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research report
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CYNTHIA M. CHIARELLO, PT, PhD1 • J. ADRIENNE MCAULEY, PT, DPT, OCS, FAAOMPT2
Concurrent Validity of Calipers
and Ultrasound Imaging to
Measure Interrecti Distance
D
iastasis rectus abdominis (DRA), the abnormal separation of
the right and left rectus abdominis, appears as an increase in the
width of the linea alba, or interrecti distance (IRD). The linea
alba is a meshwork of connective tissue in which the tendinous
insertions of the external oblique, internal oblique, and transversus abdominis
muscles intermingle, providing structure
and stability to the abdominal wall and
TTSTUDY DESIGN: Clinical measurement, concurrent validity criterion standard.
TTOBJECTIVE: To determine the concurrent
validity of digital nylon calipers in comparison to
ultrasound imaging (USI) for the measurement of
interrecti distance (IRD).
TTBACKGROUND: Diastasis rectus abdominis
is the abnormal increase in the width of the linea
alba, measured as IRD. A diastasis rectus abdominis can compromise mechanical trunk function in
both genders. IRD has been accurately measured
with USI but requires costly equipment and
extensive examiner training. Digital nylon calipers
are inexpensive and easy to use, but their use to
measure IRD has not been validated.
TTMETHODS: A sample of convenience of 56
individuals (11 men, 45 women) was measured. A
single examiner was assigned to each tool (calipers or USI), and IRD was measured at 2 locations
(above and below the umbilicus) under 2 conditions (abdominal muscles at rest and abdominal
muscles contracted). All measurements were
made during a single session, and examiners were
blinded to measurements with the other tool.
trunk.1 A DRA can indicate connective
tissue alterations of the linea alba,35 an
increase in muscle length with consequent altered line of muscle pull,14 damTTRESULTS: Above the umbilicus, the mea-
surements of IRD with calipers were similar to
those made with USI, with intraclass correlation
coefficients (model 3,2) of 0.79 with abdominal
muscles at rest and 0.71 with abdominal muscles
contracted. The absolute mean difference between
the caliper and USI measurements of IRD above
the umbilicus was 0.03 cm larger with the calipers
when the abdominal muscles were at rest and 0.03
cm smaller when the abdominal muscles were
contracted. The values of IRD obtained with the
caliper and USI techniques were not comparable
when obtained below the umbilicus.
TTCONCLUSION: The calipers are a valid tool
for measuring IRD above the umbilicus in males
and females. Measuring IRD with calipers below
the umbilicus should not be considered valid,
using USI as the criterion standard. This may
reflect anatomical variation of the linea alba
or a limitation of the calipers to assess IRD at
the same depth as USI. J Orthop Sports Phys
Ther 2013;43(7):495-503. Epub 30 April 2013.
doi:10.2519/jospt.2013.4449
TTKEY WORDS: diastasis recti, linea alba, rectus
abdominis
age of the fixation of the rectus muscles
in their sheaths,1 or a disruption of the
entire musculofascial abdominal wall.8,13
The anisotropic, directionally dependent
behavior of the linea alba,15 in conjunction with craniocaudal differences in the
width and thickness of the linea alba, necessitates IRD measurements both above
and below the umbilicus to document
the DRA width.2 Compromising the mechanical function of the trunk, a DRA has
been associated with decreased muscle
strength and endurance,11,21 reduced capacity of abdominal musculature to stabilize the pelvis,2,14,32 related pelvic floor
dysfunction,33,34 posture abnormalities,6
and lumbopelvic pain.29,33,37,38,42
Development of a DRA may be due to
failure of the abdominal wall to withstand
extreme overloading28 and may stem from
different causes in males and females. In
males, factors that appear to be related
to DRA include increasing age,12,23,31 obesity,23 greater abdominal circumference,12
performance of full-excursion sit-ups and
weight training,23 hernia,35 abdominal
aortic aneurysm,24,26 and lipodystrophy in
HIV-infected men.4 In addition to these
same factors, in females abdominal wall
stress frequently begins due to the uterus
enlarging during pregnancy, with 27%
of women exhibiting a DRA during their
second trimester and 66% during the
Program in Physical Therapy, Department of Rehabilitation and Regenerative Medicine, College of Physicians and Surgeons, Columbia University, New York, NY. 2Department of
Physical Therapy, University of New England, Portland, ME. This study was approved by the Columbia University Medical Center Institutional Review Board. The authors certify
that they have no affiliations with or financial involvement in any organization or entity with a direct financial interest in the subject matter or materials discussed in the article.
