Anatomical and functional relationships between the external abdominal oblique muscle and the posterior layer of the thoracolumbar fascia

Abstract

The abdominal muscles are important for the stability of the lumbar region through the thoracolumbar fascia (TLF). However, there is not full agreement regarding the posterior continuity of the external abdominal oblique muscle (EO) with the TLF. To clarify this point, computed tomography (CT) images from 10 cadavers and 27 subjects were used to evaluate the continuity of the TLF with the abdominal muscles. The width of the fascial continuity of the EO with the posterior layer of TLF along the posterior border of the EO was also measured (40.70±3.92 mm). The epimysial fascia of the EO was in direct continuity with the posterior layer of the TLF in eight cadavers and 23 CT images, whereas in two cadavers and four CT images the epimysial fascia of the EO first fused with the fascia covering the latissimus dorsi, and then both fasciae were in continuity with the posterior layer of the TLF. Therefore, the fascial continuity of the EO could explain the transmission of tension from the EO to the posterior layer of the TLF and its importance in maintaining the stability of the lumbar spine through a hydraulic effect. Regarding fascial continuity in the trunk, and taking the EO into consideration, the TLF is formed by the fascia of all the abdominal muscles as the rectus sheath. In this manner, myofascial continuity between the TLF and the abdominal muscles is achieved through the aponeurosis and fascia, which ensures synchronization between the erector spinae and the rectus abdominis.
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Original Communication
Anatomical and Functional Relationships
Between External Abdominal Oblique Muscle
and Posterior Layer of Thoracolumbar Fascia
CHENGLEI FAN,
1
CATERINA FEDE,
1
NATHALY GAUDREAULT,
2
ANDREA PORZIONATO,
1
VERONICA MACCHI,
1
RAFFAELE DE CARO ,
1
AND CARLA STECCO
1
*
1
Institute of Human Anatomy, Department of Neurosciences, University of Padova, Via Gabelli 65,
35127, Padova, Italy
2
School of Rehabilitation, Faculty of Medicine and Health Sciences, University of Sherbrooke, 3001, 12e
Avenue Nord, Sherbrooke, J1H 5N4, Canada
The abdominal muscles are important for the stability of the lumbar region
through the thoracolumbar fascia (TLF). However, there is not full agreement
regarding the posterior transversal continuity of the external abdominal oblique
muscle (EO) with the TLF. To clarify this point, 10 cadavers and computed
tomography (CT) images from 27 subjects were used to evaluate the transver-
sal continuity of the TLF with the abdominal muscles. The width of the fascial
continuity of the EO with the posterior layer of TLF along the posterior border of
the EO was also measured (40.70 3.92 mm). The epimysial fascia of the EO
was in direct continuity with the posterior layer of TLF in eight cadavers and
23 CT images, whereas in two cadavers and four CT images, the epimysial fas-
cia of the EO first fused with the fascia covering the latissimus dorsi, and then,
both fasciae were in continuity with the posterior layer of TLF. Therefore, the
transversal fascial continuity of the EO could explain the transmission of tension
from the EO to the posterior layer of TLF and its importance in maintaining the
stability of the lumbar spine through a hydraulic effect. Regarding fascial conti-
nuity in the trunk, and taking the EO into consideration, the TLF is formed by the
fascia of all the abdominal muscles as the rectus sheath. In this manner, myo-
fascial continuity between the TLF and the abdominal muscles is achieved
through the aponeurosis and fascia, which ensures synchronization between the
erector spinae and the rectus abdominis. Clin. Anat. 9999:1–7, 2018. © 2018 Wiley
Periodicals, Inc.
