Introduction

Musculoskeletal ultrasonographic imaging (USI) has become a valuable tool in physiotherapy research and practice [1,2,3,4,5]. Among others, it has been used to measure the width of the fibrous linea alba, also called the inter-recti distance (IRD) [6, 7]. The most common purpose of this measurement is to investigate pregnancy-related diastasis recti abdominis (DRA) or monitor the impact of specific exercises on this condition [8,9,10,11,12,13]. DRA mostly develops in advanced pregnancy and manifests itself as an overstretched linea alba leading to the excessive separation of the recti abdominis muscles. In approximately 40% of women, the condition persists after pregnancy [14, 15] showing increased linea alba laxity [8, 10] that is accompanied by decreased thickness, strength, and impaired function of the abdominal muscles [16,17,18]. Due to its appearance, persistent DRA negatively impacts women’s self-esteem and body image [19, 20]. If pronounced, the condition may result in insufficient soft tissue protection of the uterus in successive pregnancies and the formation of umbilical/epigastric hernia that may require surgical intervention [21,22,23,24]. Possible pregnancy-related DRA risk factors and the association of post-pregnancy DRA with chronic low back pain or support-related pelvic floor dysfunction are still under investigation [25,26,27,28,29,30]. According to the systematic reviews, DRA may be associated with impaired abdominal muscle strength [31], health-related quality of life [31, 32], body image satisfaction, abdominal pain [32], low back pain severity [31], and pelvic organ prolapse [32]. No significant association was observed between the DRA and lumbopelvic pain or urinary incontinence [31, 32]. However, further investigation is needed to verify these relationships because of the weak methodological quality of the existing studies and their heterogenicity regarding study design and population [31, 32]. Numerous studies have investigated the impact of specific therapeutic exercises or other physiotherapy methods on pregnancy-related DRA [8, 33,34,35,36,37]. Physiotherapy has been shown to improve the condition [38,39,40]. However, according to a recent systematic review and meta-analysis, there is currently no high-quality scientific evidence on the most effective exercise programs in the treatment of postpartum DRA [41]. Besides research on pregnancy-related DRA, IRD measurements have also been performed in studies on athletes [42, 43] and individuals with tendinopathy, dysmenorrhea, or lumbopelvic pain [44,45,46].

In physiotherapy research involving IRD measurement, musculoskeletal USI has been widely used owing to its high reliability [7, 18, 47,48,49,50]. However, review studies have indicated that IRD measurement procedure has not been standardized [39, 40]. The main discrepancies between the study protocols concern measurement sites (supraumbilical and infraumbilical) and diagnostic criteria for DRA [14, 38,39,40]. They also apply to the examinee’s body position, way of abdominal muscle activation, or breathing phase during image capturing. Differences in the measurement procedure may impact study outcomes and make the comparison of findings obtained by different research centers difficult or impossible [40]. Therefore, systematic mapping of physiotherapy-related studies involving IRD measurement with USI and discussions on specific aspects of the measurement procedures in the light of existing knowledge on musculoskeletal USI are warranted. All these should help formulate recommendations on best practices in performing and describing IRD assessment methods. If future studies follow such recommendations and use standardized measurement procedures, reports on DRA incidence or treatment effects could be compared. The recommendations may also help physicians and physiotherapists make more accurate DRA diagnoses and decide what conditions might benefit from physiotherapy. This, in turn, may prevent adverse effects of untreated DRA.

Therefore, this scoping review aimed to collect all peer-reviewed publications related to physiotherapy and physical exercise/training that describe IRD measurement procedures using USI. Based on discussions on similarities and differences of the procedures, and the completeness of their descriptions, it was aimed to formulate recommendations on the IRD measurement protocol standardization, to be considered in designing future physiotherapy studies and used in physiotherapy practice.

Methods

This scoping review was designed and conducted according to the Preferred Reporting Items for Systematic Reviews and Meta-Analyses, Extension for Scoping Reviews (PRISMA-ScR) [51, 52]. The PRISMA-ScR checklist is provided in Additional file 1: S1.

Protocol

The study protocol was jointly developed a priori by two review leads (A.O.B. and D.C.). It was registered at the International Platform of Registered Systematic Review and Meta-analysis Protocols (INPLASY). The registration number is 202290116.

Eligibility criteria

The eligibility criteria were developed by two reviewers (A.O.B. and D.C.) with experience in research and practice on women’s health physiotherapy and IRD assessment using USI. Articles published in English starting with the earliest available until the last database entry were considered for review. To assure high-standard measurement procedure descriptions, the included publications were peer-reviewed research papers or registered study protocols. Table 1 presents detailed inclusion and exclusion criteria.

Table 1 Inclusion and exclusion criteria used in the two-phase selection process to identify publications describing the measurement of inter-recti distance (IRD)/diastasis recti abdominis (DRA) with ultrasonographic imaging (USI)
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Information sources

To identify potentially relevant publications, the PubMed, Embase (Elsevier), and Ovid (Medline) bibliographic databases were searched by two independent reviewers (D.C. and A.O.B.). The most recent electronic literature search was conducted on August 31, 2022. The search was supplemented by hand searching and scanning the reference lists of the included or relevant sources of evidence.

