Issue Modeling Skeletal Muscles

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  • jlooft
    Junior Member
    • Aug 2016
    • 2

    Issue Modeling Skeletal Muscles

    Hello FeBio Support,
    We are researchers at the University of Minnesota looking at modeling skeletal muscle using FeBio. We have created multiple models and have searched through the Message boards, but we have been unable to find the appropriate solution to our problem: Our skeletal muscle is not behaving correctly when we apply a "prescribed isotropic active contraction". Instead of maintaining volume our muscle is shrinking and shortening, instead of expanding at the constant rate of shortening. We believe this is occurring due to our fiber definitions, but are having difficulty defining our fibers based on the manual examples. Secondly, the literature mostly defines skeletal models using hex elements, but we are unable to create a volume mesh from our imported STLs using hex elements in FeBio. What software would you recommend for accomplishing this task as it has proven more difficult than first thought?
    Our muscle material definition is provided below.
    We look forward to your response.
    Cheers,
    John
    <material id="2" name="Muscle" type="solid mixture">
    <mat_axis type="local">1,4,5</mat_axis>
    <solid type="trans iso Mooney-Rivlin">
    <density>1</density>
    <c1>30000</c1>
    <c2>10000</c2>
    <k>60000</k>
    <c3>0</c3>
    <c4>0</c4>
    <c5>0</c5>
    <lam_max>1.8</lam_max>
    </solid>
    <solid type="fiber-exp-pow-uncoupled">
    <alpha>0.3579</alpha>
    <beta>2</beta>
    <ksi>5.3992</ksi>
    <theta>20</theta>
    <phi>90</phi>
    <k>5399.2</k>
    </solid>
    <solid type="fiber-exp-pow-uncoupled">
    <alpha>0.3579</alpha>
    <beta>2</beta>
    <ksi>5.3992</ksi>
    <theta>-20</theta>
    <phi>90</phi>
    <k>5399.2</k>
    </solid>
    <solid type="prescribed isotropic active contraction">
    <T0 lc="7">26000</T0>
    </solid>
    </material>
  • maas
    Lead Code Developer
    • Nov 2007
    • 3400

    #2
    Hi John,

    The first thing I would check is the orientation of the fiber vectors. You can store the fiber vectors in the plot file by adding the following to your plotfile section:

    Code:
    <var type="fiber vector"/>
    In PostView, you can then add a vector plot and choose the "fiber vector" field to visualize the fiber vectors.

    You are also mixing coupled and uncoupled formulations in your material model. The solid mixture expects coupled material formulations, yet the trans-iso MR and the fiber models use uncoupled formulations. If the bulk moduli of the individual materials is relatively high, this may not be a big issue, but the bulk modulus for the trans-iso MR is relatively small compared to c1, c2, so this may be a concern. If you wish to use uncoupled formulations, I advise the "uncoupled solid mixture". We also have coupled formulations for the trans-iso MR and the fiber models.

    Regarding the meshing, we sometimes use TrueGrid or Ansa and have used Cubit as well for hex meshing, but these are commercial applications. I'm not aware of any open-source hex meshing software.

    I hope this helps. Let us know if you have any further question.

    Cheers,

    Steve
    Department of Bioengineering, University of Utah
    Scientific Computing and Imaging institute, University of Utah

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