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

Abstract

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 = 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.