To enable Plug-in Gait to finalize trial data, including filtering the data, detecting gait events, and producing a .c3d file for export from Nexus, you can run a Plug-in Gait Dynamic pipeline on the reconstructed and labeled movement trials.
Before you run the pipeline on your movement trials, ensure you have gap-filled and if necessary cropped the trials (see Review trials and fill gaps).
If required, you can change the properties for each of the operations that are included in the Plug-in Gait Dynamic pipeline. You can also run them individually, and/or run other operations at this stage too. To help you understand the different operations typically run on Plug-in Gait movement trials and the options available, each operation is described in the following topics, together with the Delete Unlabeled Trajectories pipeline operation, which you may also find useful:
When you are familiar with these operations, you can run all of the data processing in a single step by creating a custom pipeline and adding all the operations described in this section to it, as explained in Create a pipeline. When you create your new pipeline, be sure to include the operations in the above order.
Run the Dynamic Plug-in Gait pipeline
To run the Dynamic Plug-in Gait pipeline:
- On the Data Management tab, ensure the dynamic trial you have gap-filled is open (for details, see Fill gaps in trial data).
- On the Pipeline tab of the Tools pane, from the Current Pipeline list, select Dynamic Plug-in Gait.
- Ensure that the required operations are selected and that you have configured their properties appropriately (see below for details).
Click the Play button.
A green check mark appears in the list to the left of each completed operation.
In the 3D Perspective view, the axis for each segment is displayed, together with the Foot Strike and Foot Off.
- In the Subjects Resources pane, expand the subject's tree and ensure that the model outputs are listed.
- To save the data in .c3d format, on the Vicon Nexus toolbar, click the Save button.
If you don't need to perform any further processing, you can now export your data. For more information, see Export trial data.
Delete Unlabeled Trajectories pipeline operation
This pipeline operation runs on a fully labeled trial to delete any remaining unlabeled trajectories. To choose the maximum length of unlabeled trajectory to delete, select the Delete using Max Length option and specify the Max Length to Delete.
Filter Trajectories - Woltring pipeline operation
This pipeline operation filters the data using the Woltring filter to ensure smooth trajectories for calculating kinetics. This routine is based on a fifth-order spline-interpolating function (for details on this function and its filtering algorithms, see Plug-in Gait references).
In the Properties pane for this operation, you can specify these settings:
| Property | Description |
|---|---|
| First Frame | First frame to which filtering is applied |
| Last Frame | Last frame to which filtering is applied |
| Filter mode | Choose between: GCV The filtering routine makes an automatic estimate of the noise to find the optimal smoothing parameter. MSE You can change the level of smoothing by entering a different value for the Smoothing option below. |
| Trajectories | Enables you to select the trajectories to filter. Can be All trajectories, All labeled trajectories, or Linked trajectories. |
| Smoothing | Enables you to specify the level of smoothing for the MSE option. |
Detect Events From Forceplate pipeline operation
This pipeline operation automatically detects gait cycle events such as footstrikes and adds them to the time bar throughout the trial using vertical ground reaction forces (GRFs) measured by a force plate connected to the Vicon system. The operation includes the option to set the force threshold and the label of the markers attached to the front and back of the foot for both sides.
In the Properties pane for this operation, you can specify these settings:
| Property | Description |
|---|---|
| First Frame | First frame in which events are detected |
| Last Frame | Last frame in which events are detected |
| Force Threshold (N) | The force threshold for automatically detecting heel strike and toe off events. The default is 20. |
| Left anterior marker label | Marker used for event detection. You are recommended to leave this at the default setting. |
| Left posterior marker label | Marker used for event detection. You are recommended to leave this at the default setting. |
| Right anterior marker label | Marker used for event detection. You are recommended to leave this at the default setting |
| Right posterior marker label | Marker used for event detection. You are recommended to leave this at the default setting. |
Autocorrelate Events pipeline operation
This pipeline operation detects the pattern of the tracked marker at the set events and defines these events for the rest of the trial. The available parameters are: marker being tracked; the X, Y, or Z component of the marker; and the position, velocity, or acceleration of the marker. Run this operation after Detect Events from Forceplate or manually entering events on the timebar.
In the Properties section for this operation, you can specify these settings:
| Property | Description |
|---|---|
| First Frame | First frame in which events are detected |
| Last Frame | Last frame in which events are detected |
| Only after existing | If a force plate is used and you select this option, Vicon Nexus automatically correlates force plate data events from the time of the first strike on the force plate and forward. If you do not select this option, Vicon Nexus automatically correlates events both forward and backward from the first force plate strike. If a force plate is not used, you can still select this option, but you must make a thorough visual check of the events placed. |
| Correlation Method (Advanced property) | Enables you to choose the correlation method to automatically place events at the correct location in the time bar: Legacy, Least Squares or Pearson Coefficient (see Correlation Method parameters in the Vicon Nexus Reference Guide). |
| Marker (Left and Right) | Enables you to change the marker used for correlation if necessary. |
| Component (Left and Right) | For walking trials, leave the Left and Right values at the default value (Z). For running and other motor tasks, change as appropriate. |
| Derivative (Left and Right) | For walking trials, leave the Left and Right values at the default value (Position). For running and other motor tasks, change as appropriate. |
Process Dynamic Plug-in Gait Model pipeline operation
This pipeline operation runs the dynamic Plug-In Gait model on the active subject of the current trial.
