This study was undertaken to quantify resting vaginal closure force (VCF^REST), maximum vaginal closure force (VCF^MAX), and augmentation of vaginal closure force augmentation (VCF^AUG) when supine and standing and to determine whether the change in intra-abdominal pressure associated with change in posture accounts for differences in VCF.

Thirty-nine asymptomatic, continent women were recruited to determine, when supine and standing, the vaginal closure force (eg, the force closing the vagina in the mid-sagittal plane) and bladder pressures at rest and at maximal voluntary contraction. VCF was measured with an instrumented vaginal speculum and bladder pressure was determined with a microtip catheter. VCF^REST was the resting pelvic floor tone, and VCF^MAX was the peak pelvic floor force during a maximal voluntary contraction. VCF^AUG was the difference between VCF^MAX and VCF^REST. T tests and Pearson correlation coefficients were used for analysis.

VCF^REST when supine was 3.6 ± 0.8 N and when standing was 6.9 ± 1.5 N—a 92% difference (P < .001). The VCF^MAX when supine was 7.5 ± 2.9 N and when standing was 10.1 ± 2.4 N—a 35% difference (P < .001). Bladder pressure when supine (10.5 ± 4.7 cm H₂O) was significantly less (P < .001) than when standing (31.0 ± 6.4 cm H₂O). The differences in bladder pressure when either supine or standing did not correlate with the corresponding differences in VCF at rest or at maximal voluntary contraction. The supine VCF^AUG of 3.9 ± 2.7 N, was significantly greater than the standing VCF^AUG of 3.3±1.9 N.

With change in posture, vaginal closure force increases because of higher intra-abdominal pressure and greater resistance in the pelvic floor muscles.