Search strategy

The search strategy was developed by two reviewers (D.C. and A.O.B.) who gained their knowledge through video tutorials and closely cooperated with an experienced librarian from the Medical University Library. For PubMed, the final search strategy used was as follows: ((interrect*[tiab]) OR (inter-rect*[tiab]) OR (linea alba[tw]) OR (recti abdominis[tiab]) OR (recti muscle*[tiab]) OR (rectus muscle[tw]) OR (rectus abdom*[tiab]) OR (recti[tiab]) OR (rectus abdominis[mh])) AND ((diastasis[tiab]) OR (separation[tw]) OR (width[tw]) OR (distance[tiab]) OR (widening[tw])) AND ((USI[tw]) OR (ultrasound imaging[tiab]) OR (ultrasonography[tiab]) OR (ultrasonography[mh]) OR (US[tw]) OR (ultrasound[tiab])); filter: English.

All search results were saved, and duplicate publications were removed manually. To reduce the risk of error, two researchers (D.C. and M.R.Z.) independently removed duplicates and compared the outcomes of their selection.

Selection of sources of evidence

Two standardized forms were developed (using the Microsoft Forms software, Office 365) to guide the screening of the articles identified through the search strategy. Forms I and II concerned title and abstract, and full-text screening, respectively (Additional file 1: S2 and S3). These forms, along with the explanation and elaboration documents (Additional file 1: S4), were developed and refined by two review leads (D.C. and A.O.B.) during the prescreening of 50 random publications. The prescreening was a continuous interactive process of refining the questions to improve their appropriateness, accuracy, and comprehensiveness and ensure that filling out the forms will enable capturing all relevant publications. After both forms had been prepared, four reviewers (D.C., A.O.B., M.R.Z, and J.N.) performed calibration exercises, independently screening a sample of 30 titles and abstracts and filling out Forms I. The Kappa coefficient showed an interrater agreement of 0.80–0.93. The discrepancies between the reviewers’ answers and any unclear issues were discussed until a consensus was reached. Minor refinements were then made to Form I and to the explanation and elaboration document. As the interrater agreements were satisfactory [52], the screening of the titles and abstracts of all remaining publications was continued independently by two reviewers (D.C./J.N. and M.R.Z.). After completing Forms I for all publications, the reviewers compared the answers, discussed and resolved any disagreements by consensus. In cases of uncertainty, an additional reviewer (A.O.B.) was consulted to make the final decision.

The same steps were followed for full-text screening and filling out Forms II. In this phase, the calibration exercises concerned screening a sample of 20 full texts. The interrater agreements were excellent (Kappa coefficient = 1).

Data charting process

A Data-charting Form was jointly developed by two review leads (A.O.B. and D.C.) to extract relevant information on the IRD measurement procedure from the included sources of evidence (Additional file 1: S5). Two reviewers (M.R.Z./J.N. and A.O.B./D.C.) independently copied appropriate extracts from full texts and pasted them into separate forms. To facilitate comparisons, extracts by both reviewers were pasted in two adjacent columns of the final version of the Data-charting Form.

Data items

The following data items on the IRD measurement procedure using USI were sought: publication characteristics (first author, publication year, country/countries of the research center, main study objective); population characteristics (sex, age, body mass index [BMI; mean ± standard deviation], DRA presence, and for females, the number of pregnancies/deliveries, mode of delivery); specific aspects of the measurement procedure (examinee’s body position during the assessment at rest, muscle activation/task during the examination, the respiratory phase/pattern during image capturing (Table 2), the examiner’s profession and experience (hours of training and years of practice), ultrasonographic scanner (brand, type, mode, and field of view) and transducer (type and frequency), measurement site/sites; cutoff values for “normal” IRD or DRA (i.e., IRD considered normal or indicative of a pathological condition), number of images taken at each measurement site and whether measurements were averaged, image processing and measurement methods (online or offline); Table 3).

Table 2 Study objectives, participants, and conditions for performing the inter-recti distance (IRD) measurement using ultrasonographic imaging (USI) in 49 study protocols
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Table 3 Characteristics of the examiners, ultrasonographic imaging (USI) systems, and inter-recti distance (IRD) measurement methods in 49 study protocols
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Synthesis of results

To present descriptions of specific aspects of the IRD measurement procedure using USI from individual publications, three reviewers (A.O.B., D.C., and M.C.) compared charted extracts, discussed uncertain issues, resolved disagreements, and jointly made summaries of the extracts. If the studies followed the measurement procedure described in previous publications, “same as….” was stated. The citations are presented in Tables 2 and 3 in alphabetical order.

Results

Selection of sources of evidence

Details on the selection are presented in the PRISMA flow diagram (Fig. 1).

