Techniques in Coloproctology

, Volume 16, Issue 6, pp 437–443 | Cite as

Defecation 1: Testing a hypothesis for pelvic striated muscle action to open the anorectum

  • P. PetrosEmail author
  • M. Swash
  • M. Bush
  • M. Fernandez
  • A. Gunnemann
  • M. Zimmer
Original Article



We conducted an observational study to assess the hypothesis that the pelvic muscles actively open the anorectal lumen during defecation.


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.


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 defecation, 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.


Our results are consistent with the hypothesis that pelvic striated muscle actively opens the rectal lumen, thereby reducing internal anorectal resistance to expulsion of feces. Controlled studies of electromyographic activity would be useful to further test this hypothesis.


Mechanism of defecation Constipation Fecal incontinence Pelvic floor disorders Anorectal Resistance 


Conflict of interest

The authors declare that no conflict of interest exists.

Supplementary material

Supplementary material 1 (MP4 108912 kb)


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Copyright information

© Springer-Verlag 2012

Authors and Affiliations

  • P. Petros
    • 1
    • 2
    • 8
    Email author
  • M. Swash
    • 3
    • 4
  • M. Bush
    • 2
  • M. Fernandez
    • 5
  • A. Gunnemann
    • 6
  • M. Zimmer
    • 7
  1. 1.St Vincent’s HospitalUniversity of New South WalesSydneyAustralia
  2. 2.School of Mechanical and Chemical EngineeringThe University of Western AustraliaPerthAustralia
  3. 3.Department of Neurology, Royal London HospitalQueen Mary University of LondonLondonUK
  4. 4.Institute of Molecular MedicineUniversity of LisbonLisbonPortugal
  5. 5.Department of RadiologyClinica Las CondesSantiagoChile
  6. 6.Klinik für Urologie und Kinderurologie Klinikum LippeDetmoldGermany
  7. 7.Klinik für diagnostische und interventionelle Radiologie Klinikum LippeDetmoldGermany
  8. 8.Elizabeth BayAustralia

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