Vicon Model Outputs and Virtual Markers

[Sydney Dreves]

Hello,

I am collecting data using Nexux 2.6.1 and I am doing all of my post-processing on Nexus 2.7, to analyse kinetics and kinematics during a single leg countermovement jump onto a force platform. I am using a plug-in gait lower body cluster marker set (that was made up using ProCalc).

Here is the issue, for some of my trails where the Right leg is the leg performing the single leg countermovement jump, the model outputs RKneeForce and RKneeMoment are empty while the Right foot is in contact with the force platform. However, the LKneeForce and LKneeMoment model outputs are present for those frames; which could not be correct since that leg is not in contact with the force platform during those frames. It seems as though Vicon is mixing up left and right model outputs, I have double checked that my skeleton is labeled properly, I have also checked the force platform data and it does not seem to be the issue. Any ideas as to why this may be happening to some of these trails and not others, and how I can fix it?

Secondly, for some of my trails a marker drops out towards the end of the trial and never comes back into view, which makes it impossible for me to 'fill' those gaps. Is there a way to create a Virtual marker based off of another trials, or a static capture? I have created Virtual markers in Motion Analysis based off of the static capture but I am unsure how to do so on Vicon.

Best regards,
Sydney Dreves

[Kyle Chadwick]

Re: Vicon Model Outputs and Virtual Markers

Hi Sydney,

Vicon support can probably give you more definitive help - but I've dealt with both of these issues so I'll give my input.

Vicon introduced some changes in when it assigns the force plate inputs to each limb. The intention was to improve auto-detection but I found sometimes it ignored strikes or assigned to the wrong limb. The easy fix for this is just right-clicking on the force plate and assigning the proper limb before running the pipeline.

There isn't a super easy way to fill missing markers at either end of the trial, but when it was absolutely necessary - I've done pretty much what you are proposing. I have written simple bodybuilder scripts to record offsets between the marker of interest and 3 markers on the same segment during a static trial. I then use a 2nd similar bodybuilder script to use that offset to place the missing marker back into the dynamic trial. You can either apply it to the whole trial or crop to the very end and apply it to a few frames so that you can use conventional means of filling the remaining gap (my preference). If you need some sample bodybuilder scripts to go off of email me and ill send you some kchadwick@chla.usc.edu

Kyle

Hello,

I am collecting data using Nexux 2.6.1 and I am doing all of my post-processing on Nexus 2.7, to analyse kinetics and kinematics during a single leg countermovement jump onto a force platform. I am using a plug-in gait lower body cluster marker set (that was made up using ProCalc).

Here is the issue, for some of my trails where the Right leg is the leg performing the single leg countermovement jump, the model outputs RKneeForce and RKneeMoment are empty while the Right foot is in contact with the force platform. However, the LKneeForce and LKneeMoment model outputs are present for those frames; which could not be correct since that leg is not in contact with the force platform during those frames. It seems as though Vicon is mixing up left and right model outputs, I have double checked that my skeleton is labeled properly, I have also checked the force platform data and it does not seem to be the issue. Any ideas as to why this may be happening to some of these trails and not others, and how I can fix it?

Secondly, for some of my trails a marker drops out towards the end of the trial and never comes back into view, which makes it impossible for me to 'fill' those gaps. Is there a way to create a Virtual marker based off of another trials, or a static capture? I have created Virtual markers in Motion Analysis based off of the static capture but I am unsure how to do so on Vicon.

Best regards,
Sydney Dreves

[Sydney Dreves]

Re: Vicon Model Outputs and Virtual Markers

Hi Kyle,

Thank you so much for the help, I tried this and it has worked great! I will follow up via email about the bodybuilder scripts.

Best,
Sydney

[Paulo Lucareli]

Re: Vicon Model Outputs and Virtual Markers

Hi Kyle,

Thank you so much for the help, I tried this and it has worked great! I will follow up via email about the bodybuilder scripts.

Best,
Sydney

Dear Sydney,

Do you mind to share you procalc script to use clusters and calculate angles using 6Dof model?

Best

Paulo Lucareli