External Load "Body Force" Setting

[Edward Syrett]

Hi there,
I'm trying to apply some GRF values to my model using the external load tool. When converting from C3D, I get problems with the .sto file, so I have been using the setting to set the point to the origin of the force plate. However, this creates problems when applying the force to the foot (not appropriate inverse dynamics). So I wanted to set it to apply as a body force to the calcaneus.
When I click the radio button to set the force as a "Body Force" rather than "Point Force", it greys out the point columns. However, when I save the file or return to the window, it defaults back to "Point Force". I cannot seem to get the body force setting to stick. Is this something that anyone else has experienced? Or does anyone know the underlying .xml syntax to apply is as a body force that I can just add manually?
Thanks!

Dan

[Josh Carter]

Hi,

Did you ever manage to resolve this issue? I am also interested in applying Ground Reaction forces directly to the body frame rather than through a centre of pressure point. However, I seem to be experiencing the same issue as you where it does not save this selection to the XML, and also has no reference to this selection in the XML.

Thanks,
Josh

[Johannes Amend]

Any solutions yet? I have the same problem

[Nicholas Bianco]

Hi all,

If you set the applied force to a "Body Force", it will apply the force at the origin of the body frame, so there is no need to specify the "Point Columns". In addition, the selections in "Force Expressed In" and "Point Expressed In" will be ignored. See here for the full documentation.

Best,
Nick

[Ross Miller]

Here is a nice post from Ton van den Bogert on Biomch-L that explains the difference between these options (point force vs. body force) for GRF well: https://biomch-l.isbweb.org/forum/biomc ... #post39057

Essentially the "body force" definition replaces the center of pressure with the moment produced by the GRF about the axes of the force platform. Like if you set a weight on the force platform, it will register a vertical force, and this force produces moments about the axes of the plate. The magnitude of that vertical force will always be the same, but the moments will be different depending on where the weight is on the plate. It is generally "better" in my opinion than the "point force" definition, for the reason Ton highlights (no division by zero). Can always calculate the center of pressure later if it's needed as an outcome variable.

In either case, point force or body force, I think the free moment still needs to be applied for correct inverse dynamics. The moments on other axes should be zero for a point force and non-zero (generally speaking) for a body force.

If using the "c3dExport" script to convert C3D files to OpenSim MOT and TRC files, the second argument to the osimC3D function specifies how you want the GRF represented in the MOT file: 0 = body force, GRF is applied to the foot at the force platform origin; 1 = point force, GRF is applied to the foot at the center of pressure; 2 = GRF is applied at point of wrench application (I'm not sure what that is).

The only situation I use point forces for inverse dynamics is with AddBiomechanics, where it seems to be expecting point forces as inputs (maybe Keenon will see this and can confirm).

Ross