Key words: external abdominal oblique muscle; fascia; lumbar; computed
tomography
INTRODUCTION
The abdominal muscles [external abdominal oblique
(EO), internal abdominal oblique (IO), transversus
abdominis (TrA) and rectus abdominis (RA)] are all
important in the stability of the lumbar region of the
vertebral column (Tesh et al., 1987). They create the
torque necessary to flex, rotate, and bend the spine
laterally (McGill, 1991; Arjmand et al., 2008), and
stiffen the abdominal cavity and lumbar spine (Prats-
Galino et al., 2015; Creze et al., 2018) during simple
*Correspondence to:Carla Stecco, Institute of Human Anatomy,
Department of Neurosciences, University of Padova, Via Gabelli
65, 35127 Padova, Italy. E-mail: carla.stecco@unipd.it
Received 27 June 2018; Revised 16 July 2018; Accepted 31
July 2018
Published online 00 Month in Wiley Online Library (wileyonlineli-
brary.com). DOI: 10.1002/ca.23248
© 2018 Wiley Periodicals, Inc.
Clinical Anatomy (2018)

tasks such as standing, sitting, and walking (Callaghan
et al., 1999; Masani et al., 2009) and dynamic loading
and heavy lifting (Cholewicki and McGill, 1996; El
Ouaaid et al., 2009). They also assist in the expiration
of air in challenged breathing (Campbell and Green,
1953). Nevertheless, relatively little is known about
the specific physiology and mechanics of these mus-
cles, which act individually and together as a compos-
ite, multifunctional structure in stabilizing the lumbar
region (Brown et al., 2010). In particular, there is not
full agreement regarding the anatomical connection of
the EO and the TLF.
The TLF is an essential structure in biomechanics
and there is much evidence that it works to connect the
latissimus dorsi (LD) and gluteus maximus (Gmax),
thus functionally linking the arm to the leg (Vleeming
et al., 1995; Mekonen et al., 2016; Wilke et al., 2016).
Actually, the connection between the TLF and the EO is
still a topic of debate. Indeed, Bogduk and Macintosh
(1984) and Gray’s Anatomy report that the superior
layer of the TLF is not in continuity with the EO or the
trapezius (Bogduk and Macintosh, 1984; Strandring,
2016), and Schuenke et al. (2012) stated that the
loose attachment of the EO to the underlying aponeu-
rosis does not support a major role of the EO in load
transfer to the lumbar spine, because there is no direct
contribution of the EO aponeurosis to the lateral border
of the TLF. This contrasts with the results of Barker
et al. (2004), who demonstrated the connection of the
EO to the lateral margin of theTLF at the level of L2. In
addition, electrophysiological (EMG) studies of animal
and human abdominal muscles have revealed conflict-
ing results under different experimental conditions,
especially activation of the EO. Massé-Alarie
et al. (2016) reported that IO/TrA activation measured
by EMG was modulated across the kinesiophobic
phases of trunk flexion/extension in chronic low back
pain and pain-free subjects, but not the EO. However,
Escamilla et al. (2010) found that roll-out and pike
were the most effective exercises for activating the RA,
EO, IO, and LD muscles, while minimizing the activities
of lumbar paraspinal and rectus femoris EMG
(Escamilla et al., 2010). Schuenke et al. and Vleeming
et al. also clearly described the anatomical connection
of the abdominal muscles (the TrA and IO) and TLF
through the “lateral raphe”(LR). The aponeurosis of
the TrA and IO bifurcates into anterior and posterior
laminae. The anterior lamina contributes to the middle
layer of the TLF. The posterior lamina contributes to the
deep lamina of the posterior layer of the TLF (three-
layer model). The junction of the TrA aponeurosis with
the paraspinal retinacular sheath (PRS) creates the
fat-filled lumbar interfascial triangle (LIFT), which is at
the core of the LR (Schuenke et al., 2012; Vleeming
et al., 2014). Willard et al. (2012) demonstrated that
the LIFT can function in the distribution of laterally
mediated tension to balance various viscoelastic pro-
prieties along the TFL. However, the results of anatomi-
cal and EMG studies of the connection of the EO and
the TLF are not in agreement. The transversal continu-
ity of the EO with the TLF and its contribution to spinal
stability has yet to be fully understood.
The purpose of this study was therefore to analyze
the posterior transversal continuity of the EO with the
TLF for better understanding of the transmission of
EO tension, and to elucidate how abdominal muscles
cooperate in influencing lumbar stability.
