Passive Mechanical Testing of Female Human Levator Ani and Superficial Perineal Muscles

The levator ani muscles (LAMs) and superficial perineal muscles (SPMs) are critical to pelvic organ support. While few studies have investigated the female human LAMs via ex vivo mechanical testing, even fewer have investigated the SPMs. In this study, we evaluated female human LAMs and SPMs via ex vivo uniaxial tensile testing, hypothesizing that the LA would have a distinct response compared to the SPMs. SPM-bulbocavernosus (BC), ischiocavernosus (IS), transverse perinei (TP)-and LAM samples were obtained from 8 fresh-frozen female human cadavers. LAMs and SPMs were dissected en bloc as a pelvic floor complex, stored frozen, and then individual muscle specimens were dissected from thawed complexes for mechanical testing. Extension-to-failure tests were performed at a displacement rate of 5 mm min $_{}^{-1}$ , strains were measured using digital image correlation, and load-displacement and stress-strain curves (axial and transverse) were generated. The LAMs exhibited the most compliant behavior, divergent from that of the SPMs, while the IS demonstrated the stiffest response on average. Though SPMs were more similar to one another than to the LAMs, average stress-axial strain curves distinguished between individual SPMs. These results support our hypothesis that the passive mechanical behavior of the LAMs is distinct from that of the SPMs, suggesting it is inappropriate to use LAM material properties to describe SPMs. Additionally, distinction between stress-axial strain curves suggests muscle-specific mechanical properties should be considered even within the SPMs. This study provides novel data critical to improving our understanding of female human LAMs and SPMs and our ability to accurately simulate them and motivates future studies to further investigate their mechanical properties.