Fig. 1
figure 1

PRISMA-ScR flow diagram

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Characteristics of sources of evidence

All 49 included publications (published between 2008 and August 2022) were related to physiotherapy or physical exercise/training and described IRD measurements using USI.

Eight studies were from Canada [8, 17, 20, 33, 48, 53,54,55] and nine from Spain [11, 42,43,44,45, 56,57,58,59]. Countries with two studies were Brazil [60, 61], the USA [9, 62], the UK [16, 63], Norway [13, 37], New Zealand [47, 64], and China [65, 66]. Countries with one study were Israel [27], Australia [67], Japan [50], Italy [10], Sweden [68], Taiwan [18], Portugal [36], Korea [69], and Poland [70]. Six 2-center studies were from Portugal and Norway [15, 35, 49, 71,72,73], and one from Italy and Canada [12], Australia and Canada [34], the UK and Canada [46], and Spain and Canada [74]. One registered study protocol was from Norway [75].

Results of the individual sources of evidence

Table 2 presents study objectives, populations, and IRD measurement conditions; Table 3 shows the examiners’ experience, USI systems, and IRD measurement methods.

Synthesis of the results

The main objectives of the 49 studies are presented in Additional file 1: S6—Objectives of the Studies.

Population

Of the 49 publications, 36 (73.5%) concerned women in their perinatal periods: In 31, the assessment was scheduled after pregnancy/pregnancies in [8, 10, 12, 13, 16,17,18, 20, 27, 33, 34, 36, 37, 48, 49, 53,54,55,56, 60, 63,64,65,66,67,68,69,70,71, 73, 74], in four during and after pregnancy [15, 35, 72, 75], and in one during pregnancy (2.8%) [61]; 17 publications concerned women with increased IRD/DRA [8, 10, 13, 15, 17, 20, 27, 33, 34, 37, 48, 53, 64, 65, 68, 69, 75]. In 13 of the 49 studies (26.5%), the study populations comprised of men and/or women (nulliparas and/or paras): Three were validity/reliability studies [47, 50, 62], three investigated the impact of specific tasks on IRD [9, 11, 58], five assessed the IRD in medical conditions not related to pregnancy [44,45,46, 57, 59], and two investigated the IRD in athletes [42, 43] (Table 2).

Body position, muscle activity, and breathing during the IRD examination

In all 49 studies, the IRD was measured in the supine position: in 26 (53.1%), at rest and during muscle activity/task [8, 9, 11, 13, 27, 34,35,36,37, 47,48,49, 53, 55,56,57,58,59,60, 62, 64, 67, 70, 71, 73, 74], in 21 (42.9%), only at rest [10, 12, 15,16,17,18, 20, 33, 42,43,44,45,46, 50, 54, 65, 66, 68, 69, 72, 75], in one (2%), only during muscle activity/task [61], and in one (2%), in supine (at rest), and in sitting and standing [63]. One of the studies also performed measurements in side plank position [64].

Of the 27 studies that examined IRD during specific muscle activity/task in supine, some used more than one task. The tasks were head lift [8, 13, 48, 53, 55], trunk flexion with scapulae raised fully or partially off the coach (referred to as trunk flexion [60, 61], partial curl-up [9, 47, 62], semi curl-up [8, 74], curl-up [13, 27, 34, 56, 64, 70], abdominal crunch [11, 35, 36, 49, 71, 73], modified abdominal sit-up [67], and abdominal contraction [56]), transversus abdominis contraction/abdominal drawing-in maneuver (ADIM) [13, 35, 37, 49, 56, 58, 73, 74], curl-up/abdominal crunch with preactivated transversus abdominis (by ADIM [11, 73, 74] or pelvic floor muscles (PFM) contraction [34]), exclusive PFM contraction [13, 37], combined PFM contraction and curl-up [13], combined PFM contraction and ADIM [13, 37, 64], abdominal hypopressive exercise (AHE) [11, 74], pelvic tilt [13], twisted curl-up (to the left and right) [13], calf muscle maximal isometric contraction [59], Sahrmann early level single leg raise combined with ADIM, ADIM with Tubigrip and ADIM with taping, curl-up with Tubigrip, and curl-up with taping [64].

Of the 49 study protocols, 14 (28.6%) mentioned supporting the examinee’s head on a pillow during IRD measurement in the supine position [8,9,10, 20, 34, 37, 47, 48, 54, 55, 57, 62, 63, 75]. Thirty-one (63.3%) of the 49 protocols specified the examinees’ lower extremity position in supine: In 30, the lower extremities were flexed (for the measurements at rest [10, 15,16,17,18, 45, 63, 68, 72, 75], at rest and during specific task/tasks [9, 11, 13, 27, 34,35,36,37, 47, 49, 56, 57, 60, 62, 64, 67, 71, 73, 74], and during specific tasks only [61]), and in one they were extended (for the measurements at rest and during head lift) [53].

