are available in the LifeMOD library. These include data for objects or environment, full body and lower body. Figure 4 displays the panel to input motion data from the library.

Figure 4: Panel used to import motion capture data from the library.

The panel in Figure 5 is used to import the motion capture data contained in the SLF file using the format specified in the appendix. This file is used to bring in motion data to drive the complete set or subset of the base segments in the model. The panel creates data splines with the prefix assigned in the panel. The full file or a portion of the file may be brought into LifeMOD/BodySIM. At times only a subset of the motion data is free from artifacts and other inaccuracies. The user may use only a portion of the data by selecting Use PARTIAL Data Set with a data start time and end time.

Figure 5: Panel used to import motion capture data.

The data may be filtered using a high/low pass butterworth filter. To filter the imported motion data check the filter box to bring up the filter panel. Set the filter parameters and select apply. It is useful to view a sample data spline before and after the filtering. Figure 6 displays the panel to filter the motion data with a Butterworth filter.

Figure 6: By checking the Filter Tools box the filtering panel is displayed (in box). In this panel the Butterworth filter parameters may be entered. In addition, the resulting filtered data may be viewed graphically as a curve (spline).

Creating a Base Set of Motion Agents on the Model

Motion -> Create BASE Motion Agent Set
In order to create motion agents for the body, MOCAP data must be read in using the panel Motion -> Import Motion Capture Data (Figure 5). MOCAP data is available from an external file or from the library.

The panel in Figure 7 specifies the motion agent weights. These weights are multipliers on the stiffness of the springs between the agent and its rigidly-attached point on the segment. Additionally, global translational stiffness and damping properties are specified. A data prefix is entered to allow management of multiple sets of MOCAP data within LifeMOD.

The MOCAP data may be specified to the global frame or with respect to another reference frame. The other reference frame may be specified by creating a marker.

The motion agents are created by selecting "APPLY."

Figure 7: Motion agent creation panel for Body Segment CG marker set

Figure 8: Motion agent creation panel for base set.

Most often there will exist an offset between the location of the marker in the MOCAP data source and the segment attachment location established when the body segments were created (see Figure 9). To account for this, the user would run an equilibrium analysis, to minimize the distance between the motion agent locations and the respective attachment locations on the body segments. This is followed by SYNCHRONIZE BODY MARKER LOCATIONS WITH DATA button to move the body marker locations to be equivalent with the data source.

Figure 9: Offsets between the MOCAP data locations and the segment attachment locations

Augment a Motion Agent Set

Motion ->Augment Motion Agent Set
Motion agent sets may be augmented with additional motion agent sets. Using this method, additional motion agents may be added to the body to augment any base marker set. This method may also be used to create motion agents on environments or mechanical systems. For an example of applying motion agents to a mechanical system (tennis racket) see the Tennis Player Tutorial.

To input the motion capture data for the augmented motion agent set, a specific SLF file format is necessary and is detailed in the appendix. This specific format for the SLF file includes a section to specify the name of the part (or body segment) each new motion agent would be assigned to.

The panel in Figure 10 displays the input panel to create the augmented marker set. It specifies "Locate Motion Agents with First Data Point" to create the motion agents and the connections to the part at the location specified by the first location in the SLF file.

Figure 10: Panel used to create the augmentation marker set, featuring the option to locate the motion agents and segment attachments at the location specified by the first data point in the SLF file.

The panel in Figure 11 displays the option to create the motion agents at the data locations with attachments to the parts at user specified locations. It also displays a portion of the SLF file where the part numbering sequence is displayed. Figure 12 displays the resulting three motion agent augmentation set attached to the tennis racket in the model as a result of the inputs from the panel in Figure 11.

Figure 11: Panel used to create the augmentation marker set, featuring the option to locate the motion agents at the location specified by the first data point in the SLF file and the attachments to the part at a user specified location. In this example Parts 1, 2 and 3 designate the same part (racket).

Figure 12: The example continued from Figure 11 displaying the locations of the motion agent attachments and the motion agents themselves.

Creating an Individual Motion Agent

Motion -> Create Individual Motion Agent
The panel in Figure 13 allows the user to create an individual motion agent on a segment of the model. Each axis may be specified separately as either fixed, free or driven using a pre-existing data spline. Using this method, motion agents can be used to specify single axis motion guidance (see Force-Based Knee and Cervical Spine examples).

Figure 13: Panel to create single motion agent

Creating the Tracker Agent

Motion -> Create Tracker Agent
A tracking agent is a motion agent used during a forward-dynamics simulation. It is used to guide the model by applying small spring forces via the connector bushing to account for various minor instabilities in the model. Instabilities could occur due to mathematical round-off error, model imbalance, etc. See Figure 15 for the location of the tracking agent. For examples of the tracker agent being used see Gait Analysis, Walking, Hip Replacement and Dancing tutorials.

Figure 14: Panel to create the tracker agent

Figure 15: Location of the tracking agent

Using a New Set of Motion Data

Motion -> Edit Base Set
This panel allows the replacement of the motion data in the existing motion agents. This feature is especially useful for models with several motion trials to be analyzed. There must be a motion agent set in the model for this feature to be utilized. For an example of this feature, see the Lifting Tutorial.

Figure 16: Panel to replace motion data in the existing motion agents

Deleting Motion Agents

Motion-> Delete
the panel in Figure 17 is used to delete the motion agent(s) from the model. The user has the option of deleting a single agent, all agents or the tracker agent

Figure 17: Panel to delete motion agents