Structure–function relationship of the human external anal sphincter

Introduction and hypothesis: Obstetrical external anal sphincter (EAS) injury and subsequent dysfunction are leading risk factors for female fecal incontinence (FI). Limited knowledge of the EAS structure–function relationship hinders treatment optimization. We directly measured functionally relevant intrinsic parameters of human EAS and tested whether vaginal delivery alters the EAS structure–function relationship. Methods: Major predictors of in vivo EAS function were compared between specimens procured from vaginally nulliparous (VN, n = 5) and vaginally parous (VP, n = 7) cadaveric donors: operational sarcomere length (L_s), which dictates force–length relationship; physiological cross-sectional area (PCSA), which determines isometric force-generating capacity; fiber length (L_fn), responsible for muscle excursion and contractile velocity; and muscle stiffness. Data were analyzed using unpaired and paired t tests, α < 0.05. Results are presented as mean ± SEM. Results: The VN and VP (median parity 3) groups were similar in age and BMI. No gross anatomical defects were identified. EAS L_s (2.36 ± 0.05 μm) was shorter than the optimal L_so (2.7 μm), at which contractile force is maximal, P = 0.0001. Stiffness was lower at L_s than L_so (5.4 ± 14 kPa/μm vs 35.3 ± 12 kPa/μm, P < 0.0001). This structural design allows active and passive tension to increase with EAS stretching. EAS relatively long L_fn (106 ± 24.8 mm) permits rapid contraction without decreased force, whereas intermediate PCSA (1.3 ± 0.3 cm²) is conducive to maintaining resting tone. All parameters were similar between groups. Conclusions: This first direct examination of human EAS underscores how EAS intrinsic design matches its intended function. Knowledge of the EAS structure–function relationship is important for understanding the pathogenesis of FI and the optimization of treatments for EAS dysfunction.