minor inconsistencies with the Arnold's model in OpenSim 4.0

[Arash Mohammadzadeh Gonabadi]

Hello all,

I have a question about Arnold's model (Lower Limb Model 2010). The following link is the model:
https://simtk.org/projects/lowlimbmodel09

=> When I run this model in OpenSim 3.3 (GUI) and extract the IK results, everything seems good.

=> But, When I run this model in OpenSim 4.0 (GUI) and extract the IK results, there are some conflicts and issues.
I noticed that there are some minor inconsistencies with the Arnold model. For example, it appears that the joint angle and joint angular velocity for the left hip and right knee are stored in degrees, and the right hip and the left knee is in radians.

I was wondering if someone could help me. Also, I have attached the files.

OpenSim - Arnold's model.rar

(378.41 KiB) Downloaded 21 times

Thank you so much.

Best,
Arash

[jimmy d]

Hi, Arash-

There have been changes in 4.0 with how motion types are specified which cause the behavior your describing. The details are in the OpenSim changelog with the relevant part below;

The MotionType of a Coordinate is now fully determined by the Joint. The user cannot set the MotionType for a Coordinate. There are instances such as in the leg6dof9musc and Rajagopal2015 models, where a Coordinate was assigned an incorrect type (e.g. when a coordinate of a CustomJoint is not a measure of a Cartesian angle). In 4.0, the coordinate is correctly marked as Coupled since a function couples the coordinate value to the angular displacement of the patella in Cartesian space. NOTE, this causes issues (e.g. opensim-org/opensim-gui#617, #2088) when using kinematics files generated in 3.3 (or earlier) where Rotational coordinates have been converted to degrees. Because OpenSim 4.0 does not recognize the coordinate's MotionType to be Rotational it will not convert it back to radians internally.

Cheers,
-James

[Arash Mohammadzadeh Gonabadi]

Thanks a lot, James.
It was really helpful. I really do appreciate your consideration.

Best,
Arash