Internal Obliques - By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (Image: Bartleby.com: Gray's Anatomy, Plate 395, Public Domain, https://en.wikipedia.org/wiki/Abdominal_internal_oblique_muscle#/media/File:Gray395.png)

Anatomy

What’s in a name?

  • Internal (adj.) - early 15c., from Medieval Latin internalis or from Latin internus "within, inward," expanded from pre-Latin *interos, *interus "on the inside, inward." (Etymology Online)
  • Oblique (adj.) - early 15c., from Middle French oblique (14c.) and directly from Latin obliquus "slanting, sidelong, indirect," from ob "against" (see ob-) + root of licinus "bent upward" (see limb (n.1)). As a type of muscles, in reference to the axis of the body, 1610s (adj.), 1800 (n.). (Etymology Online)
    • "the internal muscle that runs slanting"

Attachments and Innervation

  • Origin: The anterior fibers arise from the lateral 2/3rds of the inguinal ligament, anterior superior iliac spine, and anterior 1/3rd of the intermediate line of the iliac crest (1-5). The lateral fibers arise from the middle 1/3rd of the intermediate line of the iliac crest and the middle layer of the thoracolumbar fascia (1-6).
  • Insertion: The anterior fibers invest in a conjoint tendon (along with the transverse abdominis) into the crest of the pubis, medial part of the pectineal line, and into the linea alba via the abdominal aponeurosis (1-6). The more lateral fibers invest into the cartilages at the borders of the 10th through 12th ribs, with potential investment into the 9th rib, and again, the linea alba via the abdominal aponeurosis (1-6).
  • Nerve: Segmentally innervated by branches of the intercostal nerves from T7 - T12, with contribution from the iliohypogastric (L1) nerve and the ilioinguinal (L1) nerves (1-6).

Relative Location

  • Depth: The internal obliques are located deep to the external obliques, but superficial to the transverse abdominis, forming the second/middle layer of the lateral abdominal wall (3, 5). The muscle fibers are oriented from inferior/lateral to superior/medial, the direction your fingers would face if your hands were placed in your back pockets, and are nearly perpendicular to the external obliques (1-3, 5-8),
  • Lateral (and superior): Laterally, the internal obliques attach to the anterior 2/3rds of the intermediate line of the iliac crest. The posterior fibers of the internal obliques pass upward and laterally, attaching to the inferior borders, tips, and the cartilages of the 10th-12th ribs (and potentially the 9th rib). The attachments between the floating ribs may merge or exhibit fascial continuity with the internal intercostal muscles (5-6).
  • Posterior: In between the ribs and iliac crest, the internal obliques continue to course posterior toward the lumbar spine, investing in the lateral raphe of the thoracolumbar fascia. The lateral raphe is the lateral portion of the thoracolumbar fascia where the posterior and middle layers merge into one thick sheath.
  • Medial: Above the arcuate line (the level of the umbilicus), the internal obliques course to the semilunar lines, where the fascia continues to the linea alba by splitting anterior and posterior to the rectus abdominis, reinforcing both the anterior and posterior rectus sheaths (5-9, 19). Below the arcuate line the fascia of the internal obliques, external obliques and transverse abdominis converge at the semilunar lines and continue to course only anterior to the rectus abdominis (5-9, 19). The linea alba itself is comprised of a crisscross of fibers from the fascia of the external obliques, internal obliques, transverse abdominis and rectus abdominis resulting in a tendon-like structure that adds strength to the abdominal wall (3). Although this implies that the internal obliques on one side may invest into the internal obliques on the other side, the way the linea alba fibers are organized suggests it would be an exaggeration to say that the fibers from the internal oblique fascia on one side continue specifically into the fascia of the internal obliques on the other side.
  • Caudal: The cuadal fibers of the internal oblique originate from the inguinal ligament and course transversely, reinforcing the lateral one-third of the anterior wall of the inguinal canal (6, 8). The fascia of the internal obliques continues to fuse with the aponeurosis of the transverse abdominis to form the conjoint tendon, which reinforces the roof and and medial posterior wall of the inguinal canal before continuing on to invest in the crest of the pubis and medial part of the pectineal line (5-8).
  • Deep: Deep to the internal obliques lies the horizontal fibers of the transverse abdominis and the deepest layer of abdominal fascia (6, 9). Deep to thef abdominal fascia is the perotineum and viscera.