Address correspondence to Dr Cynthia M. Chiarello, Columbia University, Program in Physical Therapy, 710 West 168th Street, 8th Floor, New York, NY 10032. E-mail: cmc3@
columbia.edu t Copyright ©2013 Journal of Orthopaedic & Sports Physical Therapy ®
1
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TABLE 1
Participant Characteristics*
Women
Age, y
Height, m
Men (n = 11)
Nulliparous (n = 22)
Parous (n = 23)
Total (n = 56)
37.5  9.8 (19.8-56.1)
28.5  6.0 (23.0-49.9)
39.6  9.8 (25.8-64.0)
34.8  9.8 (19.8-64.0)
1.80  0.09 (1.63-1.92)
1.63  0.06 (1.53-1.76)
1.62  0.06 (1.47-1.74)
1.67  0.10 (1.47-1.92)
Mass, kg
97.2  13.7 (74.5-119.3)
62.2  11.7 (45.8-94.4)
59.5  6.7 (49.0-72.7)
67.9  17.8 (45.8-119.3)
Body mass index, kg/m2
30.0  3.9 (23.7-33.9)
23.4  4.0 (17.2-34.1)
22.6  1.8 (20.1-27.8)
24.3  4.3 (17.2-34.1)
*Values are mean  SD (range).
third.6 Although IRD decreases during
the initial postpartum months, it does not
completely resolve and typically remains
significantly larger than IRD in nulliparous women.6,13,21 In postpartum women,
Liaw et al21 found that abdominal muscle
function and IRD improved at 6 months
but were not restored to nulliparous values. At 12 months postpartum, Coldron
et al13 reported that 48% of women had
a significantly larger IRD than nulliparous controls. Ranney30 found that 39%
of 1738 parous women undergoing a hysterectomy several years postpartum still
exhibited a DRA. Other reported risk
factors for DRA in women include multiparity, multiple gestation, older maternal age, large pregnancy weight gain, and
larger babies.10,22
Although there is agreement that a
DRA is abnormal, there is no consensus
in the literature on the optimal IRD for
all adults.3,27,31 Documentation of the size
and location of a DRA is important in examining patients with any type of musculoskeletal trunk dysfunction to determine
stability and load acceptance, the efficacy
of exercise intervention programs, as well
as the need for abdominoplasty.3,20 Typically, DRA is measured by placing palpating fingers between the medial borders of
the right and left rectus abdominis muscles, parallel to the linea alba. DRA size is
determined by indicating the number of
fingers that fit between the 2 muscle bellies at locations above, below, and at the
umbilicus while the individual performs
a partial curl-up. Clinically, this palpation
technique is easy to perform but has not
demonstrated adequate reliability9 and
validity.19 A modification of this palpation
technique using nylon dial or digital calipers to measure the IRD has been shown
to be reliable in pregnant and postpartum women.7,11,17
The validity of ultrasound imaging
(USI) to measure IRD above the umbilicus has been established. Mendes et al25
found no statistically significant difference between supraumbilical measurements taken with USI and those directly
measured during surgery for abdominoplasty. Good intrarater reliability of IRD
measurements made with USI in females
has also been demonstrated.21 Therefore,
USI is an accurate3,13,21 and valid25 method of measuring IRD size and location;
however, it requires expensive equipment and extensive examiner training
and expertise.41
Given the importance of measuring
IRD during an examination of abdominal function, an inexpensive, accessible,
and easy-to-use tool is necessary. Digital nylon calipers meet the criteria of
reliability and ease of use; however, to
date, we have found no studies validating digital nylon calipers for measuring
IRD. The primary purpose of this investigation was to determine the concurrent
validity of digital nylon calipers in comparison to USI for the measurement of
IRD. For calipers to be a clinically useful
tool, they must be valid across gender,
parity, and size of the IRD. A secondary purpose of the study was to describe
the IRD seen in typical adult males and
females.
METHODS
Participants
A
sample of convenience, consisting of an ethnically diverse group
of 56 English-speaking individuals
(11 men, 22 nulliparous women, 23 parous women) between 19 and 64 years of
age (TABLE 1), was recruited from physical therapy private practices in New York
City and the student populations at Columbia University and Touro College.
