Muscle-Tendon Length - Coordinates Function

[Axel Koussou]

Dear OpenSim Experts,

I'm using the Rajagopal's model, and I would like to know if there is a way to obtain the analytical expression of the function between the Muscle-Tendon Length and the joint angle. For instance, there is a way to obtain the detail of the function between the left semitendinosus and the left knee angle.

I'm aware that in the GUI, it is easily possible to plot the muscle-tendon length with respect to the joint angle, but I can't find a way to obtain the function behind this plot.

Thanks in advance,

Regards

[Thomas Uchida]

If you would like a function that describes muscle-tendon length vs. joint angle, you will need to generate it yourself. OpenSim computes the length of the muscle-tendon actuator from the geometric description of the muscle path and the pose of the model.

[Axel Koussou]

OpenSim computes the length of the muscle-tendon actuator from the geometric description of the muscle path and the pose of the model.

Yes, indeed, and that's exactly what I want. For example, is it possible to know the function which compute the length of the bi-articular muscle, the gastrocnemius, with respect to the knee and ankle angles ?

Thanks,

Regards

[Thomas Uchida]

You can set the pose of the model and then call the getLength() method (https://simtk.org/api_docs/opensim/api_ ... c99ba70093). If you need muscle-tendon length expressed as a function of a joint angle, you can repeat for all poses in which you are interested and then fit a curve to the points. Depending on the model, your research question, etc., you may also be able to find this information in a paper.