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 Subsubsection Mooney-Rivlin Up Subsection 4.1.2: Uncoupled Materials Subsubsection Ogden Muscle Material

This material model implements the constitutive model developed by Silvia S. Blemker [18]. The material type for the muscle material is muscle material. The model is designed to simulate the passive and active material behavior of skeletal muscle. It defines the following parameters:
<g1> along fiber shear modulus [P]
<g2> cross fiber shear modulus [P]
<p1> exponential stress coefficients [P]
<p2> fiber uncrimping factor [ ]
<Lofl> optimal fiber length [ ]
<smax> maximum isometric stress [P]
<lambda> fiber stretch for straightened fibers [ ]
<k> bulk modulus [P]
<active_contraction> activation level
The main difference between this material formulation compared to other transversely hyperelastic materials is that it is formulated using a set of new invariants, originally due to Criscione [23], instead of the usual five invariants proposed by A.J.M. Spencer [52]. For this particular material, only two of the five Criscione invariants are used. The strain energy function is defined as follows: The function is the strain energy contribution of the muscle fibers. It is defined as follows: where, and The values and are determined by requiring and continuity at .
The parameter is the activation level and can be specified using the active_contraction element. You can specify a loadcurve using the lc attribute. The value is interpreted as a scale factor when a loadcurve is defined or as the constant activation level when no loadcurve is defined.
The muscle fiber direction is specified similarly to the transversely isotropic Mooney-Rivlin model.
<material id="1" type="muscle material">
  <fiber type="vector">1,0,0</fiber>

 Subsubsection Mooney-Rivlin Up Subsection 4.1.2: Uncoupled Materials Subsubsection Ogden