MATERIALS AND METHODS
Anatomical study
An anatomical study (approved by the local ethical
committee) was carried out on 10 non-embalmed
cadavers (five male and five female, mean age at
death 68.4 years) managed by the “Body Donation
Program”of the Institute of Anatomy, University of
Padova following the framework of the Anatomical
Quality Assurance Checklist (Tomaszewski et al.,
2017; Henry et al., 2018). All cadavers displayed nor-
mal skin appearance without evidence of thoracolum-
bar region pathologies or surgery. A longitudinal
cutaneous incision was made in the midline in the thor-
acolumbar region. The skin, subcutaneous tissue, and
superficial fascia were removed in order to reach the
surface of the TLF, which was then exposed to allow its
characteristics and its relationships to the abdominal
muscles to be examined. The connection with the EO
was studied and, where there was such a connection,
the longitudinal width was measured unilaterally with a
measuring tape (1 mm). The posterior layer of the
TLF was then sectioned 2 cm laterally to the midline
and all the erector spinae (ES) were removed to facili-
tate analysis of the anterior layer of TLF and its con-
nections with the IO and TrA. The posterior portions of
the EO, IO and TrA belly were manually tractioned to
determine the effect in the TLF layers and to ascertain
whether specific lines of tension in the TLF could be
identified.
Computed tomography (CT) study
Twenty-seven subjects (n = 27) with no low back
pain or musculoskeletal pain were selected (12 male
and 15 female, mean age 59.5 years) from the
archives of a radiology center. Computed tomography
(CT) (Philips Medical Systems; Best, The Netherlands)
was used to assess the anatomical relationships
between the TLF and the abdominal muscles at various
vertebral levels. ImageJ was used for all morphometric
analyses of images. The following measurements were
recorded bilaterally: length of the common aponeuro-
sis (L
CA
) between the abdominal muscle and the lateral
border of the ES and the lumbar interfascial triangle
area (LIFT) (Fig. 4A).
Statistical analysis
Results are expressed as mean and standard devia-
tion (SD) and ranges; Student’st-test was used to com-
pare the right and left sides. Graph Pad Prism
6 (GraphPad Software Inc., San Diego, CA) was used to
test for statistically significant differences (P < 0.05).
2 Fan et al.

RESULTS
Anatomical study
In the lower lumbar region (L4/L5), the EO was
covered by a thin epimysial fascia in continuity with
the posterior layer of the TLF laterally and directly in
eight subjects (Fig. 1A). In these eight specimens,
the aponeurosis of the LD had a more medial attach-
ment. In the other two subjects, it had a more lateral
attachment to the iliac crest. Consequently, in the lat-
ter cases the epimysial fascia of the EO and the LD
fused first, and then, both were in continuity with the
posterior layer of the TLF (Fig. 1B). In these two
arrangements, the fascia of the EO, the LD and the
aponeurosis of the LD lay over the LR posteriorly. In
the upper lumbar region (L1/L2/L3), the epimysial
fascia of the EO was still fused with the epimysial fas-
cia of the LD; then the muscular fibers of the EO
passed down the LD and were inserted in ribs V-XII.
However, continuity was conserved between the EO
fascia and the fascia of the inferior part of the serratus
posterior inferior muscle (SPI), both of which were in
continuity with the superior part of the posterior layer
of the TLF in all specimens (Fig. 2A). The fascial conti-
nuity width of the EO with the posterior layer of the
TLF along the posterior border of the EO was
40.70 3.92 mm (mean SD). There was no signifi-
cant difference in the fascial continuity width between
males and females (P = 0.83) (Table 1). The multilay-
ered organization of the posterior layer of the TLF was
also evident macroscopically (Fig. 3A).
In the present study, we used the terminology of
the two-layer model of the TLF. It is very similar to the
three-layer model: the anterior layer of the two-layer
model becomes the middle layer of the three-layer
model, and the fascia of the two-layer model becomes
the anterior layer of the three layer. The common
aponeurosis and fascia of the IO and the TrA, espe-
cially the aponeurosis and fascia of the IO, bifurcated
into anterior and posterior laminae in all specimens.