The ethnicity of the sample was 66%
Caucasian, 12% Asian, 9% Hispanic, 9%
mixed, 2% African American, and 2%
Middle Eastern. Participants were excluded if they were pregnant; had scarring from previous abdominal surgery
such that, on observation of the abdomen, the linea alba or umbilicus was obscured or deformed; had rheumatological
or connective tissue disease; or had any
medical condition that would prohibit
active abdominal muscle contraction.
The study protocol was approved by the
Columbia University Medical Center
Institutional Review Board, and participants signed an informed-consent form
following a detailed explanation of all
study procedures.
Instrumentation and Examiners
Each tool was used by 1 examiner for all
data collection, and each examiner was
blinded to the measurements by the other
examiner with the other tool. The measurements of IRD using the nylon digital
calipers (Mitutoyo America Corporation,
Aurora, IL) were made by 1 examiner,
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TABLE 2
Intrarater, Within-Session Reliability Data for Interrecti Distance
Measurements With Ultrasound Imaging and Calipers
Ultrasound Imaging (n = 6)
Measurement Condition and Location
Calipers (n = 8)
ICC3,1*
SEM, cm
MDD, cm
ICC3,1*
SEM, cm
MDD, cm
PAU
0.97 (0.95, 0.99)
0.005
0.015
0.99 (0.98, 0.99)
0.001
0.003
AAU
0.98 (0.92, 0.99)
0.012
0.032
0.98 (0.95, 0.99)
0.006
0.016
PBU
0.90 (0.89, 0.99)
0.017
0.048
0.97 (0.91, 0.99)
0.005
0.014
ABU
0.94 (0.93, 0.99)
0.009
0.024
0.94 (0.81, 0.99)
0.041
0.113
Abbreviations: AAU, activated abdominal muscles above umbilicus; ABU, activated abdominal muscles below umbilicus; ICC, intraclass correlation coefficient; MDD, minimal detectable difference; PAU, passive abdominal muscles above umbilicus; PBU, passive abdominal muscles below umbilicus; SEM,
standard error of measurement.
*Values in parentheses are 95% confidence interval.
who had 10 years of experience with this
tool. The USI measurements of IRD were
performed using the LOGIQ Book XP
ultrasound unit (GE Healthcare, Waukesha, WI), with a 5-MHz curvilinear
transducer, by 1 examiner with advanced
training and 7 years of clinical USI use.
High intrarater reliability for measuring
IRD for both USI and the calipers7,11 has
been reported.21
To establish our own reliability, a
separate study was performed, in which
8 subjects were measured with calipers
and 6 subjects with USI. Three measurements for each location (above and below
the umbilicus) and each condition (abdominal muscles at rest and contracted)
were performed on the same day. Intrarater reliability for USI was assessed by
measuring 3 separate images for each location and condition. In this preliminary
work, intrarater reliability was found
to be very high for each tool-examiner
combination, with intraclass correlation
coefficients (ICCs) ranging from 0.90 to
0.99. The minimal detectable difference
(MDD) for each tool-examiner combination ranged from 0.003 to 0.113 cm
(TABLE 2).
Procedures
IRD measurements using both tools were
made during a single session. To ensure
that both tools measured IRD at precisely the same location, the participant
was positioned comfortably in supine and
the desired measurement locations were
marked with a water-soluble pen 4.5 cm
above the umbilical midpoint and 4.5 cm
below the umbilical midpoint. These specific locations along the linea alba have
previously been used to measure IRD
with 3-D photography.14 We also selected
4.5 cm above and below the umbilicus
because measurements at these locations
have previously been reported.6,11,12,29 No
IRD measurements were made at the
umbilicus due to technical difficulties
with USI, which is consistent with other
USI studies that have not reported measurements at the level of the umbilicus.3,13
Because the ultrasound gel can blur the
water-soluble markings, making future
measurements in the same location difficult, the caliper measurements always
preceded the USI measurements.
