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 Subsubsection Huiskes Reaction Rate Up Chapter 4: Materials Section 4.13: Active Contraction 

4.12 Rigid Body

A rigid body can be defined using the rigid material model. The material type for a rigid body is “rigid body”. The following parameters are defined:
<density> Density of rigid body [M/L ]
<center_of_mass> Position of the center of mass [L]
<E> Young's modulus [P]
<v> Poisson's ratio [ ]
If the center_of_mass parameter is omitted, FEBio will calculate the center of mass automatically. In this case, a density must be specified. If the center_of_mass is defined the density parameter may be omitted. Omitting both will generate an error message.
The Young's modulus and Poisson ratio currently have no effect on the results. The only place where they are used is in the calculation of a material stiffness for some auto-penalty contact formulation. If you are using contact it is advised to enter sensible values. Otherwise these parameters may be omitted.
The degrees of freedom of a rigid body are initially unconstrained [K]  [K]  This is different from previous versions of FEBio where rigid bodies were initially fully constrained.. This implies that a rigid body is free to move in all three directions and rotate about any of the coordinate axes. To constrain a rigid body degree of freedom you may use the Constraints sections (Section 3.11.1↑).
<material id="1" type="rigid body">

 Subsubsection Huiskes Reaction Rate Up Chapter 4: Materials Section 4.13: Active Contraction