Palpation

  • The internal obliques cannot be reliably palpated, as it is difficult to differentiate the fibers of the internal obliques from those of the overlying external obliques and perhaps the underlying transverse abdominis (2).

Fiber arrangement of external obliques, internal obliques, and transverse abdominis - (Image: http://web.uni-plovdiv.bg/stu1104541018/docs/res/skandalakis%27%20surgical%20anatomy%20-%202004/Chapter%2023_%20Kidneys%20and%20Ureters.htm)

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Isolated Function

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Integrated Function

  • Stabilization:
    • The internal obliques are involved in stabilization of the thorax, lumbar spine, sacroiliac joint and pubic symphysis (1-2, 9-12, 30-31). Although the internal obliques perform similar actions as the external obliques, they "behave" more like the transverse abdominis. Studies have demonstrated these muscles are active during rotation (21), have the potential to contribute to side bending (22), and may contribute to flexion via transmission of force through the semilunar lines (23). However, the internal obliques exhibit increased activity with increases in respiration and gait speed (similar to the transverse abdominis) (61), are recruited bilaterally when lifting asymmetric loads (unlike the external obliques) (24), and activity increases when the spine is compressed, perhaps contributing to an “unloading/decompression” force that accompanies an increase in intra-abdominal pressure (31, 32).
  • Eccentric Deceleration:
    • Eccentric deceleration of contralateral rotation of the spine (33).
      • This has been referred to as "anti-rotation", and may be the most important function of the internal and external obliques. The ability to decelerate and stabilize the spine against rotation, and/or rotation and extension forces is paramount to preventing injury.
    • Eccentric deceleration of extension of the spine.
    • Eccentric deceleration of contralateral flexion of the spine.
    • Eccentric deceleration of pelvic anterior tilting, lumbosacral extension and sacroiliac nutation.
      • Deceleration of these actions is particularly important in preventing excessive reduction of the neural foramina.
  • Synergists:
    • Trunk Rotation
    • Lumbar Flexion and Posterior Pelvic Tilting
    • Lateral Flexion of the Spine and Lateral Tilt of the Pelvis
    • Increased Intra-abdominal Pressure and Thoracolumbar Fascia Stiffness
      • Studies have demonstrated that the internal obliques, along with the transverse abdominis, lumbar multifidus, pelvic floor and potentially the diaphragm are activated prior to any limb movement (2, 36-37). This synergy is believed to be important for lumbopelvic stabilization, and may be referred to as the Intrinsic Stabilization Subsystem (ISS). The internal obliques contribute to stabilization by investing in the thoracolumbar fascia and linea alba bilaterally, forming a "tourniquet" that wraps around the abdomen. This enables the internal obliques to contribute to lumbar stiffness via a lateral force on the spinous process (primarily below L3) via the thoracolumbar fascia, stabilization of the sacroiliac joint (SIJ) via compression (force closure), and increased intra-abdominal pressure by decreasing intra-abdominal volume (62 - 64).
    • Forceful Exhalation
      • During forceful exhalation, the internal obliques function synergistically with the external obliquestransverse abdominis and rectus abdominis to decrease the circumference of the abdominal tourniquet and depress the ribs and costal cartilage. This decreases intrathoracic space and increases intra-abdominal pressure (1-2, 23, 29-30, 35). Further, the internal obliques may merge or have fascial continuity with the internal intercostals between the floating ribs, which may suggest a synergistic relationship with these respiratory muscles.

