Muscle length in the upper limb model (arm26)

[fujibe kenshun]

I would like to derive the length of each muscle (TRIlong, TRIlat, TRImed, BIClong, BICshort, BRA) in the upper limb model (arm26.osim) as a function of the shoulder and elbow joints when they are moved. How do you derive the muscle length for each joint angle in OpneSim?

[Thomas Uchida]

You could use the plotter to plot muscle-tendon length vs. joint angle and then export the data. The "Curve Creation Panel" page in the documentation (https://simtk-confluence.stanford.edu:8 ... tion+Panel) provides some additional information about the plotter.

[fujibe kenshun]

Thank you for your reply.
The length of the muscle changes as the joint angle changes, but what formula is used to output this?

[Thomas Uchida]

The length of the muscle changes as the joint angle changes, but what formula is used to output this?

The length of a muscle-tendon actuator is the sum of the line segment lengths and arc lengths that define the muscle path. There's some information about muscle paths on the "Muscle Editor" page in the documentation (in the "Muscle Paths and Muscle Points" section): https://simtk-confluence.stanford.edu:8 ... cle+Editor. Additional information about the muscle models can be found in the "Muscle Model Theory and Publications" section (https://simtk-confluence.stanford.edu:8 ... blications). In the simplest case, where a muscle path is defined only by origin and insertion points, the length of the muscle-tendon actuator is the length of this one line segment---specifically, the length of the vector between the two points in space.

[fujibe kenshun]

Sorry for the late reply.

Thanks to you, I was able to derive the muscle length function by reducing it to a quadratic plane for now.

Next, I will extend it to 3 dimensions.

Thank you very much.