Previous investigations of DRA using
USI have measured IRD with the abdominal muscles at rest,3,14 whereas previous studies using palpation7 and caliper
techniques12 have reported IRD measurements in a hook-lying test position, with
the subject actively performing a partial
curl-up. In this study, we elected to perform the measurements with the abdominals both at rest and activated. For the
caliper measurements with the abdominal muscles at rest, the participant was
positioned in hook-lying, arms down by
the side, with 1 pillow placed beneath the
head. The examiner palpated the medial
borders of the right and left rectus abdominis muscle bellies at the marked locations. The inside measurement jaws of
FIGURE 1. To measure interrecti distance 4.5 cm
above and below the umbilicus (indicated with skin
markings), the inside jaws of the calipers are placed
between the medial edges of the right and left rectus
abdominis muscles as palpated by the examiner.
the digital nylon calipers were positioned
at the locations of the palpating fingers,
perpendicular to the direction of the
muscles, and adjusted to the perceived
IRD width (FIGURE 1). The palpation and
caliper measurement procedure was the
same for both measurement locations,
above and below the umbilicus. To measure the IRD with abdominal muscles
contracted, each participant crossed the
arms over the chest and raised the head
until the spine of the scapulae was off the
table surface. The participant maintained
this partial curl-up while the examiner
palpated and measured the IRD with the
calipers, as described just above. Once
the caliper measurements were completed, the participant remained in the same
hook-lying position and the first examiner left the room.
Upon entering the room, the second
examiner placed the 5-MHz curvilinear
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TABLE 3
]
Descriptive Data for Interrecti Distance*
Men (n = 11)
Women: Nulliparous
(n = 22)
Women: Parous
(n = 23)
AAU
1.45  0.95
0.88  0.45
1.69  0.92
PAU
1.62  1.04
0.75  0.43
2.03  1.05
AAU
1.54  0.76
0.84  0.30
1.60  0.96
PAU
1.63  0.69
0.81  0.53
2.04  1.61
4.5 cm above umbilical midpoint
Ultrasound imaging
FIGURE 2. Placement of the ultrasound imaging
transducer, perpendicular to the linea alba, to
measure interrecti distance 4.5 cm above and below
the umbilicus (indicated with skin markings).
Calipers
4.5 cm below umbilical midpoint
Ultrasound imaging
3.5
*
3.0
*
IRD, cm
2.5
ABU
0.48  0.58
0.34  0.36
0.72  0.45
PBU
0.74  0.89
0.22  0.29
1.05  0.65
Calipers
2.0
ABU
1.39  0.37
1.18  0.33
1.63  1.04
1.5
PBU
1.68  0.54
1.69  0.68
2.36  1.43
1.0
0.5
0.0
PAU
AAU
USI
PBU
ABU
Calipers
FIGURE 3. Mean  SD IRD measured with USI
and digital nylon calipers 4.5 cm above and below
the umbilicus, with abdominal muscles resting
and contracted. *Statistically different between
measurement tools (P<.0001). Abbreviations: AAU,
activated abdominal muscles above umbilicus; ABU,
activated abdominal muscles below umbilicus; IRD,
interrecti distance; PAU, passive abdominal muscles
above umbilicus; PBU, passive abdominal muscles
below umbilicus; USI, ultrasound imaging.
transducer perpendicular to the abdominal surface at each marked location along
the linea alba (FIGURE 2). The onscreen
image of the IRD was captured using a
conventional grayscale brightness mode.
The measuring feature of USI was used
to measure the IRD by first capturing the
image, followed by the examiner determining the location of the medial borders
of the rectus abdominis muscles and using the onscreen cursor to mark the distance between the right and left muscle
bellies. Participant position and instruction for the 2 conditions (with abdominal
muscles at rest and with abdominal muscles contracted) were identical to those
described for the caliper procedures.
Abbreviations: AAU, activated abdominal muscles above umbilicus; ABU, activated abdominal
muscles contracted below umbilicus; PAU, passive abdominal muscles above umbilicus; PBU, passive
abdominal muscles below umbilicus.
*Values are mean  SD cm.
Statistical Analysis
Statistical analyses were performed in
consultation with the Columbia University Center for Social Sciences and the Department of Biostatistics, using JMP 9.0
software (SAS Institute Inc, Cary, NC)
and SPSS for Windows Version 18 (SPSS
Inc, Chicago, IL). Descriptive statistics
for participant characteristics and IRD
at both locations and under both conditions were calculated. To determine if
there was a significant difference in IRDs
measured by the 2 tools, separate paired
t tests were computed for each location
and condition, with the statistical significance set at P<.05. To graphically examine the level of agreement between USI
and caliper measurements of IRD, systematic bias, and random error, BlandAltman5 plots were constructed. We
calculated the mean difference between
tools and the 95% limits of agreement as
2 standard deviations of the mean differences.5 The concurrent validity of the
digital nylon calipers as compared to USI
was computed using ICC3,2 and standard
errors of measurement, calculated as SD
× 1 – ICC. MDD was calculated using
a 95% confidence level (standard error
of measurement × 1.96 × 2).40 ICC levels of agreement of IRD measurements
were interpreted as follows: low, 0.0 to
0.5; moderate, 0.5 to 0.7; high, 0.7 to 0.9;
and very high, greater than 0.9.16 For descriptive purposes and to ensure that the
validity of measurements using calipers
was generalizable to a greater population, the participants were divided post
hoc into male, nulliparous female, and
parous female subgroups.