http://clinicalgate.com/anatomy-and-mechanics-of-the-abdominal-muscles/

Layering of the abdominals demonstrating the dept of the internal obliques compared to the external obliques. - http://clinicalgate.com/anatomy-and-mechanics-of-the-abdominal-muscles/

Fascial Integration

Anteriorly, the internal oblique fascia is continuous with the rectus sheath, linea alba, conjoint tendon and invests in the crest of the pubis. Above the arcuate line (the level of the umbilicus), the internal obliques splits at the semilunar lines and continues to the linea alba by coursing both anterior and posterior to the rectus abdominis (5-9, 19). Below the arcuate line the fascia of the internal obliques, external obliques and transverse abdominis merge at the semilunar lines and only run anterior to the rectus abdominis (5-9, 19). The most caudal fibers of the internal oblique fascia continue to fuse with the aponeurosis of the transverse abdominis to form the conjoint tendon, which aids in reinforcing the inguinal canal before continuing on to invest in the crest of the pubis and medial part of the pectineal line (5-8). The attachments to the pubis may imply some fascial continuity with the adductor tendons, in particular the adductor longus and pectineus. The more cranial fibers and fascia of the anterior portion of the internal obliques may exhibit fascial continuity with the internal intercostals between the floating ribs. Posteriorly, the internal obliques invest in the middle layer of thoracolumbar fascia along with the transverse abdominis (1-6).

Core Subsystems

The internal obliques are part of the Intrinsic Stabilization Subsystem (ISS). The ISS is a myofascial synergy that includes the anterior and middle layers of the thoracolumbar fascia, posterior layer of abdominal fascia, the fascia of the diaphragm and investing fascia and the pelvic floor muscles (levator ani, coccygeus). This fascial continuity may integrate the function of the internal obliques, transverse abdominis, multifidus, rotatores, interspinales and intertransversarii, pelvic floor and diaphragm (12-13, 39-43). This subsystem plays a significant role in increasing intra-abdominal pressure, tension in the thoracolumbar fascia, force closure/stabilization of the sacroiliac joint (SIJ), and segmental stability and alignment of the lumbar vertebrae. Although the ISS does not actively contribute to motion, increased intra-abdominal pressure may aid in decompressing/unloading the spine during motion. Further, an increase in intra-abdominal pressure, segmental rigidity and thoracolumabar fascia stiffness may aid in eccentrically decelerating spinal flexion and lateral flexion. The ISS is "the stabilization subsystem". Analogous to the rotator cuff of the shoulder, its optimal function may be essential for optimal performance of the other subsystems (muscles of the trunk). Inhibition of the ISS most often results in synergistic dominance of the Anterior Oblique Subsystem (AOS).

Additionally, the internal obliques may "bridge" the ISS and (AOS) due to the merging of the fascia of the lateral and anterior abdominal muscles at the semilunar lines, anterior rectus sheath and linea alba below the arcuate line, the fascial continuity between the internal obliques and adductor tendons at the pubis and pectinial line, and the fascial continuity between the external obliques and internal obliques at the lateral raphe of the thoracolumbar fascia.

For additional information:

Image of a cross-section (transverse) of the rectus abdominis, with layers of the rectus sheat, linea albe and obliques labeled.

Rectus Sheath - By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body. (Image: Bartleby.com: Gray's Anatomy, Plate 399, Public Domain, https://commons.wikimedia.org/w/index.php?curid=561473)

