Architectural design of the pelvic floor is consistent with muscle functional subspecialization

Introduction and hypothesis: Skeletal muscle architecture is the strongest predictor of a muscle's functional capacity. The purpose of this study was to define the architectural properties of the deep muscles of the female pelvic floor (PFMs) to elucidate their structure-function relationships. Methods: PFMs coccygeus (C), iliococcygeus (IC), and pubovisceral (PV) were harvested en bloc from ten fixed human cadavers (mean age 85 years, range 55-102). Fundamental architectural parameters of skeletal muscles [physiological cross-sectional area (PCSA), normalized fiber length, and sarcomere length (L_s)] were determined using validated methods. PCSA predicts muscle-force production, and normalized fiber length is related to muscle excursion. These parameters were compared using repeated measures analysis of variance (ANOVA) with post hoc t tests, as appropriate. Significance was set to α = 0.05. Results: PFMs were thinner than expected based on data reported from imaging studies and in vivo palpation. Significant differences in fiber length were observed across PFMs: C=5.29±0.32 cm, IC=7.55±0.46 cm, PV=10.45±0.67 cm (p <0.001). Average L_s of all PFMs was short relative to the optimal L_s of 2.7 μm of other human skeletal muscles: C=2.05±0.02 μm, IC=2.02±0.02 μm, PC/PR=2.07±0.01 μm (p =<0.001 compared with 2.7 μm; p =0.15 between PFMs, power=0.46). Average PCSA was very small compared with other human muscles, with no significant difference between individual PFMs: C=0.71±0.06 cm², IC=0.63±0.04 cm², PV=0.59±0.05 cm² (p =0.21, power=0.27). Overall, C had shortest fibers, making it a good stabilizer. PV demonstrated the longest fibers, suggesting that it functions to produce large excursions. Conclusions: PFM design shows individual muscles demonstrating differential architecture, corresponding to specialized function in the pelvic floor.