RESULTS
Interrecti Distance
T
he mean IRDs for all subjects,
as measured by both USI and calipers for each measurement condition and location, are depicted in FIGURE 3.
Parous women exhibited the largest IRD
above and below the umbilicus, both with
and without an active abdominal muscle
contraction. Nulliparous women exhibited the smallest IRD for all conditions
and locations, except for the measurement with calipers below the umbilicus
with the abdominal muscles at rest. Men
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A
B
4
AAU Difference, cm
PAU Difference, cm
3
2
1
Mean = 0.03 cm
0
–1
3
2
1
Mean = –0.03 cm
0
–1
–2
–3
–2
0
1
2
3
4
5
0
1
2
Mean PAU
3
4
Mean AAU
C
D
4
4
2
ABU Difference, cm
PBU Difference, cm
3
Mean = 1.43 cm
1
0
3
2
Mean = 0.74 cm
1
0
–1
–1
–2
0
1
2
3
4
0.0
0.5
Mean PBU
1.0
1.5
2.0
2.5
3.0
Mean ABU
FIGURE 4. Bland-Altman plots of interrecti distance (cm), with difference in values between ultrasound imaging
and caliper techniques on the y-axis and the average between ultrasound imaging and caliper values on the x-axis.
Lines are mean differences and 95% limits of agreement. (A) PAU, (B) AAU, (C) PBU, (D) ABU. Abbreviations: AAU,
activated abdominal muscles above umbilicus; ABU, activated abdominal muscles below umbilicus; PAU, passive
abdominal muscles above umbilicus; PBU, passive abdominal muscles below umbilicus.
TABLE 4
Level of Agreement Between Interrecti
Distances Measured With Calipers
and Ultrasound Imaging*
Measurement Condition/Location
ICC3,2
SEM, cm
MDD95, cm
PAU
0.79† (0.64, 0.88)
0.01
0.04
AAU
0.71† (0.51, 0.83)
0.17
0.48
PBU
0.40 (–0.02, 0.65)
0.52
1.45
ABU
0.43 (0.03, 0.67)
0.50
1.38
Abbreviations: AAU, activated abdominal muscles above umbilicus; ABU, activated abdominal
muscles below umbilicus; ICC, intraclass correlation coefficient; MDD95, minimal detectable difference
at the 95% confidence level; PAU, passive abdominal muscles above umbilicus; PBU, passive abdominal muscles below umbilicus; SEM, standard error of measurement.
*Values in parentheses are 95% confidence interval.
†
P<.0001.
exhibited the smallest IRD for the measurement with calipers below the umbilicus with the abdominal muscles at rest
(TABLE 3).
Validity
The validity of the measurements made
with the digital nylon calipers, as compared to those made with USI, was dif-
ferent above and below the umbilicus.
Above the umbilicus, there was no statistically significant difference (P>.05)
in IRD values between the 2 tools under
either the contracted or at-rest condition
(FIGURE 3). As illustrated in the BlandAltman plots, for measurements made
above the umbilicus, the majority of the
data points fell within the limits of agreement (FIGURES 4A and 4B). On average,
compared to the calipers, the IRD measured with USI was 0.03 cm larger with
the abdominal muscles at rest and 0.03
cm smaller with the abdominal muscles
contracted (FIGURE 4). Above the umbilicus, there was a high level of agreement
between IRD measurements made with
calipers and USI with the abdominal
muscles at rest (ICC3,2 = 0.79, P<.0001)
and contracted (ICC3,2 = 0.71, P<.0001)
(TABLE 4).