Of the 30 protocols using flexed lower extremities, in 26 examinees’ feet were supported on the examination table [9,10,11, 13, 15, 17, 27, 34,35,36,37, 45, 47, 49, 60,61,62,63,64, 67, 68, 71,72,73,74,75] (16 specified that knee flexion angle was 90° [11, 15, 27, 35,36,37, 47, 49, 63, 64, 67, 68, 71,72,73, 75]), in 3, one [56] or two pillows [16, 18] were placed under the examinees’ knees, and in one the method of obtaining lower extremity flexion was not specified [57] (Table 2).

Of the 49 studies, in 32 (65.3%), IRD images were captured during a specific moment of the breathing phase: in 27 at the end of normal expiration [8, 11,12,13, 15, 17, 18, 27, 35,36,37, 42, 43, 45,46,47, 49, 50, 59, 63, 67, 68, 71,72,73, 75], in two in a neutral moment immediately after expiration [10, 64], in two during expiration [54, 61], and in one the breathing pattern/phase was related to a specific exercise [74] (Table 2).

Examiners and examination tools

Of the 49 protocols, 36 (73.5%) specified the examiner’s profession: In 31 protocols, it was a physiotherapist [1, 8, 11, 12, 15, 17, 18, 35, 37, 42,43,44,45,46,47,48,49,50, 53, 55, 56, 58, 63, 64, 67, 70,71,72,73,74,75], in two registered sonographers [20, 33], in one physicians [66], in one a physician specializing in gynecological USI [27], and in one a radiologist [65]. Of the 49 protocols, 41 (83.7%) included information on the examiner’s training and/or experience in USI [8,9,10,11,12,13, 15, 17, 18, 20, 27, 33, 35, 37, 42,43,44,45,46,47,48,49,50, 53,54,55,56,57,58, 61,62,63,64,65, 67, 70,71,72,73,74,75] (34 described type of examiner’s training/expertise [8,9,10,11,12,13, 15, 17, 18, 20, 27, 33, 35, 37, 43, 44, 47,48,49, 53,54,55,56, 58, 62, 63, 65, 67, 70,71,72,73,74,75] and 25 training and/or experience duration [8,9,10, 12, 17, 18, 33, 35, 43,44,45,46,47,48, 53,54,55,56, 58, 62, 65, 67, 70, 74, 75]). Twelve studies reported the reliability of the IRD measurements using USI [8, 18, 46,47,48,49,50, 54, 55, 62,63,64]. (Six of them were reliability studies [47,48,49,50, 55, 63].)

Forty-eight (97.9%) of 49 protocols described ultrasound transducer type: 43 chose linear probe [8, 10,11,12,13, 15,16,17,18, 20, 33,34,35,36,37, 42,43,44,45, 47,48,49,50, 53,54,55,56,57,58,59,60,61, 63, 65,66,67,68, 70,71,72,73,74,75], three curvilinear [9, 46, 62], one both types [64], and one provided unclear information [69]. Forty-six protocols specified probe frequency (3–18 MHz); 27 provided probe size (3–11 cm) [12, 15,16,17,18, 33, 35, 36, 42,43,44,45,46, 49, 53, 54, 58, 59, 63, 66,67,68, 70,71,72,73,74]. Eleven studies used additional features, settings, or products (panoramic imaging [13, 20, 33, 54, 56, 68], trapezoid mode [10, 17, 53], and acoustic standoff pad [48, 54, 55]) that enabled field of view extension, one compared IRD measurements using low and high ultrasound resolutions [47] (Table 3).

Measurement sites

Of the 49 protocols, 16 (32.6%) used one measurement site [12, 15, 16, 36, 42,43,44,45,46, 50, 54, 58, 59, 68,69,70]; another 16 (32.6%) two sites [9,10,11, 13, 34, 37, 47, 49, 53, 56, 57, 62, 64, 71, 73, 75]; nine (18.4%) three sites [8, 17, 27, 35, 61, 63, 66, 72, 74]; seven (14.3%) four sites [18, 20, 33, 48, 55, 60, 67]; and one (2%) five sites [65]. However, of the 39 studies on pregnancy-related IRD, only seven (17.9%) used one measurement site [12, 15, 16, 36, 54, 68, 69], while of the 10 studies on non-pregnancy-related IRD, one measurement site was used in nine (90%) protocols [42,43,44,45,46, 50, 58, 59, 70].

In 26 (53.1%) of the 49 study protocols, the IRD measurements were made at the superior umbilical border (16/26, 61.5%) [8, 11, 16,17,18, 20, 27, 33, 34, 36, 45, 48, 54, 55, 63, 67] and/or at the inferior umbilical border (9/26, 34.6%) [18, 42,43,44, 46, 50, 58, 59, 67] (two studies used both sites [18, 67]), and directly at the umbilicus (3/26, 11.5%) [61, 65, 66].

