Modeling permanent deformation during low-cycle fatigue: Application to the pelvic floor muscles during labor

Abstract

Low cycle fatigue (between two to hundreds of cycles) is associated with large permanent strains and the development of microdamage that can start as soon as the first cycle. It has been identified as a possible damage mechanism in the anterior cruciate ligament and recently was evaluated as a potential damage mechanism at the origin of the pubovisceral muscle (a pelvic floor muscle) near the end of labor. The purpose of this work is to provide a numerical framework that incorporates the effects of permanent deformation due to low-cycle fatigue. Based on experimental evidence, we hypothesize that the microdamage accumulation due to repetitive sub-maximal loading continuously induces macroscopic permanent deformations that adversely affect the tissue mechanical properties and thereby cause stress softening. The constitutive model is suitable for 3D finite element analysis. The model was first tested against simple numerical examples, where quadratic convergence and objectivity were verified. Next, we applied the approach to a human vaginal delivery, employing a transversely isotropic visco-hyperelastic baseline model. It demonstrated the potential of this numerical framework for simulating permanent deformation in pelvic floor muscles during birth. With robust experimental data, this can be an important tool to analyze the obstetrical risk factors for the pelvic floor permanent deformation associated with the development of pelvic organ prolapse later in life.