Below the umbilicus, IRD measurements using calipers were significantly
larger (P<.0001) than those using USI
for both abdominal muscles at rest and
contracted (FIGURE 3). As indicated in the
Bland-Altman plots, below the umbilicus, the mean difference in IRD between
the tools was 1.43 cm with abdominal
muscles at rest (FIGURE 4C) and 0.74 cm
with abdominal muscles contracted (FIGURE 4D). Systematic bias was seen for measurements made below the umbilicus, as
most points were located above the zero
line, indicating that the calipers consistently overestimated IRD when compared to USI (FIGURES 4C and 4D). There
was low agreement between the 2 measurement tools for IRD measured below
the umbilicus under both the at-rest and
contracted conditions of the abdominal
muscles (TABLE 4).
DISCUSSION
T
o our knowledge, this is the
first study to examine the concurrent validity of measuring IRD with
digital nylon calipers compared to the
best available clinical tool, USI. Caliper
measurements above the umbilicus were
comparable to the USI measurements,
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FIGURE 5. Image and diagram showing the potential source of discrepancy between measurements of interrecti
distance made with USI and the digital nylon calipers. As thickness of subcutaneous tissue increases, palpation
of the inner edge of the RA is potentially made more difficult, leading to overestimation of the distance when using
calipers. Abbreviations: RA, rectus abdominis; USI, ultrasound imaging.
whereas below the umbilicus there was
overestimation of the IRD size when
measured with the calipers.
We found good to excellent validity of measurements with the calipers
when performed above the umbilicus,
both with abdominal muscles at rest and
contracted. The mean difference in IRD
between methods was 0.03 cm, which is
less than the MDD95 and not likely clinically significant. Although almost all data
points were within the limits of agreement, a slightly higher measurement
error was seen when IRD was measured
with the abdominal muscles contracted
compared to at rest (TABLE 4). This slightly
greater difference between measurement
techniques may be related to the variability of the intensity of muscle contraction
during the measurements, as the 2 measures were done on separate trials. The
high level of intrarater reliability demon-
strated in this study for measurements
above the umbilicus concurs with previously reported results for both the calipers7,11 and USI.21 Therefore, given these
intrarater reliability and validity results,
we are confident that calipers can be used
as an accurate assessment of IRD for locations above the umbilicus.
For IRD measurements below the umbilicus, the values obtained with the calipers were not similar to those obtained
with USI (FIGURE 3, TABLE 4). Greater measurement error was also demonstrated
with the abdominal muscles at rest, as
a third of the points on the corresponding Bland-Altman plot were outside the
limits of agreement (FIGURE 4C). Using the
calipers, there may be a greater tendency
to overestimate IRD for widths greater
than 2.5 cm (FIGURE 4C). However, as only
a few subjects exhibited such a large
IRD, this requires further investigation.
]
We initially thought that using calipers
would lead to smaller IRD values than
those obtained with USI, due to overlying skin and subcutaneous tissue. But the
measurements made with the calipers
were, on average, 1.43 cm greater than
those made with USI with the abdominal muscles at rest (FIGURE 4C) and 0.74
cm greater with the abdominal muscles
contracted (FIGURE 4D). These larger IRD
measurements when using calipers may
be due to the inaccurate identification
of the medial edges of the right and left
rectus abdominis muscles using manual
palpation. It is possible that the USI and
caliper measurements were not taken at
equivalent depths. The measurement of
IRD with USI is of the closest distance
between the muscle bellies, regardless
of depth from the skin. Below the umbilicus, there may be more subcutaneous
tissue interfering with the examiner’s
ability to detect the medial borders of the
muscle by palpation. Consequently, the
palpating fingers may be at a more superficial location and more lateral than the
most medial aspect of the muscles (FIGURE 5). It is also possible that the caliper
jaws produce a slight outward pressure
against the muscle belly, thus increasing IRD—particularly if the tissue is
more extensible, as is likely in those with
greater IRD. This slight outward pressure may also explain why the caliper
measurements were larger than the USI
measurements when taken with the abdominal muscles at rest. Our results are
in agreement with those by Mendes et
al,25 who compared USI to direct surgical
measurement and found excellent validity for measurements above the umbilicus
and poor validity for those below the umbilicus. In women who had undergone a
cesarean delivery, Mendes et al25 found a
significantly larger IRD below the umbilicus at surgery than IRD measured with
USI, which they attributed, in part, to a
loss of definition of the posterior layer of
the recti muscles and fibrosis, leading to
difficulty in determining the medial rectus borders with USI.