Arthrokinematics

Spine

Motion of the spine could be described as arthrokinematic motion at each facet (superior, inferior, lateral and medial glide), which results in osteokinematic motion (flexion, extension, lateral flexion and rotation) when the movement at each facet is combined. That is, extension is inferior glide of several facets of the spine, and flexion is superior glide of several facets of the spine (3). Like the rectus abdominis and external obliques, the internal obliques produce anterior shear and compression on the vertebral column, as well as facet joint distraction, anterior glide, and superior glide (superior on inferior facet) which are synonymous with forward bending/flexion of the spine (44). The forces of anterior shear (glide) and compression may be correlated with pathologies including posterior and posterior lateral herniation of lumbar discs, degenerative joint/disc disease (DJD and DDD) of the lumbar spine, and compression fractures in those with osteoporosis (3). A unilateral contraction of the internal obliques may laterally flex the spine contributing to facet joint inferior glide and compression; however, a unilateral contraction resulting in contralateral rotation and flexion would result in superior glide and anterior shear. Considering the mechanisms of herniation, DJD or DDD, and the opposing arthrokinematic motions that may result from unilateral contraction of the obliques, it seems unlikely that over-activity of the internal obliques would be a significant contributor to dysfunction of the lumbar facets.

Rib Cage

The rib cage forms a closed chain comprised of the sternum, ribs and thoracic vertebrae (30). The costovertebral joints are synovial joints formed by the head of a rib, the two adjacent vertebral bodies (superior and inferior to the rib), and the intervertebral disc (30). The costotransverse joints are synovial joints formed between the costal tubercle of the rib, and the costal facet of the transverse process of the corresponding vertebrae (30). The slightly convex demifacets of the ribs on the concave facets of the vertebral body, results in the rib facets gliding superiorly on the vertebral facets as they roll anteriorly during spinal flexion (and consequently internal oblique activity) (45). The motion of the costotransverse joints of ribs 1-6 is similar to the costovertebral joints; however, the facets on ribs 7-10 are planar and oriented in the anterolateroinferior to posteromediosuperior plane leading to a posterior, medial and superior glide during flexion and internal oblique activation (46).

Dysfunction of the costovertebral and costotransverse joints, especially those in the lower thoracic spine (inferior glide of the rib on both joints; or inferior glide of the rib at the costotransverse joint and superior glide at the costovertebral joint), likely leads to reflexive inhibition of the internal obliques on the same side. This is likely to result in limited spine rotation to one side. Costovertebral and costotransverse joint pain can be quite debilitating, resulting in transversospinalis and intercostal muscle spasm and pain during inhalation. Considering the research discussed below under "postural dysfunction", it seems plausible that a relationship may exist between Upper Body Dysfunction (UBD), AOS subsystem dominance, under-activity of the internal obliques and costovertebral/costotransverse joint dysfunction.

Illustration of costovertebral and costotransverse joints

By Henry Vandyke Carter - Henry Gray (1918) Anatomy of the Human Body (See "Book" section below)Bartleby.com: Gray's Anatomy, Plate 313, Public Domain, https://commons.wikimedia.org/w/index.php?curid=85402

Behavior in Postural Dysfunction

The internal obliques are prone to under-activity and an increase in length (3, 39, 47-50). As mentioned above, although the internal obliques perform the same actions as the external obliques (with the exception of rotation in the opposite direction), the internal obliques behave like the transverse abdominis and muscles of the Intrinsic Stabilization Subsystem (ISS). Studies discussed below demonstrate that dysfunction is likely to result in, or result from, transverse abdominis and internal oblique under-activity, and the opposing change from the external obliques which have a propensity to become over-active.

