Is there a quick way to match red (plug-in-gait) and blue (c3d) markers initially?


Basically the title. I am conducting a motion capture experiment and I am using AnyMoCap for this. I have a good quality c3d data for multiple subjects. I am a little short on time and I realize most of my time is spent on matching the red and blue spheres to set the initial conditions.

Is there at least any way to find the center of the blue spheres so that I can match them quickly at the MarkerProtocol.any with the red spheres?

Thanks in advance.

Hi Ayberk,

Normally, you would set some of the markers on the anatomical landmarks and then they should be scaled accordingly with the subject anthropometry. By default, the ScaleMarkerPosOnOff is set to 1. This means that the sRelOpt for the marker will be scaled using the scaling function of the segment.

For markers placed on soft tissues, you can use the parameter optimization to optimize their location.

Please see this tutorial. Please also see this link for more info.

Normally, you would need to spend some time on adjusting the MarkerProtocol for your lab. But this should be a one-time process if you use the same marker protocol for all your subjects.

Then, there is the AutoPosPelvis that normally loads the human model so that pelvis is positioned according to the marker data. If you can process your c3d file in a way that they have similar starting postures, then you can set the load-time posture of the manikin to be close to this posture.

I hope this helps.

Best regards,

Hi Divyaksh,

Thank you for your reply. I used 21 markers for the upper body only, including RASI, LASI and SACR markers for the pelvis. But none for the lower body.

Unfortunately, I realized later that AutoPosPelvis does require RPSI marker and cannot use the SACR. So I will give it a shot next time for sure.

Do you think that if I use the AutoPosPelvis, it will also help determining the leg length of the subject i.e. the distance from pelvis to force plate contact area?

As I understand from your reply, once I make some adjustments for the marker protocol I used (even though I use plug-in-gait, there are some small changes or errors etc.), I can use the same markerprotocol.any for my other trials too because AMS optimizes the anthropometrics automatically. Is that the case?

Thanks for the links.

Hi Ayberk,

You can also use the SACR marker in AutoPosPelvis. Substitute BACK=RPSI with BACK=SACR where the class template is initialized in the model.

Also, it's not exactly necessary to use AutoPosPelvis. If your subjects were at the same starting position/posture during the data collection, then you can just enter approximate values for PelvisPos and PelvisRot instead of using AutoPosPelvis. It should be good enough in most cases.

Strictly speaking, AutoPosPelvis doesn't determine the leg length. It just approximates the 3D position and orientation of the pelvis from the c3d data. Yes, you can get some idea of the distance of the pelvis from the force plate, but that doesn't determine leg length, which depends also on the posture of the legs.

You are right about maker protocol. In the marker protocol, you can tell the model which markers are present in the c3d file and where are these markers located on the human body. Then, the anthropometrics can be optimized automatically (typically once for every subject using a short standing trial). Please note that you also need to be careful about optimization directions in marker protocol. You can decide which markers are optimized and in which directions. Typically, you will keep the markers on anatomical landmarks fixed (and these can serve to optimize the anthropometrics).

I hope this helps.

Best regards,

Thank you very much for your assistance. The information you provided really helped me for the current experiments I am working on.