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FEBio implements an 8-node trilinear hexahedral element. This element is also known as a brick element. The shape functions for these elements are defined in function of the isoparametric coordinates $r$ , $s$ and $t,$ and are given below. $$\begin{equation} \begin{aligned}N_{1} & =\frac{1}{8}\left(1-r\right)\left(1-s\right)\left(1-t\right)\\ N_{2} & =\frac{1}{8}\left(1+r\right)\left(1-s\right)\left(1-t\right)\\ N_{3} & =\frac{1}{8}\left(1+r\right)\left(1+s\right)\left(1-t\right)\\ N_{4} & =\frac{1}{8}\left(1-r\right)\left(1+s\right)\left(1-t\right)\\ N_{5} & =\frac{1}{8}\left(1-r\right)\left(1-s\right)\left(1+t\right)\\ N_{6} & =\frac{1}{8}\left(1+r\right)\left(1-s\right)\left(1+t\right)\\ N_{7} & =\frac{1}{8}\left(1+r\right)\left(1+s\right)\left(1+t\right)\\ N_{8} & =\frac{1}{8}\left(1-r\right)\left(1+s\right)\left(1+t\right) \end{aligned} \,.\label{eq375} \end{equation}$$ The following integration rule is implemented for this element type.