Induced Acceleration

NOTE: This feature it is no longer supported.

Induced Acceleration analysis and Induced Power analysis provide an intermediate stage of analysis between inverse dynamics and forward dynamics that focuses on the interpretation of the motion capture data for gait.

An overview of Induced Acceleration and Induced Power analysis in the context of equations of motion.

To run an Induced Acceleration Analysis (IAA) you need to have the Induced Acceleration plugin installed in the correct Visual3D plugin folder. The file name for the plugin is MAMBO_IAA. dll and it must be placed in the folder specified in Visual3D:

Edit --> Preferences --> Plugin Folder in the Default Folders tab

PowerPoint Lecture Notes

Lecture notes used by Tom Kepple for a class at the University of Delaware.

Induced Acceleration Lecture Notes (ppt)

Data files associated with the lecture notes.

Induced Acceleration Example Data Files (cmo)

Note: The classroom materials are not for the current version of Visual3D, and were produced years ago, outside of C-Motion using different plug-in libraries.

Running an Induced Acceleration

To run the sample IAA for left stance phase you run the Visual3D script Mambo_IAA.v3s. (Note: in the script the user can chose to set the frames in which the feet are fixed relative to the ground. This is usually done during the foot-flat interval of stance. The parameters right_start, right_end, left_start and left_end are used to do this. ) It is also critical that you select the trial you want run the IAA on in the file selection box and then check the "EVENT_LABEL" folder in the data tree.

After the scripts complete a new folder, titled IAA, will be created in the data tree. This contains the output of the IAA. (Note: angular accelerations are in radians /s/s.) The names of the output signals are a bit confusing. The naming convention is:

SOURCE_ON_JOINT

or

SOURCE_PW_JOINT

The first part of the name represents the joint torque component (or gravity) which drives the model. For example output starting with RANKLEX would indicate a right ankle X (dorsi/plantar flexion) torque is driving the model.

The middle portion of the name indicates if the output is induced acceleration (ON) or induced power (PW) that is output. The IAA will also use the ON when outputting the ground reaction force that is induced by each source. (Induced ground reaction forces can be used to determine the contributions of the various joint moments and gravity to support and propulsion.)

The final part of the name represents the joint or segment to be accelerated. In the case of induced accelerations this will be the joint that is accelerated. In the case of induced power this will be the power transferred to the named segment.

Some examples of output names would be:

RANKLEX_ON_RHIP - This is the acceleration produced by the X component (plantar/dorsi flexion) of the right rankle moment (RFTX) at the right hip (RTH).

LHIPY_ON_LKNEE - This is the acceleration produced by the Y component (ab/adduction) of the left hip moment (LTHY) at the left knee (LSK).

GRAV_ON_RANKLE - This is the acceleration produced by gravity (GRAV) at the right ankle (RANKLE).

RANKLEX_ON_RGF - This is the ground reaction force produced by the X component (plantar/dorsi flexion) of the right rankle moment (RANKLEX) under the right foot (RGF).

LKNEEX_PW_LSK - This is the segmental power produced by the X component (plantar/dorsi flexion) of the left knee moment (LSKX) at the left shank (LSK).

Running an Induced Acceleration – Simulation Mode

Note: IAA works without the Simulations tab. That tab is only for visualizing the effects of a subset of the results and is not required. The tab is enabled via a special license key in the Visual3D license file. Contact C-Motion if you need a new license file created with the simulations tab enabled.

The results of an Induced Acceleration Analysis can sometimes be counter-intuitive. The Simulation Mode viewer is designed to help the user visualize how the net joint moments (or gravity) produce the computed accelerations. To use the Simulation Mode viewer select the Simulation Mode tab and follow the below instructions:

1) Select the MAMBO_IAA. dll plugin from the "Simulation Plugins" box.

2) If you want to fix the left foot to the ground check the "Left Fixed" Button. (If not selected the foot rotates about the center of pressure during stance and is totally free during swing.)

