ID giving vertical ground reaction force profile

[Paige Kendell]

Hi All,

I’m working on a project that uses MotionMonitor as our motion capture software. The version we have is not capable of giving us C3D files so we have created a set of MatLab functions that write the .mot and .trc files for inverse kinematics and well as the ground reaction force .mot file for inverse dynamics. OpenSim has no trouble reading the .trc and .mot files during the inverse kinematics and the we are happy with the joint angle results we are looking at (knee, specifically). But when we attempt to do the inverse dynamics, the results contain moment values too high for typical knee moments during gait. When graphed, the right knee moment data is essentially the same as the vertical ground reaction force data that ID is supposedly reading in. The numbers follow the ground reaction force trend versus a typical trend for moments in the knee while walking.

Has anyone else run across this problem before? I suspect the issue is within the MatLab code that is writing the file, but I can’t seem to find any differences between our file and the ground reaction force motion file that comes with the Gait2354 which I’ve been using as a primary resource.

Thanks for any help,
Paige Kendell

[John Rogers]

Paige,
This sounds like the same problem I have. See my post:
https://simtk.org/forums/viewtopic.php?f=91&t=4855
John

[jimmy d]

Hi Paige,

This often happens when the units of force are incorrect (Nmm instead of Nm). When the forces are (apparently) so high, the mass of the model doesnt really matter and the resulting moments at the knee look very similiar to the grf's. In my own Vicon pipeline, I have to divide the forces by 1000 to get them into the correct units.

John, Im not sure if this is the same issue you are having. Was on holiday so will get to it soon.

Let us know if that helps,
-james

[Paige Kendell]

Hi James,

My ground reaction forces have units of Nm.

However, the marker trajectory data in my .trc files is in m whereas I've noticed Gait2354 and Gait2392 both have marker data in terms of mm. Could this unit difference cause any issues in the subsequent steps of OpenSim?

Thanks again,
Paige

[Mathew Varre]

Hi Paige,

I don't know if this would be of any help now. From my experience with running ID, I have noticed two things that would a huge make difference in the moments/force output.

1) If you're using directly IK without running RRA, filtering the data makes a huge difference in the output.

2) I have used the second header given in the 'preparing your data', which is both SIMM and OpenSim header. The .mot file given in gait2354/2392 used the first type of header. I used the second header form and it made my results look better. With regards to units, I used Force in Newtons, point of force application (COP in your case) in meters ( I divided my PFA by 1000 for this) and moments in Newton-meters. This makes units consistent for all the input variables. But if your marker data is already in meters, you don't have to make this division for PFA.

http://simtk-confluence.stanford.edu:80 ... t%29+Files

Hope this helps,
Mathew