$\newcommand{\Dev}{\operatorname{Dev}}$ $\newcommand{\dev}{\operatorname{dev}}$ $\newcommand{\grad}{\operatorname{grad}}$ $\newcommand{\divg}{\operatorname{div}}$ $\newcommand{\Ei}{\operatorname{Ei}}$ $\newcommand{\tr}{\operatorname{tr}}$

The idea behind this update rule is to enforce the fiber stretch induced by the initial prestrain gradient. As with the in-situ stretch generator option, this rule has an isochoric version and an uniaxial version for the update rule.

<constraint type="in-situ stretch">
  <tolerance>0.01</tolerance>
  <isochoric>1</isochoric>
</constraint>