3) If you want to fix the right foot to the ground check the "Right Fixed" Button. (If not selected the foot rotates about the center of pressure during stance and is totally free during swing.)

4) Select the Frame Number that you want examine in the "Frame:" edit box. This will display your model exactly as it is found in that video frame. (Note: event labels are not supported; this must be a frame number.)

5) Using the "Steps" edit box select the number of steps that you want to drive the IAA model for. A value of 100 is usually pretty good. (This value sets the number of steps used in your Simulation; the greater the number of steps the longer it will take to display the simulation. )

6) Using the "Time" edit box select the length of time you want to drive the model. This value will vary depending on the Source (see #7 below) but 0.1 (seconds) is a good place to start. (If this value is too small you may not be able to see the model move. If you allow the simulation to run too long the model eventually start to do some very strange things!)

7) Give your simulation any name you want in the "Simulation Name” box. (Example: my_simuation.)

8) Select a joint moment to drive the model with from the list of sources. (The source names are coded. The first letter is R or L for right or left. The next part of the name is the joint torque. The last letter gives the component of the joint torque. For example: RANKLEX would drive the model with the right ankle flexion/extension torque as computed by the inverse dynamics.)

9) After making all the above selections hit the "Update Simulation" button.

10) Make sure the PLAY button on the bottom left corner of the program is selected. (If Play is selected than a Pause button should appear.)

For example if "Right Fixed" is checked. "Frame" is 80. "Steps" is 100. "Time" is 0.1. And RKNEEX is selected. The simulation would should what would happen if the model, starting at the position in frame 80 and with the right foot fixed to the ground, was driven by the knee flexion/extension torque for a period 0.1 seconds. (Since we have 100 steps each step size in the Simulation would 0.001.)

IAA with IK or 6 DOF Pose Estimation

The IAA will work whether the data is run through Visual3D IK or not. However you will get slightly different results with and without the Visual3D IK. Here is what happens in the two cases:

IAA without Visual3D IK

In this case the segments’ positions and orientations are computed with the traditional Visual3D 6 dof method and exported to the IAA plug-in. For the pelvis (the IAA model root) all position and orientation information are used by the IAA model. However, at the joints all the joint translation computed by Visual3D are ignored. Also at all joints three Euler angles are computed; however at the knees the ab/aduction and axial rotation components get set to zero, and at the ankles the toe-in/toe-out angles get set to zero. After the joint translations are removed and the knee and ankle rotations are constrained the IAA is computed for the model. Note that although the IAA has all joint translations and some ankle and knee rotations removed this is done by setting the values to zero and not through an optimization process (like used in Visual3D IK).

IAA with Visual3D IK

The IK was actually first added to the Visual3d so that the data exported out of Visual3D to the IAA would match the IAA model. To make this work you would first run an IK model that starts with the pelvis as the root (6 dof) but allows no translations at the hips, knees and ankles. All three rotations at the hip are allowed while at the knee only flexion/extension is allowed and at the ankle plantar- and dorsi-flexion and inversion/eversion are allowed. Using this IK model matches the Visual3D output to the IAA model.

Now here the advantages of each approach:

Advantage of IAA with Visual3D IK

The model used to generate the position and orientation data coming out of Visual3D exactly matches the IAA model. In forward dynamics and IAA this generally a good thing to do.

Advantage of IAA without Visual3D IK

If you have really noisy target data (particular in the proximal segments) the Visual3D IK can cause the entire model to shift away from the center of pressure which would change the value of the joint moments used to drive the IAA. Thus if you suspect your target data at the proximal segments are bad you may either want to skip the Visual3D IK.

In our opinion (and it is nothing but an opinion), you are probably better off running the Visual3D IK before calling the IAA plugin. However, that said, usually the results are pretty similar either way, and rarely will the use of IK, or not, in Visual3D affect the interpretation of the IAA results.