The simultaneous contraction of two or more muscles is denominated ‘co-contraction’. When this contraction occurs in a synergistic way, enhance the motor activity [1]. The presence of a synergistic co-contraction between pelvic floor muscles (PFM) and transverse abdominal muscle (TrA) has been reported in young asymptomatic women, which seems to favor urinary continence mechanisms [2]. However, there is still no consensus about the relationship between these muscles in women with stress urinary incontinence (SUI).
We hypothesized that SUI women present an uncoordinated action between these muscles, characterized by a lower PFM response during TrA contraction, when compared to continent women. Thereby, the aim of this study was to analyze the effect of TrA contraction on pelvic floor ultrasound biometric parameters as well as to compare these findings between continent and incontinent women.

A cross-sectional study was carried out in accordance with the Helsinki declaration and after its approval by the local Human Research Ethics Committee. Initially, 302 women were recruited. Eighty-three of these women were excluded due to: current urinary tract infection (n=3), other types of incontinence rather than predominant SUI (like urgency urinary incontinence alone or mixed urinary incontinence with predominant urgency incontinence) (n=22), physical or neurological disorders that would hinder their participation in the evaluation (n=3), previous oncology treatment as brachytherapy or neovagina (n=6), both SUI or/and pelvic organ prolapse surgery history (n=15), presence of any pelvic organ prolapse exceeding the vaginal opening (n=8) and PFM contraction grade either zero or one according to the Modified Oxford Grading Scale (n=26). The continence status was ascertained using the International Consultation on Incontinence Questionnaire Urinary Incontinence – Short Form (ICIQ UI-SF), resulting in 69 continent women and 148 predominant SUI women.
At first, the participants were taught how to correct contract their TrA using ultrasound technique, as previously described by Hodges et al. (2003) [4]. Then, their PFM were evaluated using 4D translabial ultrasound (4D TLUS), at rest and during TrA contraction. Offline analyses of ultrasound volumes were performed obtaining the following parameters: change in levator-symphysis distance, change in levator plate angle, bladder neck elevation, hiatal area narrowing and puborectal strain. The Kolmogorov-Smirnov, Mann-Whitney and Unpaired t tests were used to compare the findings between groups, adopting a significance level of 5%.

Most participants were white (77.6%), married (64.8%) and with higher education level (55.7%). Participants’ mean age at examination was 45.8 years old (range from 20 to 70) and mean body mass index was 25.7 kg/m2 (range from 15.2 to 46.0). Regarding previous obstetric data, the participants’ median number of pregnancies was 2.00 (range from 1 to 6), being most of their babies delivered vaginally (0.9±1.2 vaginal deliveries versus 0.8±1.0 cesarean section). The incontinent women presented predominant SUI (ICIQ-UI SF average score was 9.43±7.1).
A significant difference regarding age (p<.0001), body mass index (p=0.03) and parity (p<.0001) was found between groups. When compared the effect of TrA contraction on PFM’s  ultrasound morphological changes, we found no significant differences between continent and incontinent women (Table 1).
Legend Table 1:  &Calculated using this formula: Elevation = √{(Cy - Ry)2 + (Cx - Rx)2}, where is calculated the difference between the values obtained at rest (R) and during contraction (C) in x and y axies. εPuborectal strain was alculated during contraction in relation to rest, by means of this formula: εcont = Ccont-Crest / Crest-lb, where εcont = deformation during contraction, Ccont = hiatal circumference during contraction, Crest = hiatal circumference at rest and lb = bone arch of the hiatal circumference. M = mean; SD = standard deviation; cm = centimeter; cm2 = square centimeter; % = percentage. 1Unpaired t test. 2Mann-Whitney test.

Some authors have been discussing the relationship between PFM and TrA, since a coordinated and simultaneous activity between these muscles had been reported, in such a way that they respond as a single functional unit. In our study, we found that during TrA contractions, pelvic floor morphological changes occurs, reinforcing this concept. However, we did not find a significant difference in pelvic floor response during TrA contraction, when compared continent and incontinent women, refuting our initial hypothesis. 
In this context, more studies are necessary to evaluate the pelvic floor and transverse abdominal muscles’ co-contraction in continent and incontinent women, as well as to test the significance of adding TrA contractions during PFM training as an approach for SUI treatment.

Despite the TrA contraction had promoted a change on pelvic floor ultrasound morphological parameters in both continent and incontinent women, no significant difference was found between groups.

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