simtk.org

Hi,

Hope you are all well.

I am trying to estimate muscle activations on an upper limb model. Some muscle paths have points that translate in space following a specific coordinate constraint (similar to the definitions if the Stanford Upper limb model).
Both SO and CMC seem to get stuck at the first time frame with no error.
I tried to plot muscle lengths and moment arms to verify if there are some trajectory issues, but all I get is an empty plot for these constrained muscles moment arms.

Are such constrained too rigid for the model to converge successfully or am I missing something ?

Thank you in advance for your help.

Najoua

Hi,

I think I see the problem. Since the muscle points are connected to massless bodies, the dynamics might be problematic.
Is my understanding correct?

Najoua

Hi Najoua,

Yes, you're correct, leaf bodies (in the multibody system tree) can not be massless, otherwise accelerations can be arbitrarily large. In general, if using somebody else's model it's important to know if the model is dynamics capable or not. There're many kinematics only models around and typically these would be missing mass and inertial properties. Populating these is a non-trivial task in general, so unless you're developing a new model, you better find a dynamics capable one.

Hope this helps,
-Ayman