In 38 (77.5%) of the 49 study protocols, more distant measurement sites were chosen: supraumbilical in 37 (97.4%) [8,9,10,11,12,13, 17, 18, 20, 27, 33,34,35, 37, 47,48,49, 53, 55,56,57, 60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75] and/or infraumbilical in 27 (71%) [9, 10, 13, 15, 18, 20, 27, 33, 35, 37, 47,48,49, 55,56,57, 61,62,63,64,65, 67, 71,72,73,74,75]. (26 of these protocols used both locations [9, 10, 13, 18, 20, 27, 33, 35, 37, 47,48,49, 55,56,57, 61,62,63,64,65, 67, 71,72,73,74,75].) Supraumbilical measurements were made at 2 cm [12, 13, 35, 37, 47, 49, 56, 64, 70,71,72,73,74,75], 2.5 cm [18, 69], 3 cm [8, 10, 17, 20, 27, 33, 48, 53, 55, 60, 61, 66], 4.5 cm [9, 57, 62, 67, 68], 5 cm [8, 17, 20, 33, 35, 48, 53, 55, 72], and 6, 9, and 12 cm [60] above the umbilicus; a third of the distance [63] and halfway [11, 34, 74] between the umbilicus and xiphoid, and at subxiphoid and epigastric sites [65].

Infraumbilical measurements were taken at 2 cm [13, 15, 27, 35, 37, 47, 49, 56, 61, 64, 71,72,73,74,75], 2.5 cm [18], 3 cm [10, 20, 33, 48, 55, 66], and 4.5 cm [9, 57, 62, 67] below the umbilicus, halfway between the umbilicus and pubis [63], and at infraumbilical and suprapubic sites [65].

Of the 38 protocols describing supra- and infraumbilical measurement sites, 37 indicated starting points to identify them: 25 precisely indicated these points [8, 9, 11, 13, 15, 18, 20, 33,34,35, 37, 48, 49, 55,56,57, 62, 64, 67,68,69, 71,72,73,74] (14 indicated umbilical midpoint [9, 13, 15, 35, 37, 49, 57, 62, 64, 68, 71,72,73,74], and 11 superior and/or inferior umbilical borders [8, 11, 18, 20, 33, 34, 48, 55, 56, 67, 69]); 12 less precisely indicated that the sites were measured from the umbilicus [10, 12, 17, 27, 47, 53, 60, 61, 63, 66, 70, 75] (Table 3).

Screening subjects for DRA

Of the 49 study designs, 19 required screening subjects for DRA/increased IRD: 12 studies chose USI for screening [8, 10, 13, 15, 17, 27, 34, 53, 61, 67, 69, 75]; however, only nine of them referred to normative values (eight [8, 15, 17, 27, 34, 53, 61, 67] referred to “normal” linea alba width values for nulliparas [76] and one [75], to “normal” IRD for postpartum primiparas [72]); nine studies used a palpatory clinical test [77, 78] for screening [13, 20, 33, 37, 48, 64, 65, 68, 69]. (Two performed screening using both USI and palpation [13, 69].) In one study, tape measurement DRA classification [79] was used for screening [13], and in three study protocols, abdominal wall protrusion [80] was an additional inclusion criterion for DRA [13, 37, 75]. Studies used different measurement conditions (i.e., body position, muscle activity, measurement sites) for screening and differently interpreted the palpatory examination outcomes. One study determined “normal” IRD values for primiparas in pregnancy and postpartum [72]. Another study proposed DRA pattern classification [10] (Table 3).

Number of images per measurement site. In 25 (51%) of the 49 studies, more than one image was captured at each measurement site to calculate the mean values of IRD: The mean IRD of three, five, and two images was used in 15 [8, 10, 17, 18, 42,43,44,45,46, 56, 58, 59, 61, 68, 73], five [20, 33, 48, 54, 55], and five [16, 50, 63, 64, 74] studies, respectively. One study used the best of three images at each measurement site [73]. Three studies just mentioned that two [53, 67] or three [34] images were taken per measurement site; five used single images [13, 35, 37, 49, 72]; and 15 did not specify the number of images per sites [9, 11, 12, 15, 27, 36, 57, 60, 62, 65, 66, 69,70,71, 75] (Table 3).

Image processing and measurement methods

In 38 (77.5%) of the 49 studies, the methods of frozen image processing and IRD measurement were described: 27 indicated offline processing (26 specified software type [13, 15,16,17, 27, 34,35,36, 42,43,44,45,46, 48, 49, 53,54,55, 58, 59, 63, 68, 71,72,73, 75]), three indicated the measurement tool/software integrated into the ultrasonographic scanner [8, 37, 47], and eight on-screen measurement [9, 11, 18, 62, 65, 69, 70, 74]. In four studies, the procedure included video capturing [8, 34, 36, 46] (Table 3).

Discussion

Summary of evidence

This scoping review aimed to present IRD measurement procedures using USI that have been used in physiotherapy-related research. Using the IRD measurement method descriptions from the 49 systematically mapped original peer-reviewed publications, the data were synthesized. The review indicates that the studies used different approaches regarding specific aspects of the IRD measurement procedure which, to a great extent, restrict or prevent cross-study comparisons. Based on the synthesis of the results, practical conclusions and recommendations on the standardization of the procedure for future physiotherapy research and clinical use have been made.

