force mot file

[Mohammadreza Rezaie]

Dear Experts,

I have two queries about generating the mot file from c3d and hope you would help me to find the answer.

1) Arranging the forces, COPs, and moments of more than 2 force plate in 18 columns is a little tricky. I saw some examples with 27 columns with the following headers:
1_force_vx,y,z 1_force_px,y,z 1_torque_x,y,z
2_force_vx,y,z 2_force_px,y,z 2_torque_x,y,z
3_force_vx,y,z 3_force_px,y,z 3_torque_x,y,z
... or more columns
Indeed each force plate data are in separate columns instead of arranging them for right and left foot.
If I assign 1st and 3rd force plate data to a segment (eg. calc_r), how OpenSim treat with this? does it use the sum of them? is it right to do this for more than 2 force plate data?

2) I ask this query just to be sure about it. If the time column in the mot file doesn't start from zero, does OpenSim know when to use force data for inverse dynamics calculation? In GUI, from zero to the starting point of the force file is a constant value (the first force value in mot file) and a static vector is shown until the starting point. But it is synced perfectly with markers in available times. Again is it right to generate mot files like this?

Capture.JPG (12.75 KiB) Viewed 1111 times

Your response would be greatly appreciated.

Regards

[Mohammadreza Rezaie]

Any help would be greatly appreciated.

Regards

[Mohammadreza Rezaie]

I'm greatly grateful for any suggestions or recommendations.

Cheers

[Michael Asmussen]

Dear Mohammadreza,

With respect to the picture you are showing, is this the largest magnitude you see for the GRF vector? If so, there appears to be a synchronization issue. What method are you using to synchronize the force plate and kinematic data?

With respect to you question about multiple force plates, I have done some processing related to this. I am guessing that a person is walking across multiple force plates and you want to assign the force to the right foot, for example, when the right foot contacts the 1st and 3rd force plate. Is this correct? If so, the force should be zero on force plate 3 when the foot is in contact with force plate 1 and vice versa.

In my experience and if I remember correctly, Opensim will take the sum of those forces acting on the right foot. If my assumption is correct though, this should not be an issue because one force plate should be zero when the foot is in contact with the other force plate.

All the best,

Mike

[Mohammadreza Rezaie]

Hi Mike,

Thanks a lot for your help and I'm please to see your response.

The picture i attached had cropped forces (eg. between 0.33 to 0.97 seconds). Indeed i didn't start the force from the beginning (zero second) and it has been cropped to the left foot stance and the vector in missed seconds was the first value of mot file (not the peak).

I uploaded sample trc and mot files and i was wondering if you would tell me whether the ID, RRA and CMC can be calculated correctly for these types of motion files (not only for left foot in this example, but also for the more strides with more force plates assigned to a segment)

Kindly Regards

Desktop.zip

(112.22 KiB) Downloaded 53 times

[Michael Asmussen]

Dear Mohammadreza,

Thanks for uploading the files. What model do you intend to use with this data?

Based on what you sent, I believe you should be able to perform all you intended analyses. The one part that isn't entirely clear is whether the motion data and force data are synchronized. I would just make sure you have the motion and force data entirely synched before bringing it into OS.

Another point that is worth considering if you intend to use a 3rd or 4th force plate is to ensure that when the foot is not in contact with the 3rd or 4th plate, you should set the force data to zero (i.e., remove any noise in the force plate data). If the extra force plate is quite far from the participant's foot, that could create a very large moment arm and, depending on the magnitude of the noise, create a large enough moment about the segment the force is applied to and that can create some issues in you analysis.

Anyway, I hope this information is helpful

Mike

[Mohammadreza Rezaie]

Hi Micheal,

Thanks a lot for your help.

Most of the time I use Rajagopal model. But my query was for developing my own MATLAB code for exporting OpenSim motion files from c3d files. The data were well synced (from a c3d sample of c3dextract toolbox by Tim Dorn).
Well, this method is more convenience than of arranging all forces, torques and COF in 18 columns and I appreciate your insights.

Cheers

[Michael Asmussen]

Hi Mohammadreza,

Do you still have issues or is everything sorted out now? If there are still issues, please post them.

All the best,

Mike

[Mohammadreza Rezaie]

Hi Micheal,

Thanks for your follow up,

You solved the issue and I appreciate it.

I wonder if you would help me in this post too:
viewtopicPhpbb.php?f=91&t=11732&p=32926&start=0&view=

Kindly Regards

[Allison Porter]

Hello,

I have a question relating to this issue. I also posted in the forum a few hours ago (see post "Multiple GRF Plate Processing for OpenSim Import"). If I have the force vector coordinates, COM coordinates, and moment components for 4 individual force plates, but I know that plates 1+4 and 2+3 correspond to the subject's left and right foot, respectively, how to I aggregate the COM and moment values from these 4 plates into values for the left foot and right foot individually? The method would need to account for the cases where a foot is on the boundary and overlaps onto two plates. Otherwise, is there a way to use overall force/COM/moment values (rather than needing to separate into left and right)?

Dear Mohammadreza,

With respect to the picture you are showing, is this the largest magnitude you see for the GRF vector? If so, there appears to be a synchronization issue. What method are you using to synchronize the force plate and kinematic data?

With respect to you question about multiple force plates, I have done some processing related to this. I am guessing that a person is walking across multiple force plates and you want to assign the force to the right foot, for example, when the right foot contacts the 1st and 3rd force plate. Is this correct? If so, the force should be zero on force plate 3 when the foot is in contact with force plate 1 and vice versa.

In my experience and if I remember correctly, Opensim will take the sum of those forces acting on the right foot. If my assumption is correct though, this should not be an issue because one force plate should be zero when the foot is in contact with the other force plate.

All the best,

Mike