Sexual Dimorphism in the Architectural Design of Rat and Human Pelvic Floor Muscles

Abstract

Skeletal muscle architecture is a strong predictor of in vivo functional capacity and is evaluated in fixed tissues, accommodating the study of human muscles from cadaveric donors. Previous studies evaluating the pelvic floor muscles (PFMs) demonstrated that the rat is the most appropriate small animal model for the study of female PFM architecture, but the rat's suitability for the study of male PFMs is undetermined. We aimed to determine (1) whether PFM architecture exhibits sexual dimorphism in rats or humans, and (2) if the rat is also a suitable animal model for the study of male human PFMs. PFMs were fixed in situ and harvested en bloc from male and female cadaveric donors and 3-month-old male and female Sprague-Dawley rats. Three architectural parameters influenced by species size were used to compare male versus female PFMs within species, while four size-independent measures compared species within sex. All comparisons were made with two-way analysis of variances and Tukey's multiple comparisons tests post hoc. Sarcomere length (rats and humans, p = 0.016 and = 0.002) and normalized fiber length (rats, p < 0.001) were significantly larger in male PFMs. Three of the size-independent measures exhibited similar species trends in both sexes, while the size-independent sarcomere length measure (Ls/Lso) differed between male rats and humans (p < 0.001). Thus, sexual dimorphism is present in rat and human PFM architecture, and the male rat is suitable for studies of human male PFMs.

Graphical abstract

Fig. 1

Top down (from cranial to caudal) views of a fixed female human (left) and female rat (middle) pelvis with the PFMs exposed demonstrating general differences in anatomy between species. The right human coccygeus (C), iliococcygeus (IC), and pubovisceralis (P) and the right rat coccygeus (C), iliocaudalis (IC), and pubocaudalis (P) are outlined. Note that the rat C has a dotted outline to indicate its position behind the IC and P muscles. Notable landmarks, including the symphysis pubis (SP) and coccyx in the human pelvis and the SP, spine, and tail in the rat images, are also labeled. The icons in the bottom left corners visualize the general orientation of each pelvis. To better visualize the rat PFMs and their origins, a second image was taken at an angle. A second image was not necessary for human PFM visualization.

Fig. 2

Statistical and visual comparisons between female (left box of each pair) and male (right box of each pair) (a) human and (b)rat PFM architecture data. Corresponding ANOVA p-values for the main effects (sex and muscle) and interaction terms are shown in the tables with significant values in bold. Only Tukey's comparisons evaluating potential sex differences for each muscle (C, IC, and P) were considered. Significant Tukey's p-values are denoted by asterisks, with * denoting <0.05, ** denoting <0.01, *** denoting <0.001, and *** denoting <0.0001. L_s in humans and L_s and L_fn in rats demonstrated significant sexual dimorphism when considering all PFMs collectively.

Fig. 3

Scatter plots representing the relationship between force-generating and excursion capacity in male and female (a) human and (b) rat PFMs. The legends in the top right denote how sex and individual PFMs are represented. For both human and rat data, C muscles generally had the shortest L_fn and greatest PCSA, while IC and P muscles had longer L_fn and smaller PCSA values. The rat data display more distinct sexual dimorphism, while the human data demonstrate a more apparent negative relationship between force-generating and excursion capacity.

Fig. 4

Statistical and visual comparisons between (a) male and (b) female rats (left box of each pair) and human (right box of each pair) size-independent PFM architectural parameters. Corresponding ANOVA p-values for the main effects (species and muscle) and interaction terms are shown in the tables with significant values in bold. Only Tukey's comparisons evaluating potential species differences for each muscle (C, IC, and P) were considered. Significant Tukey's p-values are denoted by asterisks, with * denoting <0.05, ** denoting <0.01, *** denoting <0.001, and *** denoting <0.0001. With regards to animal model comparisons, L_fn/L_mn, %PCSA, and %mass demonstrated similar trends in both sexes.

Fig. 5

An illustrative length–tension curve demonstrating where each species, sex, and PFM lies according to their mean L_s/L_so value. The legend depicts how species, sex, and individual PFMs are represented. While most lie along the plateau, which includes the optimal sarcomere length (where L_s/L_so = 1 and active tension is maximized), only male human values (IC and P) fall on the descending limb of the length–tension curve, while only female values (rat and human C and rat P) fall on the ascending limb.