In the Properties section for this operation, you can specify these settings:
| Property | Description |
|---|---|
| First Frame | First frame on which the operation is run |
| Last Frame | Last frame on which the operation is run |
| Marker diameter (mm) | Ensure the specified value corresponds to the diameter of the markers used (assuming they are applied directly to the skin surface). Note: If your marker has a base, to calculate the correct value, double the base width and add it to your marker diameter in mm. |
| Reaction reference frame | Determines which reference frame is used for reporting joint moments: Proximal segment: Reports all moments in the reference frame of the proximal segment. Distal segment: Reports all moments in the reference frame of the distal segment (for example, the tibia segment for the knee joint). Global frame: Reports the moment vector relative to the laboratory coordinate system. |
| Power Output | How Plug-in Gait is to output the power data: Standard: The joint power expressed as a single number (scalar). Individual Contributions from XYZ: The joint power expressed as individual X, Y, and Z planes in the segment's coordinate system |
| Anterior Thorax Tilt is Positive | Select this option to specify that the anterior thorax tilt is positive. Clear this option to specify that the anterior thorax tilt is negative. |
| Output Joint Centers | Select this option to visualize the joint centers that are calculated by Plug-in Gait. |
| Output Segment Centers of Mass | Select this option to visualize the center of mass of each segment that is calculated by Plug-in Gait. |
| Allow cross-plate strikes | Select this option when a foot may be in contact with more than one force plate simultaneously; for example, the heel may land on the far side of one plate and then the toes roll onto the near edge of the subsequent plate. If you select this option, reactions from the two plates are combined during processing, with the plate that is struck first being considered the primary. The forces reported by each plate are added together; the moments are also combined by referring the moment from the secondary plate back to the origin of the primary plate. In effect, the two plates are treated as a single force plate with the same origin as the primary plate. For more information, see Cross-plate foot strikes. Although every effort has been made to ensure the mathematical combination of forces and moments between two plates is correct, it is the responsibility of the operator / analyst to review the outcomes produced by this process and ensure they are correct. Use of this option in clinical assessments is strictly at the discretion of the operator/analyst. |
| Radius of Gyration: Pelvis and Thorax | The default radius of gyration for the specified segments. The number is scaled to the length of the segment. The Plug-in Gait default setting (0.31) means that the radius of gyration is 31 percent of the segment length. Only change this setting if you are sure that the default value is incorrect. |
| Version | Plug-in Gait version number. |
Cross-plate foot strikes
To perform an inverse reaction calculation from a foot strike, forces and moments recorded by a force plate must:
- Be assigned to a single context (left or right)
- Represent the full forces and moments produced by the subject
In some instances, a foot may be in contact with more than one force plate simultaneously; for example, the heel may land on the far side of one plate and then the toes roll onto the near edge of the subsequent plate.
Dynamic Plug-in Gait includes an Advanced option to permit calculations based on these cross-plate strikes.
Reactions from the two plates are combined during processing, with the plate that is struck first being considered the primary. The forces reported by each plate are added together; the moments are also combined by referring the moment from the secondary plate back to the origin of the primary plate. In effect, the two plates are treated as a single force plate with the same origin as the primary plate.
Dynamic Plug-in Gait can automatically detect which foot is activating a force plate by checking whether the origin of the segment is above the plate. To account for cross-plate strikes, the activating area is expanded beyond the bounds of the plate by the length of the segment (that is, a foot is considered to be above the plate if the ankle joint center is within a foot-length of the plate boundary).
You may also set the foot strike manually (see Add events to trials and also Automatically assess foot strikes ).
Note that the existing restriction that there must be only a single foot in contact with any given force plate still applies.
Invalid strike types that don't benefit from Allow cross-plate strikes
Only cross plate strikes can benefit from the Allow cross-plate strikes option; do not use other invalid strike types for kinetic calculation. The other invalid strike types to which Allow cross-plate strikes does not apply include:
- Double stance plate contacts Multiple feet simultaneously in contact with a single plate.In this example, the left and right foot contact Force Plate 1 at the same time.
Problem: An inability to assign forces to context
- Force plate to floor contacts A single foot strike that transitions from an initial contact with the force plate to the lab floor or begins on the floor and transitions to a force plate. In this example, the heel strikes the force plate, but the foot rolls forward off the plate, and prior to toe off, the foot is in contact with the floor.
Problem: Full forces/moment are not recorded as some are applied to the floor, not the plate
Export C3D
This pipeline operation exports the current state of the processed data to a .c3d file. You can then import the data into other software for further processing or report generation. For example, in Vicon Polygon you can visualize the trajectories, kinematic model elements, and kinetics data. If you manually import the corresponding .vst file into Polygon, it also visualizes the bones. For more information, see Configure the Export C3D operation.