Gender differences in the architec-
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ture2 and width1,12,31 of the linea alba in
cadavers have been reported. The effect
of gender on IRD has not yet been explored in living individuals. An increase
in IRD postpartum compared to nulliparous women has been shown.6,12 To our
knowledge, this is the first investigation
to describe IRD with respect to both
gender and parity. For the calipers to be
clinically useful to detect a DRA, gender
variation in the width of the linea alba
must be considered. To facilitate comparison of our data to the few published
reports on the typical IRD in adults, we
chose to describe our sample by gender
and parity. Modeling clinical practice,
we measured IRD with the abdominal
muscles under 2 conditions, at rest and
contracted.
To our knowledge, the only other investigation of IRD size that has included
both males and females was conducted by
Rath et al,31 with 60% of the 40 participants being males. However, Rath et al31
did not specifically report IRD values by
gender or parity but, rather, reported a
composite mean IRD of 8.3 mm midway
between the umbilicus and xiphoid and
9.3 mm midway between the umbilicus
and pubic symphysis, taking all measurements with the abdominal muscles
at rest. The IRD values for men in our
study appear similar to those reported
by Rath et al31 for the location below the
umbilicus, but not for the location above
the umbilicus (TABLE 3). For the parous
women in our study, in which 3 of the 23
participants were less than 1 year postpartum, IRD was more than double the
values reported by Rath et al31 above the
umbilicus and only slightly larger below
the umbilicus (TABLE 3). This comparison should be made with respect to the
present study’s sample of adults without
disease, measured using USI, whereas
the Rath et al31 study used computerized
scans to measure individuals with intraabdominal disease.
The IRD in women within their first
postpartum year has been shown to exceed that which is considered normal in
nulliparous women.13,21 The nulliparous
women in this study exhibited IRD values
comparable to those previously reported
(TABLE 3).3,21 To identify the normal width
of the linea alba in nulliparous females,
Beer et al3 measured 150 women between
20 and 45 years of age. In the study by
Beer et al,3 normal IRD was defined as
any value between the 10th and 90th
percentile, or greater than 22 mm at 3
cm above the cranial margin of the umbilicus and any value up to 16 mm at 2
cm below the caudal margin of the umbilicus. IRD measurements reported by
Liaw et al21 at 2.5 cm above and below the
umbilical ring in 20 nulliparous females
also fell within the normal IRD range
defined by Beer et al.3 The individuals in
our study, and those in the study by Liaw
et al,21 were within the same age range
and were measured at a similar location
along the linea alba.21 Coldron et al13 reported a larger IRD measurement with
abdominal muscles at rest for 69 nulli­
parous females, using measurements immediately cranial to the umbilicus, where
IRD is expected to be larger. Similar findings in our sample of nulliparous women
give credence to the validity data for this
group and provide further evidence that
parity and location along the linea alba
are important factors to consider when
reporting IRD values.
Boissonnault and Blaschak’s6 seminal
research described the active curl-up assessment technique for a DRA, which is
now used in common clinical practice.
However, most subsequent investigations
using objective measurement tools have
just reported the IRD with the abdominal muscles at rest. 3,13,21,30,31 IRD measured with the abdominal muscles at rest
with calipers has been shown to be more
than twice the width of that measured
during an active muscle contraction in
women who were 11 weeks postpartum.7
In our study, to mimic clinical examination, participants were not instructed on
a specific recruitment pattern or effort
during the curl-up. While it was necessary to measure IRD during an active
muscle contraction to validate the calipers as a clinical tool, this study was not
designed to examine the variation of IRD
at rest and during abdominal muscle
contraction.
Based on the results of this investigation and on our interpretation of the
literature, we recommend that clinicians
examine patients for the presence of a
DRA when presenting with abdominal or lumbopelvic dysfunction. Keeler
et al18 reported that 96.6% of women’s
health clinicians used the finger-width
technique for measuring a DRA. Finger
palpation is not accurate and reliable.9
We recommend using USI when equipment and training are available. In the
absence of USI, the easily obtainable and
inexpensive calipers are a good alternative. Both of these tools are reliable for
measuring IRD along the linea alba and
lead to similar values for measurements
made above the umbilicus. For measurements below the umbilicus, while recognizing that measurements made with the
calipers will be greater than those made
with USI, clinicians should not preclude
the use of either tool at this time.