The anterior lamina formed the anterior layer of the
TLF, attached medially to the tips of the transverse
processes of the lumbar vertebrae and the intertrans-
verse ligaments, and the posterior lamina contributed
to the posterior layer of the TLF. The aponeurosis and
fascia of the IO and TrA could be separated by blunt
dissection as far as the lateral border of the LR
(Fig. 2A, B). The LR was clearly evident in all speci-
mens, from the iliac crest to the 12th rib. The poste-
rior roof of the LR was composed not only of the
aponeurosis of the LD, IO, but also of the fascia of the
EO and LD, especially in the lower lumbar region
(L4) (Fig. 2A). After all the ES had been removed, it
was possible to appreciate the different orientation of
the fibrous bundles forming the anterior layer of the
TLF (Fig. 3B). On the transverse plane, the common
aponeurosis and fascia of the IO and TrA bifurcated
into anterior and posterior laminae, whereas the LR
was a fat-filled LIFT, mainly composed of the junction
of the common aponeurosis and the fascia of the IO
and TrA with the paraspinal retinacular sheath
(Fig. 2C).
CT study
In the lower lumbar region (L4/L5), CT imaging
confirmed the direct transversal continuity of the epi-
mysial fascia of the EO with the posterior layer of TLF
at L4 in 23 subjects (Fig. 4A); in four subjects, the
epimysial fascia of the EO fused first with the fascia of
the LD. The muscular fibers of the LD began to appear
at level L3 and the epimysial fascia of the EO was still
fused with that of the LD. The common aponeurosis of
the IO and TrA (L
CA
) passed over the QL and then sub-
divided into the anterior and posterior laminae at the
lateral border of the ES in the lower region. However,
the muscular fibers of the TrA had a more posterior
attachment in the aponeurosis, and that of the IO
bifurcated into anterior and posterior laminae in
seven subjects; in the other 20 subjects, the
Fig. 1. A: Asterisk (*):direct transversal fascial conti-
nuity of EO and posterior layer of TLF;￥: direct fascial
continuity of EO and LD in superior region. B: Asterisk
(*): first fascial continuity of EO and LD, and then both in
continuity with posterior layer of TLF. Gmax: gluteus
maximus; Gmed: gluteus medius; EO: external oblique:
LD: latissimus dorsi: PTLF: posterior layer of TLF. [Color
figure can be viewed at wileyonlinelibrary.com]
Anatomical and Functional Relationships Between the External Abdominal Oblique Muscle 3

muscular fibers of the TrA had a more anterior attach-
ment to the aponeurosis and those of the IO and TrA
merged, so that the layers of the aponeurosis and fas-
ciae of the IO and TrA could not be distinguished in
those subjects. The anterior lamina contributed to the
anterior layer of the TLF, and the posterior lamina
contributed to its posterior layer (Figs. 4B,C,E,F). At
the level of L4, the L
CA
(mean SD) between the
abdominal muscles and the lateral border of the ES
was 50.00 24.61 mm on the left side and
45.03 18.45 mm on the right. The LIFT area
(mean SD) was 44.57 8.94 mm
2
on the left side
and 44.82 9.08 mm
2
on the right. There was no sig-
nificant difference between left or right sides (L
CA
P = 0.54, area P = 0.94) (Fig. 4A, Table 2). In the
upper lumbar region (L1/L2), the muscular fibers of
the OE passed down the LD, and the epimysial fascia
of the EO could not be separated from the fascia of
the IO, TrA, LD or SPI, since they merged in all speci-
mens (Fig. 4D).
DISCUSSION
This study demonstrates the transversal fascial
continuity of the EO with the posterior layer of the
TLF, both in dissected cadavers and in CT images of
the lower lumbar region (L4/L5). This connection is
only possible via the epimysial fascia of the EO; in any
case, no direct insertion of the muscular fibers of the
EO into the TLF was observed.
