simtk.org

Hi,

I would like to find some information about how Opensim compute the moment arms.
In particular my ultimate task will be to input the moment arms in the model according with some measure coming from imaging.

Thanks
Fausto

Since there are several approaches which can calculate moment arms, I'm not positive which one OpenSim uses, but I believe it does it this way:

Moment Arm = ( Change in length of muscle-tendon unit) / ( Change in joint angle)

Note that the change in muscle-tendon length is a function of joint angle, and joint angles should be in radians (OpenSim uses degrees by default.)

Yes, thanks.

I found the same on Delp's paper of 1995.
My point is where or how I can find the function that describe the change in muscle length with respect to the joint angle.

You can easily plot the muscle length as function of the joint angle from the GUI. The instructions are described in one of the tutorials if I'm not wrong.

Luca

Hi All,

To remove the uncertainty that you (and other OpenSim users) have about how we compute moment arms, I posted a write up describing how Simbody/OpenSim compute/report moment arms on the Wiki at
http://wiki.simtk.org/opensim/FrontPage#preview and follow the link for "How Simbody/OpenSim Compute Muscle Moment Arms"

As to the question of how to "input" moment arms, the answer is that you can't. You compute moment arms based on a modeling decisions that you make when creating a model (e.g. where to attach muscles, how many muscle points/via points, do you use wrapping, and if so what type of surface/parameters) then OpenSim will calculate the moment arms for a specific set of assumptions. The reverse operation as I explained is not unique but I can imagine some GUI tools that would help guide the process in the future.

Please let me know if that answers your questions and good luck,
-Ayman

Thanks Ayman,
the document you post was very helpful.
Cheers
Fausto