To date, there is no universally agreedupon definition for DRA. It appears that
measurement location, level of activation of the abdominal musculature, gender, parity, and measurement tool must
be considered for the interpretation of
IRD values. Future research should be
designed to define DRA with respect to
these variables.
Limitations
Measurement error may account for
some of the inconsistency found below
the umbilicus between the 2 tools. Measurement error might have occurred with
either examiner. Though all efforts were
made to minimize examiner error, our
results represent not only the measurement tool but also the raters, and results
may differ among raters. The results
represent a first step in determining the
validity of caliper measurement of IRD.
Future studies should examine this technique with additional raters with varying
amounts of clinical experience.
Related to error among raters is
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[
sample size, and we recognize that a
sample size of 20 or more is necessary
to determine intrarater reliability for a
novel tool.39 As the reliability of each tool
has been previously reported in the literature,7,11,16,21 in our preliminary work we
used a small but statistically appropriate
sample size to determine our intrarater
reliability.
To accurately perform IRD measurement with calipers, the examiner must
palpate the medial borders of both rectus abdominis muscles and place the
calipers at the precise IRD locations palpated. The examiner perceives the edges
of the muscle belly through subcutaneous tissue. If the individual has excessive
subcutaneous tissue, adiposity, or a thick
and rounded muscular configuration,
it may be difficult to identify the actual
medial extent of the muscle. When IRD
is measured with USI, the examiner
delineates a line between the most medial aspects of the muscle bellies, and
the computer determines this exact distance. The subumbilical morphological
variability previously described,1,2 along
with these palpation issues, can explain
how we achieved accurate measurements
with each tool but how the measurements
between the calipers and USI were only
comparable above the umbilicus.
In the design of this study, the USI
examiner measured IRD without any
palpation of the linea alba to most accurately represent USI. This does not
represent clinical practice, and future
studies may include palpation prior to
transducer placement. When evaluating
IRD with USI, examiner judgment, training, and consistent USI usage are vital to
accurate assessment. The fascial sheaths
of the rectus appear on the USI monitor
as white, hyperechoic outlines, which
demarcate the conical borders of the
muscles against the darker, hypoechoic
muscle bellies. The quality of this visual
image can be affected by age and musculoskeletal dysfunctions such as atrophy,
which produce a fatty fibrous muscle
with diminished water content. Thus,
the muscle appears whiter throughout or
research report
presents with greater echogenicity. 36 The
participants in our study represented
a range of ages and muscular development, some of which produced very poor
images. In the participants with hard-todistinguish muscle edges, measurement
error could have been introduced.
Accurately placing the ultrasound
transducer at measurement sites identical to those of the calipers was vital
to ensure that IRD measurements were
made by both tools in precisely the same
locations. As the ultrasound gel removed
skin markings, we chose to perform the
caliper measurements first. Although
randomized testing conditions are preferable, these would have necessitated
re-marking the measurement location
with the use of each tool, introducing an
additional source of measurement error.
]
to the calipers, especially for measurements made below the umbilicus.
CAUTION: Interpretation of these results
should consider that only 2 examiners,
with extensive experience with these
measurements, collected data for this
study.
ACKNOWLEDGEMENTS: The authors thank Dr
David Madigan for his assistance in statistical analyses, and Drs Susan Ohlsen and
Loretta Verma for assistance with data collection. We also acknowledge KIMA Center for
Physiotherapy and Wellness for the use of
space and equipment, as well as the support
of Touro College in New York, NY, where Dr
McAuley served as faculty during much of the
time devoted to this project.
CONCLUSION
T
his concurrent validity study
comparing IRD measurements
made with digital nylon calipers to
those made with USI offers preliminary
evidence to support the validity of using
calipers in both males and females when
measuring IRD above the umbilicus with
the abdominal muscles both at rest and
during contraction. Measurements with
calipers, although reliable, were not comparable to those made with USI at a location 4.5 cm below the umbilicus, which
may reflect anatomical variation of the
linea alba or a limitation of the calipers
to assess IRD at the same depth as USI. t
KEY POINTS
FINDINGS: Digital nylon calipers are a reli-
able and valid tool for measuring IRD
above the umbilicus. This is in contrast
to measurements made below the umbilicus.
IMPLICATIONS: The calipers are a readily
available tool providing clinicians with
an easy method to objectively and precisely document IRD when measured
above the umbilicus. For research purposes, or when precise measurements of
IRD are necessary, USI is still preferable
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