The first synthesized aspects of the IRD measurement procedure were the examinee’s body position, specific muscle activity/task, and breathing phase during image capturing. In all 49 publications, the supine position was the basic body position for IRD examination. Depending on the study aim, this position was used to measure the IRD at rest, and/or as a starting position to perform a specific task for IRD measurement during muscle activity. Several protocols included information that in the supine position, the examinee’s head was rested on a pillow. Slight head elevation improves the examinee’s comfort and should not interfere with IRD measurements. However, pillows of unspecified sizes do not allow a uniform angle of neck flexion. To standardize the head position, an examination table with adjustable headrest that can be elevated to the desired angle (i.e., 15°) might be considered.

The majority of protocols specified the examinees’ lower extremity position during the IRD examination, and in almost all of them, it was hip and knee flexion. For the examinations at rest and/or during specific tasks, lower extremity flexion was attained by supporting feet on the examination table. The knee flexion angle of 90° was the most frequent. However, in a few studies lower extremity flexion was obtained by placing one or two pillows under the examinees’ knees. To ensure reproducibility, the lower extremity position during the IRD measurement in the supine position should be described in detail. The pillow of unspecified size placed under the examinees’ knees does not allow for reproducing the lower extremity position. Instead of pillows, the examiners may consider a foam roller. For instance, a foam roller of 15 cm in diameter ensures slight hip and knee flexion and enhances the examinee’s comfort in the supine position. The position involving slight lower extremity flexion may be useful, especially if the IRD is measured only at rest.

If the IRD is measured in supine with greater hip and knee flexion angles, this position may be accompanied by reduced anterior pelvic inclination/tilt angle [81]. It should be accounted for that as a result of the increased posterior pelvic tilt, the linea alba insertion at the pubic symphysis and its origin at the xiphoid come closer, and the anterior abdominal wall structures relax. Theoretically, this position may alter the linea alba length, tension, and width; however, to the authors’ knowledge, this has not been investigated. Considering pelvifemoral motion during hip flexion [81], we recommend that researchers provide at least the magnitude of the examinee’s knee flexion angle when using the crook-lying position with feet supported on the examination table. In one study, the authors attempted to standardize the pelvic inclination and examined the IRD after teaching the examinee to maintain a neutral pelvic position [63]. The study was also the only one that used sitting and standing positions in addition to supine for IRD measurement [63]. The measurement in vertical positions is an interesting approach from the perspective of physiotherapy. It may allow to, for example, determine the effects of specific treatments in DRA subjects on their linea alba parameters during postural abdominal muscle tone and increased load on the anterior abdominal wall structures compared to relaxed supine position.

Most protocols included information that IRD images were captured during a specific moment in the breathing phase and most commonly, it was at the end of normal expiration. During resting ventilation, the abdominal cavity volume and intra-abdominal pressure are modulated depending on the respiratory phase; therefore, the tension of the anterior abdominal wall soft tissues, including linea alba, fluctuates. It has been also suggested that normal expiration is accompanied by some engagement and increased thickness of the abdominal muscles [82, 83]. As the linea alba is formed by interlacing abdominal muscle aponeuroses [6], it can be assumed that its tension and width may also change during the respiratory cycle. Ultrasound images should therefore be captured at the same phase of the breathing cycle (specified in the description of the measurement procedure). The approach to capture images at the end of normal expiration is consistent with previous recommendations on the evaluation of the abdominal muscle parameters and IRD with USI [84, 85].

In this review, the characteristics of the examination tools, selection of IRD measurement sites, and methods of DRA screening were also synthesized. Most studies used linear transducers (a few curvilinear) with frequencies ranging from 3 to 18 MHz. In over half of the protocols, transducer’s size was provided, ranging from 3 to 11 cm (most often, 4 cm). Some protocols specified additional features, settings, or products used, such as panoramic imaging, trapezoid mode, and acoustic standoff pad to extend the field of view to measure wider DRA. Acoustic standoff pad and panoramic mode have been suggested to be valid methods to measure the IRD of up to three finger widths. However, further research is needed on the validity and reliability of the extended field-of-view methods for greater IRD [54]. Sufficient training and experience in the use of panoramic USI are also of importance [86].

This scoping review indicates researchers used 1 to 5 measurement sites during their IRD assessments. Most studies that investigated pregnancy-related IRD/DRA used more than one measurement site, while the great majority of studies on individuals with other medical conditions and athletes performed measurements only at one site. As linea alba width changes along its course [6, 10, 72, 76, 87, 88], examining more than one location is important. This review confirms its relevance in research on pregnancy-related IRD/DRA.

