Need help calculating total fiber-force given muscle-tendon length

[Ben Bolen]

Hello,
I am a mechanical engineering grad student working on a project to build a bipedal robot with pneumatic artificial muscles. I wrote the location of attachment points from GaitModel2392 and placed them in Matlab. With this information I've been able to find the total muscle-tendon lengths and confirm they match what is found in OpenSim. I now need to calculate the total fiber-force (and later on I'll need to find the moment and torque). Given the muscle specific properties of total muscle-tendon length (length), max isometric force (mif), optimal fiber length (ofl), tendon slack length (tsl), and pennation angle at optimal, along with the other properties like FMaxMuscleStrain, how can I compute total fiber force?

I've read the muscle theory pages on the OpenSim site, and the Thelen 2003 paper. Still unclear on how that differential equation is used to calculate total fiber-force, and which Thelen variables correspond to which OpenSim variables. An equation for total fiber force (and moment), along with a concrete example, would be greatly appreciated.

[jimmy d]

There are details on the Wiki Page.

You can get access to the muscle parameters and values through the API documentation.

You can get a better feel for the muscle calculations by going through the Tug-of-war example.

You can also go through the source code to see the Thelen or Millard muscle code.

[Ben Bolen]

Like I said in the previous post: I've already looked at the Wiki and it wasn't very helpful. I've also looked at the source code but am not really experienced enough in C++ or Arduino to really understand what the program is doing. It is unclear what variables like excitation, activation, and velocity have to do with finding the force in a static muscle.

I'm hoping someone in the forum can explain to me, in plain English and using examples, how to calculate force and moment given the known variables listed in the original post above.

[jimmy d]

Here are some papers for you to read:
http://e.guigon.free.fr/rsc/article/HoyEtAl90.pdf
http://nmbl.stanford.edu/publications/pdf/Delp1990b.pdf
http://nmbl.stanford.edu/publications/p ... rd2013.pdf
http://proceedings.asmedigitalcollectio ... id=1830919
http://journals.plos.org/plosone/articl ... ne.0112625
http://nmbl.stanford.edu/publications/p ... ld2013.pdf
http://biomechanical.asmedigitalcollect ... id=1409437
http://people.stfx.ca/smackenz/Courses/ ... ces[1].pdf
https://www.sciencedirect.com/science/a ... 9003000101

[Ben Bolen]

James,

Thank you for your help. After I read these papers I was able to accomplish the things I needed to do.

Sincerely,
Ben