Effect of levator ani muscle contraction on urethrovesical and anorectal pressures and role of the muscle in urination and defecation.

OBJECTIVES: To investigate the effect of levator ani muscle (LAM) (the main muscle in the pelvic floor) contraction on the anorectal and urethrovesical pressures and elucidate its role in the mechanisms of defecation and urination.
METHODS: In 18 healthy volunteers (10 men and 8 women; mean age 38.6 +/- 10.2 years), the LAM was stimulated to contract by a concentric needle electrode, and the anal, rectal, urethral, and vesical pressure responses were recorded. The test was repeated once after LAM anesthetization with Xylocaine injection and again after saline instead of Xylocaine injection into the LAM.
RESULTS: On LAM stimulation, the anal and urethral pressures had a significant decline (both P <0.05), and the rectal and vesical pressures exhibited no significant changes (both P >0.05). The anal, rectal, urethral, and vesical pressures did not show significant changes on stimulation of the anesthetized LAM; after saline injection, the pressure response was similar to that before injection (P >0.05). These results were reproducible.
CONCLUSIONS: LAM contraction lowers the anal and urethral pressures and appears to assist in the evacuation process. We therefore propose that the LAM is a muscle of evacuation and that LAM dysfunction could lead to defecation and urination disorders.