Our results show that, when the aponeurosis of the
LD is inserted into the medial border of the iliac crest
(80% of cases), the epimysial fascia of the EO is in
direct transversal continuity with the posterior layer
of the TLF. In other subjects, the aponeurosis of the
LD is inserted more laterally into the iliac crest, and
Fig. 2. A: After removal of LD, asterisk (*): fascial
continuity of EO with inferior part of serratus posterior
inferior muscle. B: After removal of LD and EO, note fas-
cia and aponeurosis continuity of IO with TLF (A); fused
fascia and aponeurosis of IO and TrA (B and C). C: Lum-
bar interfascial triangle (LIFT) at level of L4. Note fatty
composition of LIFT (‘*’). EO: external oblique; IO: inter-
nal oblique; LD: latissimus dorsi; SPI: serratus posterior
inferior muscle; Gmax: gluteus maximus; Gmed: gluteus
medius; LR: lateral raphe; ES: erector spinae; CA: com-
mon aponeurosis and fascia of abdominal muscle; PTLF:
posterior layer of TLF; ATLF: anterior layer of TLF. [Color
figure can be viewed at wileyonlinelibrary.com]
Fig. 3. A: Macroscopic aspect of posterior layer of
TLF. B: Macroscopic aspect of anterior layer of TLF. A:
anterior lamina and fascia of IO and/or TrA; B: aponeuro-
sis and fascia of TrA. Dotted lines: different orientation of
multilayered fibrous bundles; arrow: main direction of
muscular traction. EO: external oblique; IO: internal obli-
que; TrA: transversus abdominis; LD: latissimus dorsi;
Gmax: gluteus maximus. [Color figure can be viewed at
wileyonlinelibrary.com]
4 Fan et al.

consequently the epimysial fascia of the EO and LD
fused first, and then, both were in continuity with the
posterior layer of the TLF. In the upper lumbar region,
transversal fascia continuity still remains between the
EO and the lower part of the serratus posterior inferior
muscle first, then both are in continuity with the upper
part the posterior layer of the TLF in all specimens.
Indeed, there are three myofascial laminae in the
trunk (superficial, middle, and deep), The superficial
lamina envelops the LD and the EO of the trunk
through the fascial continuity that permits these mus-
cles work synergistically in spiral/rotational move-
ments. Therefore, our results confirm the work of
Schuenke et al. (2012) in that the EO shows no muscle
insertion into the TLF. However, we also demonstrate
Fig. 4. CT imagings,A: showing fascia continuity of EO and posterior layer of TLF
(red arrow ‘"’) at L4. Lumbar interfascial triangle (LIFT, red triangle) was located
between both layers of TLF. B: showing aponeurosis and fascia of IO which bifurcates
into anterior and posterior lamina (red arrow‘"’) at L3. Aponeurosis and fascia of TrA
(yellow arrow ‘"’). C: merged aponeurosis and fascia of IO and TrA, bifurcating into
anterior and posterior lamina (red arrow‘"’) at L3. D: merged fascia of EO, IO, LD and
SPI (red arrow‘"’) at L1/2. Scheme of myofascial continuity between TLF and abdomi-
nal muscle (E and F). E: aponeurosis of IO, bifurcating into anterior and posterior lam-
ina, F: aponeurosis of IO and TrA, merging and then bifurcating into anterior and
posterior lamina. EO: external oblique; IO: internal oblique: TrA: transversus abdomi-
nis; ES: erector spinae; QL: quadratus lumborum muscle; Psoas: psoas muscle; LD:
latissimus dorsi; CA: common aponeurosis length between abdominal muscles and
lateral border of ES; RA: rectus abdoninis; LR: lateral raphe. [Color figure can be
viewed at wileyonlinelibrary.com]
TABLE 1. Cadaver parameters and the fascial
continuity width (n = 10)
Attachment Gender Age Width(mm)
Directly M 70 45.00
M 62 39.00
M 79 39.00
M 64 38.00
F 83 47.00
F 59 40.00
F 73 42.00
F 63 41.00
Indirectly M 56 43.00
F 75 33.00
Anatomical and Functional Relationships Between the External Abdominal Oblique Muscle 5

that its fascia contributes to forming the posterior layer
of the TLF, indicating that the EO can also contribute to
the tension of the TLF.