Concomitantly, the comparison of findings across studies is only possible if IRD measurements are made at the same anatomical location. This scoping review indicates there is no consistency between study protocols regarding this aspect. The IRD was examined at the superior and/or inferior umbilical borders and at various levels (between 2 and 12 cm) above and (between 2 and 4.5 cm) below the umbilicus. A few protocols presented a valuable approach to supra- and infraumbilical site selection by taking into account the inter-individual variability in the linea alba length (i.e., the distances from the umbilicus to the linea alba origin and insertion). The measurements were performed at a third of the distance [63] or halfway [11, 34, 74] between the umbilicus and xiphoid, and halfway between the umbilicus and pubis [63].

This review also indicates the IRD was more frequently examined at the superior than at the inferior umbilical border, and above than below the umbilicus. In addition, most infraumbilical measurement sites were relatively close to the umbilicus. When choosing the measurement sites along the linea alba, the current knowledge on the measurement reliability at specific locations should be considered [47,48,49,50, 55, 63]. According to a systematic review and meta-analytical reliability generalization [7] and subsequent reliability studies [48, 63], the reliability of IRD measurements using USI is high; however, it is slightly lower for infraumbilical than for supraumbilical measurements. Infraumbilical measurements are more challenging because, at this anatomical location, the posterior rectus sheath is hardly visible [48, 89]. In women with DRA and history of cesarean section, the infraumbilical measurements using USI have been reported inaccurate compared to intraoperative surgical compass measurements. The researchers suggested that their difficulty in measuring the IRD at that location was due to the cesarean section-related fibrosis and loss of definition of the posterior layer of the recti muscles [89]. This is consistent with the cadaver study that indicated the posterior rectus sheath below the umbilicus consisted of much thinner collagen fibers than the fibers of the posterior rectus sheath above the umbilicus [6]. It is therefore possible that less frequent use of infraumbilical measurement sites indicated by this scoping review was due to the greater difficulty to measure the IRD at this location. Considering this, it is important to determine measurement reliability at specific sites before the primary investigation, as performed in several studies [8, 18, 46, 54, 62]. It should be noted that measurement reliability may also depend on the examiner’s experience [7, 47].

Most protocols that included IRD measurements away from the umbilicus specified the starting points from which the anatomical locations were measured. These starting points were the umbilical midpoint, and the superior and/or inferior umbilical borders (for locations above and/or below the umbilicus, respectively). To enable cross-study comparisons, we propose the superior umbilical border as the standard starting point to determine supra- and infraumbilical measurement sites. This would reduce possible discrepancies in determining these locations caused by inter-individual variability in the size of the umbilical ring.

Considering the need to standardize the measurement protocol to facilitate cross-study comparisons [7, 39, 40] and the synthesis of results in this scoping review, we propose four basic measurement sites:

  • half of the superior umbilical border-xiphoid distance;

  • a quarter of the superior umbilical border-xiphoid distance (most proximal to the umbilicus);

  • the superior umbilical border;

  • a quarter of the superior umbilical border-pubis distance (most proximal to the umbilicus; Fig. 2).

Fig. 2
figure 2

Proposed sites for inter-recti distance measurement using ultrasonographic imaging based on the scoping review results synthesis. AU1: ½ of the superior umbilical border-xiphoid distance; AU2: ¼ of the superior umbilical border-xiphoid distance; U: the superior umbilical border; and BU: ¼ of the superior umbilical border-pubis distance

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We recommend the locations of a quarter of the superior umbilical border-xiphoid/pubis distances as they roughly correspond to locations 4.5/5 cm above/below the umbilicus, which have been frequently applied. When choosing measurement sites in women with pregnancy-related DRA, it should be remembered that supraumbilical DRA patterns are the most common, and in the case of complete opening, the diastasis is usually wider above the umbilicus [10].

More than half of the protocols included information that 2–5 images were taken per site to calculate measurement means (most often the mean IRD of three images). In one study, the best of three images per site was used for the measurement [73]. Higher intrarater and interrater reliability and lower intrarater measurement error were demonstrated when using means of 2 or 3 consecutive measurements in determining specific abdominal muscle parameters with musculoskeletal USI [1, 90]. Therefore, to enhance measurement precision, we propose to use the mean value of three measurements taken at each anatomical location. This may be of particular importance when the assessment is performed by a less experienced examiner.

Our synthesis revealed researchers used different selection methods and criteria to identify subjects with or without DRA. Most researchers used ultrasound screening with cutoff values for “normal” linea alba width that had been determined by Beer et al. [76]. In that study, the examinees were in the supine resting position with the lower extremities extended; the measurement sites were at the xiphoid, 3 cm above and 2 cm below the umbilicus [76]. For consistency, these measurement conditions and sites should be followed during screening for DRA. However, some of the studies modified the procedure.

