Immediate effects of different types of abdominal exercises on the inter-rectus distance

1. Introduction

The linea alba is a complex network of connective tissue comprised mainly of collagen fibres [1]. The linea alba has different functions which include supporting the proximal anterior recti, ensuring the stability of the vertebral column, pelvis and the pelvic floor, and intervening in the function of the abdominal muscles since it serves as the anterior anchorage to the former [2, 3, 4]. The inter-rectus distance (IRD) is the linear distance between the two anterior recti abdominis and covers the width of the linea alba.

Different instruments are used to measure the IRD but there is a lack of consensus among researchers on their use [5]. Previous literature review reported one of the most used are manual assessment, but also the pachymeter and ultrasound imaging which showed a higher reliability and higher confidence levels than manual assessment [5].

Diastasis recti abdominis (DRA) is produced when the integrity of the linea alba is incorrect, i.e., a wider separation of the anterior recti than under normal conditions [3, 6, 7, 8]. There is debate over the normal width of this IRD linea alba [2, 6, 9, 10, 11, 12]. Rath et al. investigated cadavers by anatomically dissecting them and considered abdominal diastasis as an IRD greater than 10 mm above the umbilicus, 27 mm at the level of the umbilicus and 9 mm below the umbilicus [9]. Consequently, in 2009, Beer et al. studied the separation between the recti abdominis in nulliparous women (who do not give birth) using ultrasound imaging, and indicated that a normal IRD separation is equal to or less than 15 mm at the xiphoid level, 22 mm at above the umbilicus and 16 mm at below the umbilicus [10].

All these prior studies marked the importance of DRA since it may be associated with health-related quality of life, impaired abdominal muscle strength and severe low back pain [13]. Although there is a surgical intervention for DRA, in which correction is performed by means of abdominoplasty or laparoscopy [14], conservative exercise-based treatment has been mentioned as an alternative and it has been selected as the main option in order to avoid complications from surgery. However, there are no specific interventions which reduce DRA, since many exercises have been tested but authors did not come to a conclusion yet [15].

Taking into consideration that the linea alba connects the left and right sides of the abdominal wall throughout a fibrous structure [16], several authors studied the effect of the different abdominal exercises on the IRD, such as abdominal crunch and transversus abdominis activation [1, 17, 18, 19]. The words studied exercise was the abdominal crunch, which appeared to decrease the IRD [1, 17, 18, 19, 20]. On the other hand, transversus abdominis was also investigated, alone and in association with abdominal crunch, finding in both situations an increase in IRD [18, 19] and a strain on the linea alba [1]. Furthermore, hypopressive exercise has been largely studied since it was found to be effective in improving strength and endurance of pelvic floor muscles [21, 22, 23, 24, 25] and in treatment of prolapses [26]. Hypopressive abdominal exercises were developed in 1980 by Marcel Caufriez and were initially destined for women in postpartum periods. The decrease of abdominal pressure when performing hypopressive exercises produces a contraction of the pelvic floor muscles and of the deeper abdominal muscles [27, 28]. Little evidence is known about the effects of these exercises on the abdominal musculature, but Stüpp et al. [25] and Ithamar et al. [29] observed an activation of the transversus abdominis muscle when performing hypopressive exercise, measured by surface electromyography (EMG). In this regard, recent investigations studied the behaviour of IRD at rest [30] and in DRA [31], but both reported the need of assessing the effects of abdominal exercises in IRD to increase the knowledge in sports training and therapy.

Thus, the aim of this research was to analyse the IRD at rest and during abdominal crunch, abdominal crunch with transversus pre activation, and hypopressive exercise. On the basis that the linea alba has a different behaviour according to the different exercises performed, we hypothesized that abdominal crunch would decrease the IRD and transversus abdominis activation and hypopressive exercise would increase IRD.

2. Method

3. Results

The differences in IRD were calculated between the genders at both the U and UX points during each exercise (Table 1). As shown in the table, there were no inter-gender differences in the IRD during any exercise at neither the UX or U points.

The differences in IRD during rest and abdominal exercises are shown in Table 2, using mean and standard deviation (SD). As detailed in the table, there were no differences in IRD during rest or in any exercise in the UX point. However, significant differences were found in the U point, as crunch + transversus have increased the IRD in comparison with rest ( p = 0.01; ES = 0.12, medium) and in comparison with abdominal crunch ( p = 0.037; ES = 0.121, medium).

The main result of this study suggests that performing an abdominal crunch with transversus abdominis pre-activation increase IRD in U point in comparison with a condition of rest and abdominal crunch. Furthermore, IRD measurements are generalizable to both genders.

The IRD observed in our participants at rest was 1.02 ± 0.53 cm (1.06 ± 0.57 male and 1.00 ± 0.50 female) in UX point and 1.26 ± 0.49 cm (1.30 ± 0.57 male and 1.23 ± 0.44 female) at U point. These measurements agree with previous research, since the width of the linea alba increases near the umbilicus but decreases near the xiphoid [9, 10].

However, IRD measurements obtained in the U point were lower than those described in previous studies, possibly due to the characteristics of the sample since it is formed by young subjects with normal BMI and IRD has been demonstrated to increase with age and obesity [1, 9, 12, 31].