Several studies have investigated the EMG activity of the internal obliques during various exercises.  Sparkes et al. demonstrated preferential recruitment of the internal obliques, when compared to rectus abdominis activity during quadrupeds with arm raise, and quadrupeds with opposite arm/leg raise (66). Do et al. used ultrasound (US) to demonstrate increased thickness (stronger contraction) of the transverse abdominis and internal obliques during a plank, increased thickness when an unstable surface was placed under the the upper extremity, and a larger increase in thickness when an unstable surface was placed under the lower extremity (67). These studies demonstrate that quadrupeds preferentially recruit the internal obliques (and transverse abdominis), and that increasing instability may be used to progress exercise. Anderson et al. compared push-ups to push-ups with feet on a stability ball, hands on a balance board, or both, demonstrating that increasing the challenge to stability increased core muscle activity (including the internal obliques) with dual instability resulting in the most activity (68). Norwood et al. demonstrated internal oblique (and core muscle) activity increased when bench press was performed with an unstable surface under the upper or lower body, and like Anderson et al. demonstrated the largest increase in activity with dual instability (69) These studies suggest that in addition to core exercise, adding instability to resistance training exercise, may also increase internal oblique recruitment. Last, several studies investigate the abdominal drawing in maneuver (ADIM) and internal oblique recruitment. Mew et al. demonstrated that the abdominal drawing in maneuver resulted in the greatest muscle thickness in the transverse abdominis and internal obliques in standing when compared to hook lying; however, the greatest change in thickness between resting and active for the obliques was noted in the hook lying position (70). An interesting study by Chan et al. demonstrated that the ADIM increased both internal oblique activity and gluteus medius and gluteus maximus activity during clams, side-lying hip abduction and prone hip extensions (71). A study by Chon et al. demonstrated that adding dorsiflexion to exercises using the ADIM increased transverse abdominis activity, as well as internal oblique activity (72). In summary, quadrupeds preferentially recruit the internal obliques (and transverse abdominis), adding unstable environments to core or resistance training exercise increases core muscle activity, and the ADIM may increase internal oblique activity (and transverse abdominis) and optimize recruitment of stabilizing musculature during various exercises and functional tasks.

The Brookbush Institute (BI) does not attempt to address internal oblique activity in isolation, as mentioned above, studies demonstrate that the internal obliques are included in interventions intended for the integration of the Intrinsic stabilization Subsystem (ISS) and/or activation of the Transverse Abdominis. This includes "Quadrupeds and Progressions", as well as cuing the abdominal drawing-in maneuver (ADIM) during exercise and functional activities.

  • Upper Body Dysfunction (UBD) may result in the internal obliques exhibiting a decrease in activity relative to changes in core subsystem recruitment. In those exhibiting signs of UBD and a thoracic kyphosis, the Anterior Oblique Subsystem (AOS) may become over-active and synergistically dominant for relative inhibition of the Posterior Oblique Subsystem (POS) and Intrinsic Stabilization Subsystem (ISS) (including the internal obliques). The altered activity of these subsystems, in particular the inhibition of the ISS, leads to reliance on the prime movers for stability, including the latissimus dorsi and the AOS. This can be illustrated using the Overhead Squat Assessment (OHSA). When an individual with UBD performs the OHSA with hands overhead, the arms fall forward or adduct and the spine may stay relatively neutral, because the over-activity of the Anterior trunk muscles are balanced by over-activity of the latissimus dorsi. If the OHSA is performed again with hands on the pelvis, the individual will usually exhibit signs of lumbar flexion or an excessive forward lean, because the latissimus dorsi are no longer taut and resisting the activity of the over-active AOS.
  • Lumbo Pelvic Hip Complex Dysfunction (LPHCD) and low back pain are correlated with a decrease in internal oblique and transverse abdominis activity, and an increase in external oblique activity. Hodges et al. demonstrated that the internal obliques and the transverse abdominis fired before lower extremity activity in asymptomatic individuals, and both muscles exhibited latent firing patterns in those with low back pain (36). In two studies by Ng et al., decreased internal obliques activity and increased external obliques activity was noted during static and dynamic rotation of the spine in those exhibiting symptoms of low back pain (20, 51). In studies by O'Sullivan et al. and Hungerford et al., decreased internal obliques activity was correlated with "sway back" posture and SIJ pain (27, 65). Further, Cholewicki et al. demonstrated that delayed trunk muscle reactivity (including the internal obliques) was a predictor of future low back pain (52). Although they are often omitted in the discussion of muscle recruitment and the lumbar spine, activity of the internal obliques is commonly investigated in conjunction with the transverse abdominis. In summary, the internal obliques exhibit a propensity to mirror activity of the transverse abdominis, including decreased activity in those exhibiting pain or dysfunction of the low back, and/or SIJ.
  • Sacroiliac Joint Dysfunction (SIJD)  Several studies mentioned above allude to the relationship between the SIJ and ISS. Specifically, the study by van Wingerden et al. demonstrated that the muscles of the abdominal tourniquet (including the internal obliques) increased thoracolumbar fascia tension and contributed to force closure (stabilization via compression) of the sacroiliac joint (63). Further, the study by Hungerford et al. demonstrated that the internal obliques are likely to exhibit under-activity in those with SIJ pain (65). It is recommended that following specific interventions to address asymmetry in mobility, stiffness and stability of the SIJ and hip joints, that Intrinsic stabilization Subsystem (ISS) exercise are performed to aid in optimal stabilization of the SIJ.
  • Lower Extremity Dysfunction (LED) similar to UBD, activity of the internal obliques may be altered in conjunction with altered recruitment of the core subsystems. As in UBDLED is likely to result in AOS over-activity, relative to inhibition of the POS and ISS; however, in LED this altered behavior contributes to, or is correlated with an excessive forward lean. A prospective study Zazulak et al., demonstrated that a delay in trunk muscle reactivity (including the internal obliques) was correlated with a higher risk of future knee injury (59). This likely implies that core exercise consistently be part of exercise programs designed for optimizing recovery or performance of the lower extremity.

