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Date:
2014-05-07 15:09
Priority:
3
State:
Open
Submitted by:
Allison Kinney (alhall20)
Assigned to:
Ayman Habib (aymanh)
Summary:
Unexpected Moment Arm Values

Detailed description
I am using a subject-specific model of a subject with a knee replacement. The muscle lines of action and attachment points are based on the Arnold 2010 model.

When I run the analyze tool to calculate the moment arms for all of my degrees of freedom, I have some unexpected results for certain muscles. For example, the gaslat_l muscle wraps around two wrapping surfaces (femoral_component_obstruction and Gastroc_at_shank_l). The resulting knee flexion moment arm from the Analyze Tool has unexpected behavior (spikes where the moment arm changes magnitude very quickly) when the muscle comes into or out of contact with both wrapping surfaces (see red line in attached figure). If the moment arm is calculated with either (but not both) wrapping surface, the moment arm is smooth (see green and blue lines in attached figure). I would expect that when the muscle contacts both wrapping surfaces the resulting moment arm would be some average between the green and blue lines when they are both in contact, but that is not the case.

The non-smooth moment arm seems non-physiological. This may not be a bug, but I wanted to bring this to the attention of the OpenSim team in case it warranted investigation.

I have attached the model files, the knee flexion moment arm analyze file, and the figure showing the moment arms.

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