Query regarding Millard2012AccelerationMuscle Model for Static Optimization Simulation

[Kaushik Mukherjee]

Hello everybody,

I am working with the mouse hindlimb model of Charles et al. (2016) available at https://simtk.org/projects/mousehindlimb. I am interested in predicting musculoskeletal force during normal walking of the mouse. I have successfully incorporated the motion data and performed the inverse kinematics and inverse dynamics simulation. However, my static optimization simulation is failing. I am getting the following two specific messages whenever I am trying to run the static optimization:

1) WARNING: Millard2012AccelerationMuscle initialization exception caught:
SimTK Exception thrown at MuscleFixedWidthPennationModel.cpp:355:
Error detected by Simbody method MuscleFixedWidthPennationModel::calc_DPennationAngle_DfiberLength: Fiber length is below the lower bound for this muscle.
(Required condition 'fiberLength > m_parallelogramHeight' was not met.)

Continuing with initial tendon force of 0
and a fiber length equal to the optimal fiber length,
and a fiber length velocity equal to 0 ...

2) The model appears unsuitable for static optimization.
Try appending the model with additional force(s) or locking joint(s) to reduce the following acceleration constraint violation(s).

I found out that the mouse hindlimb model implements the Millard2012AccelerationMuscle Model and according to the YouTube video (https://www.youtube.com/watch?v=Q1rId1fOkjw) Millard2012AccelerationMuscle Model is not suitable for CMC. The Millard2012AccelerationMuscle Model is only recommended for static simulation and forward simulation. I was, therefore, wondering whether this muscle model is suitable for Static Optimization simulation or it is better to consider the Thelen2003Muscle Model for this purpose.

Any help in this regard will be highly appreciated.

With kind regards,
Kaushik

[Thomas Uchida]

The Millard2012AccelerationMuscle model has been used in only a few simulations and has not been thoroughly tested with all OpenSim Tools. I recommend investigating the Millard2012EquilibriumMuscle (https://simtk.org/api_docs/opensim/api_ ... uscle.html). The model is described in the following paper:

Millard, M., Uchida, T., Seth, A., Delp, S.L. Flexing computational muscle: modeling and simulation of musculotendon dynamics. ASME Journal of Biomechanical Engineering 135(2):021005, 2013. http://nmbl.stanford.edu/publications/p ... rd2013.pdf