Representation of Forces

[Kaitlyn Downer]

Hello,

I am attempting to scale my model using AddBiomechanics, but I previously discovered issues in my CoP calculations when I expressed the forces with respect to the CoP, leading to high residual moments. To move past this, I decided to use the method of applying the force and moment from the origin of the forceplate (so representing force from the forceplate origin - resulting in constant p values). I have performed scaling & RRA in OpenSim, and got residuals within the recommended range. However, my residual moments when using AddBiomechanics with the same model, marker file (TRC), and GRF file (MOT) are >300Nm.

Is it not possible to use this representation of forces within AddBiomechanics? Based on various OpenSim forum posts, I was under the impression the CoP representation was only really necessary for visualization purposes. Also, if data is uploaded as a C3D file, does it automatically use the CoP representation?

Best,
Kaitlyn Downer

[Nicholas Bianco]

Hi Kaitlyn,

You will need to use the center of pressure representation of forces because AddBiomechanics uses some heuristics under the hood to assign input forces to foot bodies to the model. If the applied forces are not coincident with the foot bodies, then they probably won't be assigned properly.

We will keep other force representations in mind as a possible future option.

Best,
Nick