On the other hand, the location of the U and UX point could influence the results since previous studies have reported significant differences in IRD according to the location, varying from 2 cm above the umbilicus [18, 19, 20, 38], 2 cm below [18, 19, 38], 5 cm above [17, 18] and 4, 5 below [17]. In this study, the U point was located just above the umbilicus, and the UX point was located halfway between the umbilicus and the xiphoid, according to Lee and Hodges [1] and obtaining similar results. Participants in this study did not present with DRA, but recent studies suggest that the effects of abdominal exercises in IRD in people with DRA are similar to those of healthy subjects in this study [31]. In this regard, prior investigations analysing immediate effects of abdominal exercises in IRD and DRA were only performed in postpartum women [18, 19, 20, 38] since pregnancy and child birth have been identified as risk factors in increasing the IRD [6, 7, 8, 12]. However, other investigations in the same vain included both men and nulliparous women in their samples, and obtained similar findings with no differences in IRD between sex [1, 17] or between athletes and healthy subjects [30].

On the other hand, the results of this study also suggest that abdominal exercises influence the IRD differently. While the abdominal crunch with transversus pre-activation appears to increase significantly the IRD in both U, abdominal crunch and hypopressive exercise do not seem to influence the IRD in any point. In fact, most research find an increase in IRD when the transversus abdominis is voluntarily activated [18, 19, 38], probably due to the due to the orientation of its fibres which could help tensing and widening the linea alba [18, 39, 40].

No significant differences were found in IRD during the hypopressive exercise, neither in comparison with rest nor with other abdominal exercises, and in any point. There is a large scientific discussion about the effects of hypopressive exercises in the IRD [22], and despite previous studies indicated that hypopressive exercises activate transversus abdominis involuntarily [25, 29], more research is needed since the voluntary activation of this muscle has not been evaluated. Moreover, this is the first study assessing the IRD with ultrasound imaging during an hypopressive exercise. In this regard, results suggest that IRD is influenced by the transversus abdominis contraction when voluntarily (abdominal crunch with transversus pre-activation), but not when involuntarily (hypopressive exercise). Previous research showed similar findings in other muscles as the pelvic floor, with differences in their contraction depending on whether the contraction is voluntary (Kegel’s exercises) or involuntary (hypopressive exercises), and finding higher effects when voluntarily [21, 23, 25, 26, 31]. These results agree with findings in this study, showing a possible explanation to the differences in IRD between the abdominal exercises assessed. Conversely, abdominal crunch has been the most studied exercise in prior investigations, and its influence in linea alba has been largely reviewed, finding a decrease in IRD in comparison with rest [17, 18, 19, 20]. Specifically, Mota et al. [18] and Chiarello et al. [17] detected a high decrease in IRD during abdominal crunch in postpartum women and also in nulliparous women, whereas Lee and Hodges [1] observed a decrease of IRD in women with DRA in comparison with a control group of healthy subjects. Consequently, abdominal crunch seems to decrease IRD in postpartum or in women with DRA, but there is no effect assessed of this exercise in IRD of men an nulliparous women, in agreement with findings in this investigation.

In summary, there is so much variability in the effects of abdominal exercises in IRD. This lead to think there could be external variables influencing the results obtained, but also point out the importance of evaluations with techniques as ultrasound imaging, and the individualisation of treatments since an abdominal exercise could influence differently between subjects in the same conditions [34].

Even though in this sample sex does not influence the results obtained, and there are no subjects with risk factors such as obesity, pregnancy and DRA [12, 31, 41], prior studies reported other factors which could influence results, such as changes in posture or pelvic instability, and they should be taken into account for future research [3, 42, 43].

Upcoming investigations could take into consideration changes in linea alba, since it could be influence by abdominal exercises, modifying its morphology and stiffness [1, 44]. Besides, it is important to continue researching the effects of abdominal exercises in DRA, since the linea alba could respond differently [1, 17, 31], but samples with the same number of women and men should be included to increase knowledge in both sexes. Additionally, variables such as pelvis instability or posture [3, 42, 43] may directly influence the IRD and their influence should be taking into consideration in future investigations.

The limitations of this study should be taken into account when interpreting the results. Abdominal exercises were not performed in a randomized order, since the first exercise was always the abdominal crunch, then the abdominal crunch with transversus pre-activation and finally the hypopressive exercise. This may have influenced the results although we believe that 5 minutes of rest were sufficient to reset the anatomical relationships. Researchers and health care professionals may find these results useful for their clinical practice since there is still no consensus about the performance of the IRD and linea alba during exercises, but measuring with ultrasound imaging grant high reliable results.

5. Conclusion

The abdominal crunch with transversus abdominis pre-activation increases the inter-rectus distance in comparison with rest and with abdominal crunch in the U point. Abdominal crunch and hypopressive exercise do not appear to influence the IRD of this sample.

Author contributions

CONCEPTION: Iria Da Cuña-Carrera.

PERFORMANCE OF WORK: Yoana González-González, Eva Lantarón-Caeiro, Alejandra Alonso-Calvete, Mercedes Soto-González and Iria Da Cuña-Carrera.

INTERPRETATION OR ANALYSIS OF DATA: Alejandra Alonso-Calvete.

PREPARATION OF THE MANUSCRIPT: Mercedes Soto-González and Iria Da Cuña-Carrera.

REVISION FOR IMPORTANT INTELLECTUAL CONTENT: Yoana González-González and Eva Lantarón-Caeiro.

SUPERVISION: Mercedes Soto-González and Iria Da Cuña-Carrera.

Ethical considerations

The project of this study was approved by the ethics committee of the Faculty of Physiotherapy of the University of Vigo in its resolution N ∘ 17072018.

Funding

The authors report no funding.