the most prevalent cause of pelvic floor muscle dysfunction in females is the attenuation 
 of the pelvic floor muscle , resulting in a state in which the pelvic floor muscle is unable 
 to be independently recognized and contracted . this results in pelvic girdle relaxation , 
 pelvic organ prolapse , or stress urinary incontinence . the pelvic floor muscle , in the form 
 of the pelvic floor and abdominal cavity , has the dual function of controlling cystic 
 incontinence and stabilizing the radial region1 . dysfunction of the pelvic floor muscle is generally associated with 
 dysuria and radial pain2 . a recent study has shown that the pelvic floor muscles are fundamentally important for 
 trunk core muscle strength and stabilization3 . perri4 reported 
 that the transversus abdominis muscle , pelvic floor muscle , and deep spine intrinsic muscle 
 function in harmony with each other , and that dysfunction in one of them can produce related 
 effects , influencing spinal stability , and causing pain . sapsford et al.5 found that during maximum contraction of the 
 pelvic floor muscles , all the abdominal muscles , including the transversus abdominis , 
 internus obliquus , externus obliquus , and rectus abdominis , were activated . recently , ultrasonography has become one of the instruments used widely in research and 
 clinical settings , since it allows muscle shape and reaction to be quantified , especially 
 the contraction of the abdominal muscles , in a non - invasive manner . ultrasonography employs 
 a novel method to evaluate the provided morphological information of deep muscles6 . it acquires objective and highly reliable 
 information , since the measurement is performed while visually checking the location and 
 shape of the deep muscles . kiesel et al.7 ) 
 reported that an ultrasonogram is useful for evaluating muscle thickness as the 
 changes in a muscle are measured related to contraction . hence , this study employed 
 unltrasonography in order to objectively evaluate abdominal muscle contractions related to 
 changes in vaginal pressure . in cases of dysfunction ( e.g. , stress urinary incontinence ) , it is generally advised that 
 the rehabilitation of pelvic floor muscles be done in isolation8 . however , it is now known that abdominal muscle activity 
 occurs in conjunction with pelvic floor muscle contraction , and there is preliminary 
 evidence that the reverse may also occur ( i.e. , pelvic floor muscle activity in response to 
 specific abdominal maneuvers ) . many clinical exercise methods have been developed to use abdominal muscles in pelvic floor 
 muscle exercises . however , studies have rarely been conducted to measure abdominal muscle 
 thickness in response to vaginal pressure generated by pelvic floor muscles . in this study , 
 normal adult females were divided into different age groups , and changes in abdominal muscle 
 thickness in response to changing vaginal pressure were comparatively analyzed . the subjects of this study were classified by age into two groups comprised of 11 subjects 
 in their 20s and 30s ( group i ) , and 13 subjects in their 40s and 50s ( group ii ) . the average 
 ages of group i and group ii were 33.55 and 49.69  years , respectively , with a significant 
 difference . however , there were no significant differences between the two groups in vaginal 
 pressure , height , weight , and body mass index ( bmi ) . this study was approved by hospital , 
 and all the participants provided their written informed consent . vaginal pressure was 
 measured with a perineum vaginal pressure meter ( perineometer , peritron 9300 , cardio design 
 australia ) . subjects were placed in a supine position with their hips flexed at approximately 60. each 
 subject was instructed to strongly tighten her vagina as if pulling it into her body . while 
 avoiding lumbar and pelvic motions , subjects received visual feedback about their 
 contraction force through a pressure gauge connected to a pressure biofeedback device . the 
 pelvic floor muscle contraction was performed at five levels : resting ( 0 cmh2o ) , 
 10 cmh2o , 20 cmh2o , 30 cmh2o , and maximum contraction . the 
 subjects maintained the target pressure for five seconds and took a 10-second rest before 
 the next measurement as the muscles may have tired during each contraction . three 
 measurements were taken for each subject , and the average values were calculated . an ultrasonography instrument ( hdi 5000 , philips atl ) was used with a linear array 
 transducer , which could be operated at 7  mhz . the static muscle cross - sectional area was 
 represented by a b ( brightness)-mode scan . to minimize inter - observer variations , a physical 
 therapist who was skilled in ultrasonography performed the measurements on the basis of 
 basic anatomical knowledge of the abdomen . gel for ultrasonography was applied between the 
 linear array transducer and the skin , and the right abdominal muscle was checked by 
 inspection through the ultrasonogram , and palpation by the hands of the tester . the 
 thicknesses of the transversus abdominis muscle , internal oblique abdominal muscle , and 
 external oblique abdominal muscle were measured at the same position , which was 1  cm from 
 the boundary of the fascia to the center of the muscle9 . the measurement position was in the anterolateral position of the 
 axillary line between the iliac crest and costal inferior horn10 . the subjects were in the supine position with a 
 triangular support under the knee joints so that the position would remain fixed for the 
 duration of the measurement . the mean and standard deviation of the general characteristics of the subjects were 
 calculated using descriptive analysis . one - way anova was performed to compare the muscle 
 thickness changes between the isometric contractions at the five levels of vaginal pressure . 