It is still unclear what IRD values (obtained by USI) should be considered physiological and pathological from the perspective of proper functioning of the abdominal wall structures [14, 38,39,40]. Normative IRD values of up to 15 mm at the xiphoid, up to 22 mm at 3 cm above the umbilicus, and up to 16 mm at 2 cm below the umbilicus were designated using the 90th percentile. The investigators suggested that DRA could be anticipated beyond these values with an error rate of 10%. The study also presented the population’s extreme linea alba widths, which were 31 mm at the xiphoid, 35 mm at 3 cm above the umbilicus, and 31 mm at 2 cm below the umbilicus. They concluded that the DRA could be anticipated beyond these values without doubt [76]. Another study proposed the cutoff values for “normal” IRD in primiparas using the 80th percentile of the values obtained in 84 females in their perinatal period. At six months postpartum, these values were 24 mm at 5 cm above the umbilicus, 28 mm at 2 cm above the umbilicus, and 21 mm at 2 cm below the umbilicus. The authors suggested that, in primiparas, the IRD might be considered “normal” up to values wider than for nulliparas [72]. Further investigation is warranted to clarify the DRA diagnostic criteria based on USI. Also, systematic mapping and verification of the quality of the existing studies that determined “normal” IRD cutoff values using different methods may provide additional information on what is already known and in what direction future research should proceed. Regarding physiotherapy research and practice, we suggest establishing normative values for measurement sites specified in Fig. 2. It is worth mentioning that musculoskeletal USI can also be used to inspect the entire linea alba from its origin to insertion to determine the DRA pattern and identify the location of the greatest IRD [10].

Several reviewed studies used a palpatory clinical test to screen subjects for DRA. This test is performed during abdominal muscle activation to facilitate palpation. The examinee lies in a supine position with the knees bent (at 90°) and feet resting on the examination table. The examinee is asked to raise the head and shoulders upward (until the spines of the scapulae leave the examination table). To palpate the IRD, the examiner places fingers horizontally across the midline of the examinee’s abdomen at the umbilicus. The same procedure is then repeated above and below the umbilicus [67, 71, 77, 78]. This clinical test has shown good [71] and very good [67] intrarater reliability, and moderate [71] or moderate-to-good [67] interrater reliability. The test result is considered negative when a maximum of two-finger widths can be placed between the medial parts of the right and left recti muscle bellies and/or when the fingers do not sink into the gap between the muscles, and/or no linea alba bulging or slumping is present [77,78,79]. This review revealed that some modifications to the test execution have been introduced by researchers, e.g., the IRD palpation during head lift instead of head and shoulder lift [20, 33, 48]. Such an approach may influence test results because abdominal muscle engagement is different between the two tasks [13, 77, 78]. Another modification concerns the interpretation of the test outcome; some studies classified two- and not more than two-finger widths as DRA [13, 20, 33, 37, 48].

Although the palpatory clinical test [77, 78] is the easiest and fastest screening procedure for DRA, the limits of agreement between this assessment method and USI are wide [67]. We believe palpatory test results of DRA = IRD > two-finger width [77, 78] should be used as indicative only. The two-finger width equals approximately 3 cm; however, it may differ especially between male and female examiners. The original interpretation of the test result as positive or negative also fails to consider the fact that normal linea alba is narrower above and below the umbilicus compared to its width at the umbilicus [6]. Therefore, IRD palpation should be combined with caliper measurement, which is a reliable and valid method [62, 67]. It would be necessary though to determine “normal” IRD values during abdominal muscle activity under standardized conditions using a caliper. Notwithstanding, the strength of the palpatory method is that it enables identifying linea alba slumping or bulging during abdominal muscle activation [79]. It also enables examination of the entire linea alba for the DRA pattern.

Limitations

The limitation of this scoping review is that the publications were searched for in three major databases. Therefore, some studies might have been omitted. Also, only research papers published in English have been included.

Conclusions

This scoping review indicates that most studies provided a detailed description of IRD measurement using USI. IRD examination was typically made with the subject in the supine position. A majority of the examinees had their hips and knees flexed. Most studies captured images at the end of normal expiration. Different measurement sites and starting points to determine supra- and infraumbilical sites were used. Most studies used linear transducers of widths and frequencies 3–11 cm and 3–18 MHz, respectively. To enhance the IRD measurement precision, most studies captured more than one image at each measurement site, and calculated the mean values of the measurements. Different approaches were used to screen subjects for DRA. Further investigation on USI-based measurements (following the protocol proposed in this review) is warranted to clarify the DRA diagnostic criteria. Determining “normal” IRD values using a caliper during abdominal muscle activity (following protocol for palpatory clinical test [67, 71, 77, 78]) should be also considered.

To facilitate the IRD measurement procedure reproducibility and cross-study comparisons, the following protocol standardization for the examination in supine is proposed based on result synthesis:

  • the headrest of the examination table elevated by 15°; a foam roller (15 cm in diameter) under the knees or the knees flexed to 90° with the feet on the examination table;

  • image acquisition at the end of normal expiration;

  • measurement sites: (1) half of the superior umbilical border-xiphoid distance, (2) a quarter of the superior umbilical border-xiphoid distance (most proximal to the umbilicus), (3) the superior umbilical border, and (4) a quarter of the superior umbilical border-pubis distance (most proximal to the umbilicus; Fig. 2);

  • use of mean IRD value of three images per measurement site as measurement outcome.