ORIGINAL ARTICLE Defecation 1: Testing a hypothesis for pelvic striated muscle action to open the anorectum P. Petros • M. Swash • M. Bush • M. Fernandez • A. Gunnemann • M. Zimmer Received: 22 November 2011 / Accepted: 26 June 2012 / Published online: 14 August 2012 Ó Springer-Verlag 2012 Abstract Background We conducted an observational study to assess the hypothesis that the pelvic muscles actively open the anorectal lumen during defecation. Methods Three groups of female patients were evaluated with video imaging studies of defecation using a grid or bony reference points. Eight patients with idiopathic fecal incontinence had video myogram defecography; eight with obstructive defecation had magnetic resonance imaging (MRI) defecating proctograms; and four normal patients had video X-ray or MRI defecating proctogram studies. Results In all three groups, the anorectum was stretched bidirectionally by three directional muscle force vectors acting on the walls of the rectum, effectively doubling the diameter of the rectum during defecation. The anterior rectal wall was pulled forwards, and the posterior wall backwards and downwards opening the anorectal angle, associated with angulation of the anterior tip of the levator plate (LP). These observations are consistent with a staged relaxation of some parts of the pelvic floor during defe- cation, and contraction of others. First, the puborectalis muscle relaxes. Puborectalis muscle relaxation frees the posterior rectal wall so that it can be stretched and opened by contraction of the LP and conjoint longitudinal muscle of the anus. Second, contraction of the pubococcygeus muscle pulls forward the anterior rectal wall, further increasing the diameter of the rectum. Third, when the bolus has entered the rectum, the external anal sphincter relaxes, and the rectum contracts to expel the fecal bolus. Conclusions Our results are consistent with the hypoth- esis that pelvic striated muscle actively opens the rectal lumen, thereby reducing internal anorectal resistance to expulsion of feces. Controlled studies of electromyo- graphic activity would be useful to further test this hypothesis. Keywords Mechanism of defecation Á Constipation Á Fecal incontinence Á Pelvic floor disorders Á Anorectal Á Resistance Electronic supplementary material The online version of this article (doi:10.1007/s10151-012-0861-2) contains supplementary material, which is available to authorized users. P. Petros St Vincent’s Hospital, University of New South Wales, Sydney, NSW, Australia P. Petros Á M. Bush School of Mechanical and Chemical Engineering, The University of Western Australia, Perth, Australia P. Petros (&) 31/93 Elizabeth Bay Rd, Elizabeth Bay, NSW 2011, Australia e-mail: pp@kvinno.com M. Swash Department of Neurology, Royal London Hospital, Queen Mary University of London, London, UK M. Swash Institute of Molecular Medicine, University of Lisbon, Lisbon, Portugal M. Fernandez Department of Radiology, Clinica Las Condes, Santiago, Chile A. Gunnemann Klinik fu ¨r Urologie und Kinderurologie Klinikum Lippe, Detmold, Germany M. Zimmer Klinik fu ¨r diagnostische und interventionelle Radiologie Klinikum Lippe, Detmold, Germany 123 Tech Coloproctol (2012) 16:437–443 DOI 10.1007/s10151-012-0861-2