 a least - square difference ( lsd ) the 
 independent t - test was performed to compare muscle thicknesses between the age groups . the 
 reliability of the abdominal muscle thickness measurement by the tester according to vaginal 
 pressure was analyzed using the intraclass correlation coefficient ( icc ) . all the 
 statistical processing was performed with spss version 12.0 software , and a significance 
 level of 0.05 . the iccs of the tester s three times measurements of muscle thickness at each vaginal 
 pressure ranged from 0.900.98 for the transversus abdominis muscle , from 0.870.99 for the 
 internal oblique abdominal muscle , and from 0.880.97 for the external oblique abdominal 
 muscle . i , the change in abdominal thickness dependent on vaginal pressure was significant 
 for the transversus abdominis muscle ( p<0.05 ) but not significant for the external 
 oblique abdominal muscle and the internal oblique abdominal muscle ( p>0.05 ) . the change 
 in abdominal muscle thickness dependent on vaginal pressure was significant from resting to 
 a vaginal pressure of 30 cmh2o , from resting to maximum contraction , and from 10 
 cmh2o to maximum contraction for the transversus abdominis muscle ( p<0.05 ) . however , no significant differences were found for the external oblique abdominal muscle and 
 internal oblique abdominal muscle ( p>0.05 ) ( table 
 1table 1 . abdominal muscle thicknesses of each age group ( unit = cm)gi musclevpthicknessresting10 cmh2o20 cmh2o30 cmh2omaximumtraresting0.240.01**10 cmh2o0.260.0220 cmh2o0.300.0230 cmh2o0.320.03maximum0.350.03eoresting0.590.0210 cmh2o0.610.0320 cmh2o0.610.0330 cmh2o0.600.03maximum0.590.02ioresting0.430.0410 cmh2o0.440.0420 cmh2o0.460.0430 cmh2o0.470.04maximum0.500.04gii musclevpthicknessresting10 cmh2o20 cmh2o30 cmh2omaximumtraresting0.230.01***10 cmh2o0.300.02**20 cmh2o0.340.0330 cmh2o0.380.04maximum0.420.04eoresting0.550.0410 cmh2o0.530.0420 cmh2o0.510.0430 cmh2o0.510.04maximum0.510.04ioresting0.440.02 * 10 cmh2o0.440.03 * 20 cmh2o0.470.0230 cmh2o0.590.03maximum0.530.02vp : vaginal pressure , tra : transversus abdominis , eo : external oblique muscle , io : 
 internal oblique muscle . * vp : vaginal pressure , tra : transversus abdominis , eo : external oblique muscle , io : 
 internal oblique muscle . * p<0.05 in group ii , the change in thickness dependent on vaginal pressure was significant for the 
 transversus abdominis muscle and internal oblique abdominal muscle ( p<0.05 ) but not 
 significant for the external oblique abdominal muscle ( p>0.05 ) . the change in abdominal 
 muscle thickness dependent on vaginal pressure was significant from resting to vaginal 
 pressures of 20 cmh2o , 30 cmh2o , and maximum contraction , and from 
 vaginal pressures of 10 cmh2o to 30 cmh2o and maximum contraction for 
 the transversus abdominis muscle ( p<0.05 ) . additionally , a significant change was found 
 from resting to maximum contraction , and from 10 cmh2o to maximum contraction for 
 the internal oblique abdominal muscle ( p<0.05 ) . however , no significant difference was 
 found for the external oblique abdominal muscle ( p>0.05 ) . a comparison of the muscle thickness changes in the transversus abdominis muscle and 
 internal oblique abdominal muscle dependent on vaginal pressure between the two groups 
 showed that the thickness increase was greater in group ii , but the increase was not 
 significant ( p>0.05 ) . for the external oblique abdominal muscle , while no thickness 
 change was found in group i , muscle thickness gradually decreased as vaginal pressure 
 increased in group ii , but the decrease was not significant ( p>0.05 ) . the analysis of transversus abdominis muscle thickness depending on vaginal pressure showed 
