The pelvic diaphragm is a gutter-shaped sheet of muscle formed by the levatores ani and coccygeus muscles and their covering fasciae. From their origin, the muscle fibers on the two sides slope downward and backward to the midline, producing a gutter that slopes downward and forward. A rise in the intra-abdominal pressure, caused by the contraction of the diaphragm and the muscles of the anterior and lateral abdominal walls, is counteracted by the contraction of the muscles forming the pelvic floor.
By this means, the pelvic viscera are supported and do not “drop out” through the pelvic outlet. Contraction of the puborectalis fibers greatly assists the anal sphincters in maintaining continence under these conditions by pulling the anorectal junction upward and forward. During the act of defecation, however, the levator ani continues to support the pelvic viscera but the puborectalis fibers relax with the anal sphincters.