C3D markets drop

#1

Hi

My C3D file have some markets falling off during the operation. What effect does it have on kinematics analysis? How can I solve this problem?Any help would be greatly appreciated

Best regards
zzs

#2

Hi ZZS

Sorry for the slow reply. Easter holiday here in Europe :slight_smile:

My C3D file have some markets falling off during the operation. What effect does it have on kinematics analysis? How can I solve this problem?

Marker dropouts are always a problem, but it can be handled in AnyBody. All markers can have a weight associated. That weight can be faded to zero when the marker drops out. The AnyInputC3D class can create these weight functions automatically for you based on the ‘residual’ information in the C3D file. The class has a member called ConstructWeightFunUsingResidualOnOff which can be set to on. After that all the markers need to be modified to use those weights.

Hence, the marker will have no effect when it drops out. However, it is very important that there is enough other markers to define the kinematics. But marker dropouts are not without problems. When markers suddenly jump in and out of the analysis they may cause bad acceleration which gives problems in inverse dynamics.

Handling marker dropouts is a bit difficult in the older version of AnyBody and AMS. Simply because you have to do everything your self. But in the newest versions, it is handled automatically. On of the examples in the newest AMMR even have markers that drop out, and that is handled automatically because all drivers use the weight functions by default.

That said, you should always try to avoid marker dropouts. The MoCap software I know (vicon and Qualisys) can ‘gap-fill’ short dropouts. In the case of longer drop-outs it is not a good idea to do gap-filling or try to fix the c3d file. In those cases, it is better to use the Weight functions in AMS or simply to remove the marker completely from the AnyBody model fo that particular trial. That solution is often easiest.

BW

#3

Hi melund

Thank you for your reply,this helps me a lot. I have updated Anybody to version 7.1 a few days ago.My C3D file can’t be analyzed now. In the initial position, there is also a few markets missing on the pelvis. When it runs in Mokka, there are many markets in the process that will disappear and appear again. Can the new version handle this situation automatically?

BW
zzs

#4

Hi ZZS

I have updated Anybody to version 7.1 a few days ago.

That is great. That should help a lot. I also think you may even be able to use the newest AMMR with that version of AMS. At least I know that AMMR 2.2 works with AnyBody 7.1.2. The newest AMMR is not included with your version of AnyBody, but you can download it from Zenodo where it is achieved. So give that a try.

When it runs in Mokka, there are many markets in the process that will disappear and appear again. Can the new version handle this situation automatically?

Yes. That should be handled correctly in AMS (at least 7.1.2). But you better base your model on the newest MOCAP examples from AMMR 2.2. Just to be sure the all the settings are configured correctly.

But marker dropouts are not great in general. They easily cause weird accelerations when the drop-in/out, and if too many markers are missing there may not be enough information to satisfy all the degree of freedom. Too few markers on the pelvis could be a problem.

AnyBody’s ability to handle marker dropouts are more tool to save the occasional drop-out which is hard to avoid. Not to compensate for bad data. And remember very short drop-outs are best gap-filled in the MoCap software.

BW

#5

Hi melund

Thank you very much for your help. I will try it in AMMR2.2. If the new version does not adjust automatically. What should I do. Can I use Matlab to modify C3D files?

BW

#6

Yes. You could use something like Matlab with BTK or Python with ezc3d to modify the C3D file. But you do need some programming and signal processing skills to write the code that does gap-filling for you. Which MoCap system did you use for recording your data?

#7

Hi melund

Sorry, I don’t know what system to use.I think these C3D files can only be modified in this way, the file data is missing too much.

BW

#8

There is a discussion here about possible methods for gap-filling.

#9

Thanks for your help, I will try it.:slightly_smiling_face: