Inter-Rectal Distance and Abdominal Wall Muscles in Nulligravidous Women at Rest and During Valsalva Manoeuvre: A Prospective Cohort Study Inter-Rectal Distance and Abdominal Wall Muscles in Nulligravidous Women at Rest and Valsalva Manoeuvre: A Prospective Cohort Study.

Introduction: Postpartum women often confront their health care providers with complaints due to weakened abdominal wall muscles and diastasis recti. No reference values for young, healthy and nulligravidous women are present, which can serve as references for the diagnosis and treatment of complaints in postpartum women. Study Purpose: Our study evaluated values for inter-rectal distance (IRD), thickness of the linea alba (LA), M. rectus abdominis (RA) and lateral abdominal wall muscles (LAWM) at different contraction states and levels along the abdominal wall for a cohort of young, healthy and nulligravidous women. Materials and Methods: In a prospective cohort study from 4/2015-3/2016 we measured the dimensions of the abdominal wall muscles at different levels at rest and during Valsalva manoeuvre by ultrasound in 20 healthy, nulligravidous women aged 21-35 years. Exclusion criteria were a body mass index over 30 kg/m 2 , multiple or big uterine fibroids, chronic lung disorders, collagenosis, chronic constipation, history of urine/stool incontinence, former abdominal surgery and inability to perform Valsalva manoeuvre. Descriptive statistics were done for the study population and a one-way ANOVA for the muscle values, using SPSS version 22.0 (SPSS Inc., Chicago, IL). Results: IRD is the widest at the umbilicus, wider above the umbilicus than below and decreases towards xyphoid and symphysis. The effect of rest and Valsalva manoeuvre on muscle thickness, IRD and LA is inconsistent and depends on the abdominal level and side. Valsalva manoeuvre thickens the RA and the two inner muscles of the LAWM, but not the outer muscle at almost every level. The values for RA and the LAWM are in average higher on the


Introduction
Postpartum women often confront their obstetricians, midwives, physiotherapists or even plastic surgeons with complaints due to weakened abdominal wall muscles and diastasis recti. No reference values for young, healthy and nulligravidous women are provided in the literature, which can serve as references for the diagnosis and treatment control of such complaints in that special group. It is well known, that the muscles of the abdominal wall play a major role in the stabilization of the trunk and the intraabdominal organs, in the movements of the whole body and in the biomechanics of respiration [1][2][3][4][5][6][7][8][9]. Weakened abdominal muscles and imbalance between the different structures might result in malfunction, pain or abnormal body shape [2,7,[10][11][12]. The structures that mainly contribute to the integrity of the anterior and lateral abdominal wall are the M. rectus abdominis with the linea alba between the two portions of the muscle, the M. externus and internus obliquus abdominis and the M. transversus abdominis [1,3,10]. Their shape and function are affected by genetic factors, hormonal changes and specific conditions that cause a higher intraabdominal pressure, such as sports, obesity, constipation, chronic obstructive lung diseases and especially pregnancy [10,13,14]. These factors can for example contribute to a thinning or reduced tensile resistance of the linea alba, which connects the two portions of the rectus abdominis muscle. This thinning or reduced tensile resistance of the linea alba can lead to a wider inter-rectal distance (IRD) or laxity of the abdominal wall, which becomes visible as an abdominal protrusion in some cases, in others not. It is unknown, whether this wider inter-rectal distance arises from a separation of the two rectus abdominis muscles with a stretching or laxity of the linea alba or from an overexpansion of the linea alba [10,13]. The association of such changes in the structure of the abdominal wall to lower back pain and dysfunction of the pelvic floor, for example to incontinence and descensus uteri, is conflicting, as some authors promote this association, whereas others deny it [1][2][3]8,11,[15][16][17][18][19].
Many women after birth consult their obstetricians, midwives and physiotherapists either with such complaints or for cosmetic reasons in cases with diastasis recti. As therapeutic interventions result from changes in the physiological structure and function, it is essential that the physiological state is assessed correctly for that group. However, there is lack of knowledge about the physiological values of the different abdominal wall muscles, especially in a homogenous, healthy, young and nulligravidous cohort. Regarding the anterior and lateral abdominal wall different studies exist, that aim to assess normative values of the inter-rectal distance, thickness of the M. rectus abdominis, M. obliquus externus and internus and M. transversus abdominis [3,4,12,[20][21][22][23][24][25][26][27][28][29].
Somehow, all these different publications mostly focus on single muscles, a single or small number of measurement points along the abdominal wall, different states of muscle function, different measurement techniques and tools, dead bodies or mixed cohorts regarding sex, age and health condition. As training programs for women after birth focus on exercises of the pelvic floor and the anterior and lateral abdominal wall muscles, but no reference values are provided for that special group, the aim of this study was to provide values of the abdominal wall structures at several levels along the abdominal wall in a homogenous cohort of healthy, young, nulligravidous women at different states of muscle activation.

Study Design
As part of a paramount prospective, observational study, that evaluated the influence of pregnancy on abdominal wall muscles,

Ultrasound Examination
Two well-trained ultrasound senior consultants of our obstetrical department performed all the ultrasound examinations. We

Statistics
Descriptive statistics were done for the characteristics of the study group. Continuous variables were summarized as mean and standard deviation (SD). Comparisons of the inter-rectal distances, width of the linea alba and muscle thicknesses were carried out by using one-way ANOVA for repeated measures. Statistical analysis was performed using the statistical software package SPSS version 22.0 (SPSS Inc., Chicago, IL). Statistical significance was set at p<0.05.

