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Hello,

I am running the ID tool in OpenSim version 3.1 and I am just looking to get some information about what the tag <output_body_forces_file> is giving me.

It seems that it is creating the body_forces_at_joints.sto, and in that there are 6 columns of data for each coordinate in the model. (_FX,_FY,_FZ,_MX,_MY,_MZ).

What I am hoping for clarification on is:

1. If this file is to report net joint forces, then what are the columns: _MX,_MY,_MZ reporting, and how are they supposed to be different from the other inverse_dynamics.sto output file which reports moments about each coordinate.

2. What are the references for these forces (i.e. force applied by the child body on the parent in the parents reference frame)? The description above the tag in the ID setup file reads:
"<!--Name of the storage file (.sto) to which the body forces at specified joints are written.-->"
and I have not been able to find very much supporting documentation for it.

If anyone could point me in the right direction or share some insight it would be greatly appreciated.

Thanks!

Alex

Hi,

I have the same question. I was wondering if anyone would clarify these outputs or share a link to any documentation in this regard.

Thank you in advance.

Hi Alex,

I think that enabling such tag allows you to calculate both the net resultant joint forces as well as the net joint torque with respect to the joint center (equivalent system of the one where muscles are not replaced by ideal joint torque actuators, i.e. the one used in JRA analysis). If your model has muscles:
1) the net joint force (1 force vector for each joint) results from both articular contact and musculotendon (and ligaments) actions; in this sense it does NOT represent the joint contact force that, instead, you would obtain through the JRA tool where the contribution from muscles is explicitly accounted for.
2) the net joint torque at a joint (1 component for each dof) represents the resultant moment with respect to the joint center exerted by muscles about the X,Y or Z axis.

In its default configuration, the inverse dynamics tool provides the net joint torques only. For a given joint, the magnitude of the output torque written in the inverse_dynamics.sto and in the body_forces_at_joints.sto files should be the same. You have to enable also the following tag: <joints_to_report_body_forces /> and specify which joints you are interested in (the comment indicates that the the forces are expressed in the ground frame, and I think acting on the child body).

I hope it helps you!
Matilde