Anatomical basis for contribution of hip joint motion by the obturator internus to defaecation/urinary functions by the levator ani via the obturator fascia.

The functional association between hip joint motion and defaecation/urinary function has attracted considerable research and clinical attention owing to the potential novel approaches for pelvic floor rehabilitation; however, the anatomical basis remains unclear. This study, therefore, aimed to analyse the anatomical basis of force transmission between the obturator internus, a muscle of the hip joint, and the levator ani, a muscle of the pelvic floor. Twenty-three cadavers were used for macroscopic and histological analyses. The three-dimensional structures of the muscles and fascia were recorded using a high-definition camera and a 3D scanner. The arrangement and attachment of the muscle fibres, tendons and fascia were visualised using histological sections stained with Masson's trichrome. The obturator internus and levator ani were in broad contact through the obturator fascia. The height of their contact area was 24.6 ± 9.1 mm. Histologically, the obturator internus and levator ani shared a large area of the obturator fascia, and the obturator fascia provided the attachment of several muscle layers of the levator ani. The contribution of hip joint motion to defaecation/urinary function can be explained by the broad 'planar' contact between the obturator internus and levator ani. This anatomical feature suggests that movement of the obturator internus creates the foundation for the function of the levator ani and contributes to pelvic floor support through the obturator fascia. This study provides an anatomical basis for the effectiveness of the hip muscles in improving defaecation/urinary function by enabling balanced and proper movements.