Results
We examined 20 healthy, nulligravidous women aged 21 to 35 years with a body mass index less than 28 kg/m 2 . The characteristics of the study population are shown in Table 1.

Inter-Rectal Distance (IRD) And Linea Alba (LA)
The widest IRD was found at the level of the umbilicus with a value of 14.13 mm during both Valsalva manoeuvre and at rest (Table 2). IRD in general was wider above the level of the umbilicus than below. It decreased the more distant away from the umbilicus in both directions, with values of 7.00 mm at +9 and 0.48 mm at -6 ( Table 2). When comparing the values during Valsalva manoeuvre to resting state, IRD during Valsalva manoeuvre did not change at the level of the umbilicus compared to rest, but had the tendency to decrease 9 cm above and 6 cm below the umbilicus and to increase 3cm and 6cm above and 3cm below the umbilicus ( Table   2). However, all changes between Valsalva and resting state were statistically not significant.
Our measurements revealed no significant differences of the thickness of the LA between rest and Valsalva manoeuvre, except at level +6 ( and further on to 5.29±1.62 mm at 6 cm below the umbilicus ( Table   2).

M. Rectus Abdominis (Ra)
At rest, the thickness of the RA was smaller in the middle of the abdominal wall (9.23 -9.51 mm) than near its insertion at the symphysis (9.76-10.00 mm) and rib cage (10.21 -11.09 mm) ( Table   3). During Valsalva manoeuvre, the results were more inconsistent.
The greatest thickness of the RA at rest according to the side of the body (right versus left) differed depending on the abdominal level but was in average thicker on the right side of the body, but mostly statistically not significant ( Table 3). The same effect was seen on the same levels during Valsalva manoeuvre ( with the RA on the right side at level -6, in most cases statistically significant (Table 3).

Lateral Abdominal Wall Muscles (TrA, IO, EO)
Regarding the lateral abdominal wall muscles, the thickness of the two inner muscles IO and TrA significantly increased during Valsalva on both sides of the body, whereas it significantly decreased in the outer muscle EO (Table 3). No significant difference was seen between the thickness of every muscle between the right and the left side during Valsalva or at rest (Table 3).

Inter-Rectal Distance and Linea Alba
Inter-Rectal Distance: We found the widest IRD (14.13 mm) at the level of the umbilicus during both Valsalva manoeuvre and at rest, which is in accordance to other studies [22][23][24]42]. However, the values between the few other studies and ours slightly differ.

Beer et al evaluated 150 nulliparous women between 20 and 45
years of age with a BMI less than 30 kg/m2 by ultrasound at only three reference points along the abdominal wall (xiphoid, 3cm above and 2 cm below the umbilicus) and only at rest [20]. The mean width of the IRD was 7 ± 5 mm at the xiphoid, 13 ± 7 mm above and 8 ± 6 mm below the umbilicus. These values differ from ours, as we found a smaller IRD above and below the umbilicus. We found 9.92 ± 5.36 mm at +3 and 4.2 ± 4.63 mm at -3. One reason for that might be the fact, that we had different measurement points along the abdominal wall than Beer et al. A second reason might be the fact, that the women in our cohort were younger and had never been pregnant or even given birth before. Overall, these values were also smaller than ours were. Pascoal et al found an IRD of 9.6 ± 2.8 mm at 2 cm above the umbilicus at rest in 10 nulliparous women [43], which was also a little bit less than  [22,23,[42][43][44][45]. IRD in general was wider above the level of the umbilicus than below and decreased the more distant away from the umbilicus in both directions, which is also according to other studies (20,22,24). We found only non-significant changes An explanation for the tendency of a greater LA thickness during Valsalva might be as follows. As one performs Valsalva manoeuvre, the abdominal wall muscles are activated in such a way that the two rectus abdominis muscles contract and get thicker. By that, the anterior and posterior sheets of the rectus abdominis muscles depart and the LA consecutively gets thicker as well.

M. rectus abdominis
At rest, the thickness of the RA was in average smaller in the middle of the abdominal wall than near its insertion at the symphysis and rib cage. The same effect could be seen on the same levels during Valsalva manoeuvre (  [5]. But he pointed out the positive correlation between BMI and muscle thickness, the influence of gender and stated interindividual muscle asymmetry.
In the study of Tahan, significant side differences for the lateral abdominal muscles were found and gender, age and body mass index were identified as significant factors of influence on muscle thickness in his mixed cohort of 75 males and 81 females aged 18 to 44 years [26]. Men had in general thicker muscles; age was negatively associated with muscle thickness and BMI positively.  [48]. The study of Chiarello in postpartum women, split in an exercise and a non-exercise group, showed that postpartum exercise decreases IRD, so the amount of physical activity also changes the dimensions of the abdominal wall [24].

Conclusion
In summary, we found, that inter-rectal distance is the widest at the level of the umbilicus, is in general wider above the level of the umbilicus than below and decreases the more distant away