For my OpenSim undergraduate course project, I have all my students perform static optimization using soccer kicking data collected from me. This year, I told all the students to switch their muscle models to the Millard equilibrium muscle. However, for the obliques, this muscle failed during static optimization, but when it was replaced with the old Thelen2003 muscle, everything worked fine. All the other muscles in the model remained Millard equilibrium muscles. Below is the error output from OpenSim:
java.lang.RuntimeException: SimTK Exception thrown at
Error detected by Simbody method
Equation is singular: check pennation angle
(Required condition 'abs(denominator) >= SimTK::SignificantReal' was
[catch] at java.lang.Thread.run(Thread.java:662)
My guess is that singular equations should not be happening for realistic body poses, but I could be wrong. All the files needed to recreate the problem (apart from Geometry folder, which is from Sam Hamner's full body running model) are in the attached zip file.