Passive Force

[Elizabeth Sylvestre]

Hello,

I use the same model for SIMM 5.0 and OpenSim 2.0.2. However, when using the passive force plot tool, the results obtained vary from one program to another. For example, when calculating passive force of the deltoid muscle during humeral abduction, SIMM calculates the peak force at 200 N while the results obtained in OpenSim are of 500 N for the same motion. Would it be possible to obtain an explanation on the nature of this disparity?

Thank you

[Peter Loan]

Hi Elizabeth,

The differences in passive force are due to the different muscle models in OpenSim and SIMM. Here's the history:

SIMM uses a natural cubic spline to represent the passive force-length
behavior of muscle fibers. This spline is specified in the muscle file and
can be modified by the user. The Dynamics Pipeline module in SIMM
uses a "dynamic" muscle model based on work by Lisa Schutte (muscle
model 4 in the code). This model uses an exponential equation for passive
force because allowing the force to go to zero can sometimes causes
problems for the numerical integrator. The Schutte1993Muscle model in
OpenSim uses exactly the same equations as the Dynamics Pipeline
muscle model 4.

The exponential equation for passive force produces higher forces than
the default SIMM spline. For a normalized length of 1.5, the SIMM spline
gives a normalized force of about 0.4, and the exponential gives a force of 1.0.
This is why you are seeing much higher passive forces in your model.
By the way, the Thelen2003 model also uses an exponential for passive force,
but the coefficients are different (and it's linear for large lengths). For a
normalized length of 1.5, it gives a force of 0.51. Furthermore, the coefficients
of the Thelen2003 model can be changed in the OSIM file (whereas the
Schutte ones cannot), so you might consider using that muscle model
instead.

Regards,
Peter

[Elizabeth Sylvestre]

Hello Mr. Loan,

The information you provided us was very helpful.

Regards,

Elizabeth