Assessment

Overhead Squat Assessment:

Brookbush/Grieve (56) Sacroiliac Joint (SIJ) Dysfunction Movement Assessment Cluster:

Goniometric Assessment:

  • Decreased or asymmetrical spine rotation

Related Pathology:

  • Rectus abdominis tendonitis
  • Sports Hernia
  • Diastasis Recti
  • Costovetebral and costotransverse joint pain
  • Intercostal pain
  • Low back pain
  • Thoracic pain
  • Sacroiliac joint dysfunction
  • Pubic symphysis dyskinesis

Trigger Points and Referral Pain Patterns:

Active trigger points in the internal and external obliques have the potential for referral pain patterns that extend into the chest, down toward the pelvis, and obliquely across the rectus abdominis (2). Further, active trigger points in the internal obliques can produce symptoms similar to heartburn, appendicitis, pelvic pain syndromes, and urinary tract disease (2). These visceral-like symptoms can easily confound or be correlated with visceral dysfunction (2, 57-60). Careful assessment and referral to a physician should be considered when a clear delineation cannot be made between visceral dysfunction and trigger points in the abdomen.

Release techniques for the internal obliques are generally ineffective. This is likely due to the internal obliques rarely presenting with active trigger points, the difficulty in attempting Release techniques on a muscle that has nothing to press the muscle against (e.g. bone), and pincer Release being difficult or impractical due to the closely packed layers of fascia and abdominal muscle.

The internal obliques cannot be differentiated from the external obliques or transverse abdominis with manual palpation. However, the manual therapist can evaluate the abdominal muscles as a group, for the presence trigger points. This is generally performed with the individual in supine or on their contralateral side, taking deep, diaphragmatic breaths to passively stretch the abdominal musculature and increase pressure sensitivity (2). The human movement professional can then use oblique pressure to identify acute points of increased tissue density, acute points of pain pressure sensitivity, or points that result in referral pain and a replication of the patients symptoms. Common trigger point locations specific to the internal obliques include the inferior margin of the tips of ribs 6-12, and/or close to the pubic bone (2).


Related Videos

Transverse Abdominis Isolated Activation (Quadrupeds):

Quadruped Crawl (Dynamic Quadruped):

Additional Quadruped Crawl Progressions:

For Additional Videos: Transverse Abdominis Activation

Bibliography:

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© 2020 Brent Brookbush

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