 that the resting thickness was 0.24  cm in group i , and 0.23  cm in group ii , indicating a 
 similar starting line . a significant difference was found in group i from resting to a 
 vaginal pressure of 30 cmh2o , and from resting to maximum contraction . a 
 significant difference was found in group ii from resting to 20 cmh2o , 30 
 cmh2o , and maximum contraction , and from 10 cmh2o to 30 
 cmh2o and maximum contraction . this was consistent with the finding of 
 sapsford5 that the pelvic floor muscle 
 and transversus abdominis muscle activate together when the pelvic floor muscle is 
 contracted . this result is also in agreement with the result that the muscular activity of 
 the pelvic floor muscle can be increased to a greater degree when the pelvic floor muscle is 
 contracted using the abdominal muscle rather than when only the pelvic floor muscle is 
 contracted11 , 12 . in our study , the thickness of the external oblique abdominal muscle increased slightly or 
 did not change in group i , while it gradually decreased from resting to maximum contraction 
 in group ii . the thickness of the internal oblique abdominal muscle significantly increased 
 from resting to maximum contraction , and from 10 cmh2o to maximum contraction in 
 group ii . the result of group ii is consistent with previous results that the thickness of 
 the internal oblique abdominal muscle increases by 1020% in the performance of a hollowing 
 exercise , but , that the thickness of the external oblique abdominal muscle increases only 
 slightly or decreases13 . in group ii , the 
 result of the pelvic floor muscle exercise was similar to that of a hollowing exercise , 
 indicating that vaginal pressure was increased when the internal abdominal muscle was used 
 during the pelvic floor muscle contraction . a morphometric study on the rhabdo - urethral muscle reported that the volume of the levator 
 anti - muscle decreases with age14 . a 
 histological study found that as age increases , the transverse muscles decrease along the 
 back of the urethra and the bladder neck15 . thus , we assume that the middle - aged women in this study increased 
 vaginal contraction pressure by using their abdominal muscles rather than their attenuated 
 pelvic floor muscles . a comparison of the two age groups showed that the pelvic floor muscles were contracted 
 without using the transversus abdominis muscle from resting to 30 cmh2o in group 
 i , and from resting to 20 cmh2o in group ii . the transversus abdominis muscle was 
 used at a vaginal pressure that was 10 cmh2o lower in group ii than in group i , 
 but the difference was not significant . the childbirth delivery method was not considered in the categorization of subjects . in 
 future studies , abdominal contraction during pelvic floor muscle contraction should be 
 compared by considering how the childbirth delivery method affects pelvic floor muscle 
 contraction , and studies should also be conducted with patients with pelvic floor muscle 
 dysfunction . hung et al.16 suggested the retraining 
 of the combined functions of the diaphragm , deep muscle , and pelvic floor muscles as an 
 alternative treatment for urinary incontinence . bo et al.17 also recommended the synergistic effects of the co - contraction of 
 the pelvic floor muscles and transversus abdominis in the treatment of urinary incontinence . 
 on the basis of our result , it may be necessary to develop an exercise program that 
 maximizes the effect of the contraction of the abdominal muscle and pelvic floor muscle by 
 appropriately controlling the strength of the pelvic floor muscle contraction .