Schuenke et al. (2012) reported that the aponeu-
rosis of the transversus abdominis (TrA) and internal
abdominal oblique (IO) is subdivided into anterior and
posterior laminae, which join the paraspinal retinacu-
lar sheath separately. However, on T1 MRI tracing,
the authors only stated that the aponeurosis of the
TrA subdivides into anterior and posterior laminae
(Schuenke et al., 2012). Our studies showed that the
aponeurosis and fascia of the IO (seven subjects) and
the merged aponeurosis and fascia of the IO and TrA
(20 subjects) bifurcate into anterior and posterior
laminae. The anterior lamina contributes to the ante-
rior layer of the TLF, whereas the posterior lamina
contributes to its posterior layer in CT images
(Fig. 4A, B, C, E, F). Anatomical study shows that the
common aponeurosis and fascia of the IO and TrA,
especially the aponeurosis and fascia of the IO, bifur-
cate into anterior and posterior laminae in all speci-
mens, whereas the aponeurosis and fascia of the IO
and TrA can be separated by blunt dissection as far as
the lateral border of the LR. Theobald et al. (2007)
and Schuenke et al. (2012) reported that the LR could
reduce friction between adjacent fascia under the
high tension generated by the abdominal myofascial
girdle. Our previous studies showed that the epimy-
sial fasciae of the EO, IO and TrA were separated by a
thin layer of loose connective tissue (Stecco et al.,
2011; Stecco et al., 2018). In addition, Brown and
McGill (2009) demonstrated that force generated by
abdominal muscles can be passed one to another
through connective tissue links. Therefore, the
abdominal muscles interact through connective tissue
during trunk movements. In this way, the EO medi-
ates tension in the posterior layer of the TLF, either
directly by the continuity of the epimysial fascia, or
indirectly by the interactions with the other abdominal
muscles through connective tissue. Our findings
extend these results of the relationship between the
abdominal region and the lumbar segment. Thanks to
the transversal fascial continuity of the EO with the
posterior layer of the TLF, the former is also important
in the mechanical coordination of the lumbar region.
Regarding fascial continuity in the trunk, also tak-
ing the EO into consideration, the similar organization
of the lumbar region on both sides of the body is clear.
Indeed, the TLF is formed by the fascia of all the
abdominal muscles as the rectus sheath. Above all,
when we examine the rectus sheath above the line of
Douglas, we see that the aponeurosis of the IO is
subdivided into two laminae: the upper one fuses with
the aponeurosis of the EO to form the anterior layer
of the rectus sheath, and the deep one fuses with the
aponeurosis of the TrA to form the posterior layer of
the rectus sheath (Strandring, 2016). In a similar
manner, in the lumbar region, the aponeurosis of the
IO subdivides into two laminae: the upper one fuses
with the epimysial fascia of the EO and the aponeuro-
sis of the LD to form the posterior layer of the TLF,
whereas the anterior lamina fuses with the aponeuro-
sis of the TrA to form its anterior layer. Thus, the
abdominal muscles functionally connect the RA with
the ES, permitting their activation to be synchronized.
The LR can be considered as corresponding to the Spi-
gelian line in the front part of the trunk. Lastly, since
muscle contraction inside a rigid compartment is
more efficient (the hydraulic effect described by Gra-
covetsky et al., 1981; Gracovetsky et al., 1985), con-
traction of the abdominal muscles can simultaneously
stretch both the rectus sheath and the TLF. This
mechanism can probably better explain the role of the
abdominal muscles in protecting the back.
As regards the factors that influence the degree of
continuity, our study revealed no significant difference
in fascial continuity width between males and
females. Further studies should reveal how age and
physical activity influence the degree of continuity
quantitatively and how the EO functionally cooperates
with other abdominal muscles (RA, IO, TrA) at a deep
level, especially during trunk movements.
ACKNOWLEDGMENTS
The authors express their gratitude to the donation
of cadavers to the Human Anatomy Section of the
Department of Neuroscience of the University of
Padova within the context of the Body Donation Pro-
gram. The authors declare that they have no conflict
of interests.
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6 Fan et al.

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